# From Food to You: Nutrients, Digestion and Metabolism

**Published edition** — 11 September 2026.

**Carbon Atlas · Made from two-year-old notes by Shreyam Adhikari.**

**Original notes: circa 2024 · Recovered 9 September 2026 · Refined 11 September 2026**

What’s in a meal? Where does it go? Follow nutrients through digestion, into the structures your body builds, the fuel it stores, and the energy you use tomorrow.

This article grew from my two-year-old Google Docs notes and learning from Dr. Andy Galpin's physiology lessons. This edition brings that material back into one connected explanation, with the science checked against textbooks, official nutrient references and original research. The original notes date to around 2024; this recovered edition was refined in September 2026.

[Read the original metabolism field notes](https://docs.google.com/document/d/1Gm1ZX6PRb_QBfdjxeoIPJYOlRZJazi30kODUNmqtGpA/edit?usp=sharing).

Oxidized fat can leave through your breath. **Breathing faster is not a fat-loss shortcut.** To follow that final part of the journey, we first need to know what food contains and what cells do with it.

- [01 · First, meet the ingredients](#food-map)
- [02 · The nutrient atlas](#nutrients)
- [03 · The whole journey](#big-picture)
- [04 · Digestion & absorption](#digestion)
- [05 · Carbohydrate’s route](#carbohydrates)
- [06 · Dietary fat’s route](#fats)
- [07 · Protein’s route](#proteins)
- [08 · Build, store & release](#storage)
- [09 · How cells make ATP](#energy)
- [10 · Tomorrow’s energy](#context)
- [11 · Breath & metabolism](#breath)
- [12 · Check your understanding](#quiz)
- [13 · Sources and field notes](#sources)

<a id="food-map"></a>

## 01 · First, meet the ingredients

A meal is a mixture of molecules. Some supply fuel; others become structures, help reactions happen or provide the water in which those reactions occur. An ingredient such as a lentil contains several categories at once.

Here is a useful starting classification:

| Family | What belongs here | A useful distinction |
| --- | --- | --- |
| Carbohydrates | Sugars, starches and most dietary fiber | Molecular structure and digestibility are related, but different classifications. |
| Fats and other lipids | Triglycerides, phospholipids and cholesterol | Fatty-acid types describe chain chemistry; they are not kinds of fat cells. |
| Proteins | Chains built from amino acids | Amino acids supply building material and can also contribute fuel. |
| Water | H₂O | Needed in large amounts, while providing no calories. |
| Vitamins | Thirteen recognized vitamins | A, D, E and K are fat-soluble; C and the eight B vitamins are water-soluble. |
| Minerals | Major and trace mineral elements | Major and trace describe quantities, not importance. |
| Choline | A nutrient with membrane and signaling roles | Essential, although outside the conventional 13-vitamin list. |
| Other plant compounds | Examples include polyphenols, lycopene and lutein | Biological activity does not automatically establish nutritional essentiality. |

“Macro” and “micro” describe the amounts needed, not the size of a molecule. Water is a small molecule and belongs in the larger-quantity group. This is a guided inventory rather than a complete chemical taxonomy. [USDA macronutrient resources](https://www.nal.usda.gov/human-nutrition-and-food-safety/food-composition/macronutrients), [MedlinePlus vitamin inventory](https://medlineplus.gov/vitamins.html), [MedlinePlus minerals](https://medlineplus.gov/minerals.html), [NIH choline](https://ods.od.nih.gov/factsheets/Choline-HealthProfessional/).

### Essential is a separate question

An **essential nutrient** requires a dietary supply because the body cannot make enough. It is a statement about supply, not a ranking of importance. An amino acid described as nonessential is still used by the body; ordinarily, the body can synthesize it. Some ordinarily synthesizable amino acids become **conditionally essential** when synthesis is insufficient in particular physiological or illness states. [MedlinePlus amino acids](https://medlineplus.gov/ency/article/002222.htm).

These labels cut across the family tree. Essential fatty acids belong within lipids; essential amino acids belong within protein chemistry. They are not extra families competing with fats and proteins.

### Some branches overlap

Most fiber is carbohydrate, but common dietary-fiber definitions also include lignin, which is not. Resistant starch illustrates another overlap: it is starch by structure, while escaping digestion in the small intestine. [FDA dietary-fiber definition](https://www.fda.gov/food/nutrition-food-labeling-and-critical-foods/questions-and-answers-dietary-fiber).

Plant compounds do not fit one sealed “non-nutrient” box either. Some carotenoids can supply vitamin A after conversion; others, including lycopene and lutein, cannot. A chemical's plant origin does not settle whether it is an essential nutrient. [NIH vitamin A and carotenoids](https://ods.od.nih.gov/factsheets/VitaminA-HealthProfessional/).

Now take the three main fuel-bearing families in turn. Their structures help explain why digestion and metabolism send them down different routes.

---

<a id="nutrients"></a>

## 02 · The nutrient atlas

Vitamins and minerals help reactions, structures and signals work. They supply no calories themselves. Riboflavin helps form FAD and FMN; niacin contributes to NAD and NADP; pantothenic acid is needed for coenzyme A. [24], [25], [26]

The atlas brings the pieces together: vitamins, minerals, water, choline, essential amino acids and essential fatty acids. It also separates broader food families from special cases. More is not automatically better: being required for a reaction does not mean extra amounts speed it up.

One continuous interactive table shows all 82 entries together. It includes 39 core nutrient entries, 17 food-family entries, 17 selected molecular examples or building blocks, and 9 context entries. Color groups relatives; ESS, FAM, PART and NOTE labels distinguish their meaning. Search highlights matches in place. Positions are editorial, not atomic numbers or chemical periodicity. Mineral tiles use element symbols; others use abbreviations. Vitamin K and potassium (K) are different entries.

An adult-nutrition learning map: core nutrients, food families, selected molecular examples and classification notes. These are not distinct molecules in every case, and this is not a complete inventory of the body or a supplement checklist. Family, component and context entries are not additional essential dietary requirements.

The table begins with water and choline, then follows carbohydrates, lipids, proteins, vitamins, minerals, plant compounds and special cases. Carbohydrates show single and paired sugars beside larger families. Lipids separate structure, saturation, geometry and essential fatty acids. Protein includes all twenty standard amino acids: nine indispensable and eleven usually synthesized. Small screens reflow the same complete table into readable family blocks.

**Essentiality describes supply, not importance.** Some requirements can also be met through synthesis or precursors: vitamin D can be made in skin and niacin can be formed from tryptophan. Fiber has intake guidance even though it is not a single indispensable molecule. ‘Major’ and ‘trace’ describe amounts required, not importance.

### Water & choline

Two nutrients that deserve their own place.

**Small pieces. A connected whole.** Read across each family. Then open a tile to find its role, relatives and references. A shared color means related chemistry—not the same dietary requirement.

#### Water + choline

**Water (H₂O) — Essential nutrient.** Forms body fluids, transports dissolved substances, lubricates tissues and supports temperature regulation. Water comes from drinks, food and metabolic reactions. Water is a macronutrient by quantity, but it does not supply calories. [Reference](https://medlineplus.gov/ency/article/002471.htm)

**What the name includes.** Water is the same H2O molecule in a glass or within food; this macronutrient provides volume without supplying calories.

**Where it appears.** Plain water, milk, soup, tea and the water contained in foods all contribute to total water intake.

**How the body handles it.** Water forms body fluids, carries dissolved substances and lubricates tissues; evaporation of sweat helps regulate body temperature.

**Choline (Cho) — Essential nutrient.** Used to make phosphatidylcholine, sphingomyelin and acetylcholine; also contributes methyl groups and supports lipid transport. The body makes some choline, but normally needs a dietary contribution. The body makes some choline, but dietary choline is still needed. It is not a fourteenth vitamin. [Reference](https://ods.od.nih.gov/factsheets/Choline-HealthProfessional/)

**What the name includes.** Food supplies free choline and several choline-containing compounds, including water-soluble forms and the phospholipids phosphatidylcholine and sphingomyelin.

**Where it appears.** Eggs, fish, dairy, soybeans, other beans and cruciferous vegetables are examples of foods that contribute choline.

**How the body handles it.** Water-soluble forms enter portal blood toward the liver, while some intact fat-soluble forms enter chylomicrons and travel through lymph.

### Carbohydrates

Single sugars → joined sugars → larger structures.

**Length is one question.** Digestion is another. Fiber is not simply the longest chain; starch can be very long too. [Reference](https://www.fda.gov/food/nutrition-food-labeling-and-critical-foods/questions-and-answers-dietary-fiber)

#### One sugar unit

**Glucose (Glc) — Monosaccharide · one sugar unit.** Glucose is a single sugar. Starch and glycogen are built from glucose units. [Reference 1](https://openstax.org/books/biology-2e/pages/3-2-carbohydrates) · [Reference 2](https://openstax.org/books/anatomy-and-physiology-2e/pages/23-7-chemical-digestion-and-absorption-a-closer-look)

**What the name includes.** One sugar unit. Starch digestion supplies glucose, and glucose is also present in foods such as fruit. Glucose, fructose and galactose share the formula C₆H₁₂O₆ but arrange atoms differently.

**Where it appears.** Glucose occurs in fruits and honey and is released when digestible starch is broken down.

**How the body handles it.** Absorbed glucose reaches portal blood and the liver, then other tissues. It can support ATP regeneration, glycogen storage and synthesis.

**Fructose (Fru) — Monosaccharide · one sugar unit.** Fructose is a single sugar. It has the same molecular formula as glucose but a different arrangement of atoms. [Reference 1](https://openstax.org/books/biology-2e/pages/3-2-carbohydrates) · [Reference 2](https://openstax.org/books/anatomy-and-physiology-2e/pages/23-7-chemical-digestion-and-absorption-a-closer-look)

**What the name includes.** One sugar unit with a different arrangement from glucose. Glucose and fructose are the two units in sucrose. Glucose, fructose and galactose share the formula C₆H₁₂O₆ but arrange atoms differently.

**Where it appears.** Fruit and honey contain fructose. Sucrose also provides fructose when its bond is split.

**How the body handles it.** This single sugar can be absorbed without first splitting a sugar–sugar bond. It reaches portal blood; intestinal and liver processing help integrate its carbon into metabolism.

**Galactose (Gal) — Monosaccharide · one sugar unit.** Galactose is a single sugar. Joined to glucose, it forms lactose. [Reference 1](https://openstax.org/books/biology-2e/pages/3-2-carbohydrates) · [Reference 2](https://openstax.org/books/anatomy-and-physiology-2e/pages/23-7-chemical-digestion-and-absorption-a-closer-look)

**What the name includes.** One sugar unit. In lactose, galactose is linked to glucose; digestion separates them. Glucose, fructose and galactose share the formula C₆H₁₂O₆ but arrange atoms differently.

**Where it appears.** Much dietary galactose comes from digestion of lactose in milk and milk products.

**How the body handles it.** This single sugar can be absorbed without first splitting a sugar–sugar bond. It reaches portal blood; intestinal and liver processing help integrate its carbon into metabolism.

#### Two sugar units

**Sucrose (Suc) — Disaccharide · two sugar units.** Sucrose joins two single sugars: glucose and fructose. [Reference 1](https://openstax.org/books/biology-2e/pages/3-2-carbohydrates) · [Reference 2](https://openstax.org/books/anatomy-and-physiology-2e/pages/23-7-chemical-digestion-and-absorption-a-closer-look)

**What the name includes.** Glucose + Fructose form this two-sugar combination. α(1↔2)β describes the connecting bond.

**Where it appears.** Sucrose occurs in many plants and is the sugar commonly used at the table.

**How the body handles it.** Enzymes at the small-intestinal surface split this disaccharide into single sugars before absorption. The products can then enter portal blood.

**Lactose (Lac) — Disaccharide · two sugar units.** Lactose joins two single sugars: glucose and galactose. [Reference 1](https://openstax.org/books/biology-2e/pages/3-2-carbohydrates) · [Reference 2](https://openstax.org/books/anatomy-and-physiology-2e/pages/23-7-chemical-digestion-and-absorption-a-closer-look)

**What the name includes.** Galactose + Glucose form this two-sugar combination. β(1→4) describes the connecting bond.

**Where it appears.** Lactose is the characteristic sugar in milk; amounts differ across milk products.

**How the body handles it.** Lactase splits lactose at the intestinal surface. Lower lactase activity leaves more lactose available to colonic microbes; digestion depends on the person and the amount.

**Maltose (Mal) — Disaccharide · two sugar units.** Maltose joins two single sugars: glucose and glucose. [Reference 1](https://openstax.org/books/biology-2e/pages/3-2-carbohydrates) · [Reference 2](https://openstax.org/books/anatomy-and-physiology-2e/pages/23-7-chemical-digestion-and-absorption-a-closer-look)

**What the name includes.** Glucose + Glucose form this two-sugar combination. α(1→4) describes the connecting bond.

**Where it appears.** Maltose occurs during starch breakdown, including in germinated grains and malted foods.

**How the body handles it.** Enzymes at the small-intestinal surface split this disaccharide into single sugars before absorption. The products can then enter portal blood.

#### Broader families

**Sugars (Sug) — Carbohydrate family.** Sugars include single units (monosaccharides) and pairs (disaccharides). The adjacent tiles show familiar examples. Glucose is an important body fuel, but the body can also make it; an individual sugar is not classified like an indispensable amino acid. [Reference 1](https://www.accessdata.fda.gov/scripts/interactivenutritionfactslabel/assets/InteractiveNFL_Glossary_October2021.pdf) · [Reference 2](https://openstax.org/books/anatomy-and-physiology-2e/pages/24-2-carbohydrate-metabolism) · [Reference 3](https://openstax.org/books/anatomy-and-physiology-2e/pages/23-7-chemical-digestion-and-absorption-a-closer-look)

**What the name includes.** Monosaccharides are single sugars; disaccharides join two. These are structural categories. A food can contain both, alongside starch, fiber, fat and protein.

**Where it appears.** Fruit, milk and many other foods contain sugars. Sucrose is the sugar commonly used at the table; lactose occurs in milk.

**How the body handles it.** Disaccharides must be split before absorption. Glucose, galactose and fructose use different transport and processing routes; the liver helps handle the arriving supply.

**Starch (Sta) — Carbohydrate family.** Amylose is mostly unbranched; amylopectin is branched. Digestible starch supplies glucose. Resistant starch escapes small-intestinal digestion and overlaps the fiber category. [Reference 1](https://www.accessdata.fda.gov/scripts/interactivenutritionfactslabel/assets/InteractiveNFL_Glossary_October2021.pdf) · [Reference 2](https://openstax.org/books/biology-2e/pages/3-2-carbohydrates) · [Reference 3](https://pubmed.ncbi.nlm.nih.gov/33530525/) · [Reference 4](https://www.ncbi.nlm.nih.gov/books/NBK223591/)

**What the name includes.** Amylose is mostly unbranched; amylopectin is branched. Both contain glucose, but their connections and arrangement influence how the material behaves.

**Where it appears.** Rice, potatoes, bread, beans and lentils can supply starch. Cooking, cooling and the surrounding food structure can alter its digestibility.

**How the body handles it.** Digestible starch supplies glucose. Resistant starch escapes some small-intestinal digestion and can reach the colon, where microbes may ferment it.

**Dietary fiber (Fib) — Functional food category.** Dietary fiber includes nondigestible carbohydrates and lignin. Some fibers are fermented by microbes; their effects depend on structure and properties. Cellulose, pectins, oat/barley beta-glucans, inulin/fructans, resistant starch and psyllium show different combinations of properties. Lignin is not a carbohydrate. Fiber has dietary guidance and important physiological roles; a family badge does not mean it is unnecessary. Fiber is not simply the biggest carbohydrate chain: digestibility matters, and some fibers are shorter oligosaccharides. Starch can also form very long chains. [Reference 1](https://www.fda.gov/food/nutrition-food-labeling-and-critical-foods/questions-and-answers-dietary-fiber) · [Reference 2](https://www.ncbi.nlm.nih.gov/books/NBK223591/) · [Reference 3](https://www.ncbi.nlm.nih.gov/books/NBK619141/table/ch2.tab2/?report=objectonly)

**What the name includes.** Solubility asks how fiber behaves in water. Viscosity asks whether it thickens. Fermentability asks whether microbes can break it down. These are overlapping properties.

**Where it appears.** Oats, beans, vegetables, fruit, whole grains, nuts and seeds provide different fiber mixtures. No single food stands for every fiber type.

**How the body handles it.** Fiber resists digestion in the small intestine. Microbes ferment some into short-chain fatty acids; the unfermented fraction and microbial biomass contribute to stool.

#### Body store

**Glycogen (Glyc) — Body-made glucose store.** The body builds glycogen from glucose, especially in liver and muscle. Liver glycogen can support blood glucose; muscle glycogen mainly supports that muscle. Glycogen and plant starch are related storage carbohydrates, but they are not the same structure or dietary requirement. [Reference 1](https://openstax.org/books/anatomy-and-physiology-2e/pages/24-2-carbohydrate-metabolism) · [Reference 2](https://openstax.org/books/biology-2e/pages/3-2-carbohydrates)

**What the name includes.** Glycogen is a highly branched glucose polymer. Most connections are α(1→4), with α(1→6) bonds at branch points.

**Where it appears.** Glycogen is shown here as a body-made glucose store, not a separate essential food requirement.

**How the body handles it.** Liver glycogen helps support blood glucose. Muscle glycogen is a local reserve for that muscle’s work; the two stores have different jobs.

### Fats & lipids

Structure, double-bond count and geometry are different ways to sort.

**Labels can overlap.** LA and ALA are both polyunsaturated. Omega-6 and omega-3 locate the first double bond from the methyl end: another way to classify fatty acids. [Reference](https://ods.od.nih.gov/factsheets/Omega3FattyAcids-HealthProfessional/)

#### Molecular structure

**Triglycerides (TAG) — Storage-lipid family.** Most dietary fat and stored body fat is triglyceride: a glycerol backbone joined to three fatty-acid chains. The three chains need not be identical. [Reference 1](https://openstax.org/books/organic-chemistry/pages/27-1-waxes-fats-and-oils) · [Reference 2](https://openstax.org/books/anatomy-and-physiology-2e/pages/23-7-chemical-digestion-and-absorption-a-closer-look) · [Reference 3](https://www.ncbi.nlm.nih.gov/books/NBK305896/)

**What the name includes.** Triacylglycerol and triglyceride name the same structure: glycerol attached to three fatty acids. Mono- and diacylglycerols have one and two fatty-acyl tails.

**Where it appears.** Oils, butter, nuts, seeds and many animal foods contain triglycerides with mixtures of fatty-acid tails.

**How the body handles it.** Digestion releases fatty acids and monoacylglycerols. Intestinal cells rebuild much of the long-chain triglyceride and package it in chylomicrons for lymph, then blood.

**Phospholipids (PL) — Membrane-lipid family.** Phospholipids are phosphate-containing lipids. Their water-interacting and water-avoiding regions help organize cell membranes. Phosphatidylcholine is one member. Choline is an essential nutrient used to make particular phospholipids; the whole lipid family is not one extra essential nutrient. [Reference 1](https://www.ncbi.nlm.nih.gov/books/NBK26871/) · [Reference 2](https://openstax.org/books/anatomy-and-physiology-2e/pages/23-7-chemical-digestion-and-absorption-a-closer-look) · [Reference 3](https://www.ncbi.nlm.nih.gov/books/NBK26883/)

**What the name includes.** A phosphate-bearing region interacts with water while other regions avoid it. Glycerophospholipids and sphingomyelin have different backbones; the builder uses one glycerol-based example.

**Where it appears.** Eggs, soybeans and cell-containing foods supply phospholipids. The body also synthesizes them.

**How the body handles it.** Phospholipids help form cell membranes and lipoprotein surfaces. Their components are digested, absorbed and reused; a dietary membrane does not simply become an intact body membrane.

**Sterols (Stl) — Ring-structured lipid family.** Cholesterol is a sterol used in cells and as a precursor to steroid hormones, bile acids and vitamin D. The body makes cholesterol. Having an important body function does not make dietary cholesterol essential. [Reference 1](https://medlineplus.gov/cholesterol.html) · [Reference 2](https://www.ncbi.nlm.nih.gov/books/NBK305896/)

**What the name includes.** Sterols have a ring-based structure. Cholesterol is the principal animal sterol; phytosterols are related plant molecules. Neither is a three-tailed triglyceride.

**Where it appears.** Animal foods can supply cholesterol; plant foods supply phytosterols. Humans also synthesize cholesterol.

**How the body handles it.** Cholesterol contributes to membranes, bile acids and steroid molecules. Lipoproteins carry it through blood; LDL and HDL describe particles, not separate cholesterol chemicals.

#### Double-bond count

**Saturated fatty acids (SFA) — Fatty-acid family.** Saturated fatty-acid chains have no carbon-carbon double bonds. Palmitic and stearic acids. Saturation describes bond structure; it does not summarize the nutritional profile of a whole food. [Reference](https://www.fda.gov/regulatory-information/search-fda-guidance-documents/small-entity-compliance-guide-trans-fatty-acids-nutrition-labeling-nutrient-content-claims-and)

**What the name includes.** Saturation means no carbon–carbon double bonds in a fatty-acid chain. It does not mean the chain has no flexibility, or that an entire food contains only one fatty acid.

**Where it appears.** Butter, coconut oil, meat and dairy fat contain saturated fatty acids in differing proportions. Foods also contain other fatty-acid types.

**How the body handles it.** After digestion and transport, fatty acids can be oxidized, stored or used in other lipids. Saturation is a structural feature, not a destination label.

**Monounsaturated fatty acids (MUFA) — Fatty-acid family.** A monounsaturated fatty-acid chain has one carbon-carbon double bond. Oleic acid. [Reference](https://www.fda.gov/regulatory-information/search-fda-guidance-documents/small-entity-compliance-guide-trans-fatty-acids-nutrition-labeling-nutrient-content-claims-and)

**What the name includes.** A monounsaturated fatty acid has one carbon–carbon double bond. Geometry and omega position add information; oleic acid is a cis omega-9 example.

**Where it appears.** Olive oil, avocados and many nuts contain monounsaturated fatty acids alongside other kinds.

**How the body handles it.** These fatty acids can enter fuel pathways or become parts of storage and membrane lipids. A shared bond count does not make every monounsaturated molecule identical.

**Polyunsaturated fatty acids (PUFA) — Contains essential members.** Polyunsaturated fatty-acid chains have two or more carbon-carbon double bonds. LA and ALA belong to this group. Omega numbering locates the first double bond from the methyl end. It is a second classification, alongside saturation. [Reference 1](https://www.fda.gov/regulatory-information/search-fda-guidance-documents/small-entity-compliance-guide-trans-fatty-acids-nutrition-labeling-nutrient-content-claims-and) · [Reference 2](https://ods.od.nih.gov/factsheets/Omega3FattyAcids-HealthProfessional/?redirect=false)

**What the name includes.** Polyunsaturated means at least two carbon–carbon double bonds. Omega-3 and omega-6 specify the first double bond from the methyl end, so a fatty acid can be both polyunsaturated and omega-3, or both polyunsaturated and omega-6.

**Where it appears.** Seeds, nuts, vegetable oils and fish provide different polyunsaturated fatty acids. The specific mix matters to the name, even within one omega family.

**How the body handles it.** Polyunsaturated fatty acids contribute to membranes, storage and signaling precursors as well as energy metabolism. LA and ALA are dietary essentials.

#### Bond geometry

**Cis fatty acids (Cis) — Double-bond geometry.** Cis describes the arrangement around a carbon-carbon double bond in an unsaturated fatty acid. The carbon-chain segments lie on the same side of that bond. Cis and trans describe geometry. Mono- and polyunsaturated describe how many double bonds there are; these classifications overlap. [Reference](https://openstax.org/books/biology-2e/pages/3-3-lipids)

**What the name includes.** Cis describes geometry around a double bond. Most naturally occurring unsaturated fatty acids have cis double bonds; a chain can have more than one.

**Where it appears.** Oleic acid in olive oil and linoleic acid in many seed oils are cis examples with different double-bond counts.

**How the body handles it.** The geometry influences a chain’s shape and packing. It is separate from chain length, saturation count and the omega family.

**Trans fatty acids (Trans) — Double-bond geometry.** Trans describes the arrangement around a carbon-carbon double bond in an unsaturated fatty acid. Trans is a geometry label, not a fourth double-bond-count category. It can overlap mono- and polyunsaturated structures. [Reference](https://www.fda.gov/regulatory-information/search-fda-guidance-documents/small-entity-compliance-guide-trans-fatty-acids-nutrition-labeling-nutrient-content-claims-and)

**What the name includes.** Trans geometry places groups differently around a double bond. A trans fatty acid still has that double bond and therefore remains unsaturated.

**Where it appears.** Some trans fats arise in ruminant foods; industrial partial hydrogenation can also produce them. These are origin distinctions, not different definitions of trans.

**How the body handles it.** Trans and cis chains can share a formula and double-bond count while differing in geometry. The builder compares oleic and elaidic acid to isolate this structural change.

#### Essential fatty acids

**Linoleic acid (LA) — Essential fatty acid.** Linoleic acid is an omega-6 polyunsaturated fatty acid that humans must obtain from the diet. [Reference](https://ods.od.nih.gov/factsheets/Omega3FattyAcids-HealthProfessional/?redirect=false)

**What the name includes.** LA is 18:2 omega-6, with two cis double bonds. It is both polyunsaturated and omega-6; the two labels answer different questions.

**Where it appears.** Many vegetable oils, nuts and seeds provide linoleic acid as part of mixed dietary lipids.

**How the body handles it.** LA contributes to membrane lipids and can supply material for longer omega-6 fatty acids. Humans cannot synthesize linoleic acid, so dietary supply is needed.

**Alpha-linolenic acid (ALA) — Essential fatty acid.** Alpha-linolenic acid is an omega-3 polyunsaturated fatty acid that humans must obtain from the diet. EPA and DHA are other omega-3s. Conversion from ALA is limited; they are not additional universally indispensable adult fatty acids. [Reference](https://ods.od.nih.gov/factsheets/Omega3FattyAcids-HealthProfessional/?redirect=false)

**What the name includes.** ALA is 18:3 omega-3, with three cis double bonds. EPA (20:5) and DHA (22:6) are longer members of the omega-3 family.

**Where it appears.** Flax, chia, walnuts and some vegetable oils provide ALA. Fish and algae can supply the longer omega-3s EPA and DHA.

**How the body handles it.** ALA is a dietary essential. Conversion to EPA and DHA is limited, so sharing an omega family does not make these fatty acids interchangeable.

### Proteins & amino acids

Twenty standard building blocks. Many possible sequences.

**9 + 11 = 20.** “Essential” describes dietary supply. “Nonessential” does not mean unimportant. Some needs depend on the physiological setting. [Reference](https://medlineplus.gov/ency/article/002222.htm)

#### The family

**Proteins (Prt) — Macronutrient family.** Proteins are amino-acid chains folded into working structures. The standard protein alphabet has twenty amino acids: nine indispensable in adult diets and eleven the body can usually make. A protein need not contain all twenty types. Its sequence, length and folding matter. The body needs an adequate overall supply of amino acids and nitrogen, including the nine indispensable amino acids. [Reference 1](https://www.ncbi.nlm.nih.gov/books/NBK26830/) · [Reference 2](https://medlineplus.gov/ency/article/002222.htm) · [Reference 3](https://openstax.org/books/anatomy-and-physiology-2e/pages/23-7-chemical-digestion-and-absorption-a-closer-look) · [Reference 4](https://www.ncbi.nlm.nih.gov/books/NBK26829/)

**What the name includes.** Proteins are ordered amino-acid chains. Chain length, sequence, folding, processing and assembly all matter; a short list of ingredients does not specify a working protein.

**Where it appears.** Beans, lentils, soy, dairy, eggs, fish, meat, nuts and seeds supply protein. Foods differ in amino-acid proportions and digestibility.

**How the body handles it.** Digestion supplies amino acids and small peptides. Absorbed and recycled amino acids join a shared supply for synthesis and other metabolism, with no dedicated surplus-protein tank.

#### Nine indispensable amino acids

**Histidine (His) — Indispensable amino acid.** An indispensable amino acid used in proteins. [Reference 1](https://medlineplus.gov/ency/article/002222.htm) · [Reference 2](https://www.ncbi.nlm.nih.gov/books/NBK26830/)

**What the name includes.** His (H) has the side group CH₂–imidazole. Its protonation depends strongly on pH and local environment; it is not always positively charged. Side-group chemistry is a separate classification from whether dietary supply is indispensable.

**Where it appears.** This amino acid occurs within food proteins, alongside other amino acids. Protein-rich foods supply mixtures; no single food belongs exclusively to one amino acid.

**How the body handles it.** Absorbed and recycled amino acids can be assembled into new proteins according to cellular instructions. Their side groups influence interactions and folding; a particular mixture alone does not specify a protein.

**Isoleucine (Ile) — Indispensable amino acid.** An indispensable amino acid used in proteins. [Reference 1](https://medlineplus.gov/ency/article/002222.htm) · [Reference 2](https://www.ncbi.nlm.nih.gov/books/NBK26830/)

**What the name includes.** Ile (I) has the side group CH(CH₃)–CH₂–CH₃. A branched side chain with a second stereocenter. Side-group chemistry is a separate classification from whether dietary supply is indispensable.

**Where it appears.** This amino acid occurs within food proteins, alongside other amino acids. Protein-rich foods supply mixtures; no single food belongs exclusively to one amino acid.

**How the body handles it.** Absorbed and recycled amino acids can be assembled into new proteins according to cellular instructions. Their side groups influence interactions and folding; a particular mixture alone does not specify a protein.

**Leucine (Leu) — Indispensable amino acid.** An indispensable amino acid used in proteins. [Reference 1](https://medlineplus.gov/ency/article/002222.htm) · [Reference 2](https://www.ncbi.nlm.nih.gov/books/NBK26830/)

**What the name includes.** Leu (L) has the side group CH₂–CH(CH₃)₂. A branched nonpolar side chain. Side-group chemistry is a separate classification from whether dietary supply is indispensable.

**Where it appears.** This amino acid occurs within food proteins, alongside other amino acids. Protein-rich foods supply mixtures; no single food belongs exclusively to one amino acid.

**How the body handles it.** Absorbed and recycled amino acids can be assembled into new proteins according to cellular instructions. Their side groups influence interactions and folding; a particular mixture alone does not specify a protein.

**Lysine (Lys) — Indispensable amino acid.** An indispensable amino acid used in proteins. [Reference 1](https://medlineplus.gov/ency/article/002222.htm) · [Reference 2](https://www.ncbi.nlm.nih.gov/books/NBK26830/)

**What the name includes.** Lys (K) has the side group (CH₂)₄–NH₃⁺. Usually positively charged near physiological pH. Side-group chemistry is a separate classification from whether dietary supply is indispensable.

**Where it appears.** This amino acid occurs within food proteins, alongside other amino acids. Protein-rich foods supply mixtures; no single food belongs exclusively to one amino acid.

**How the body handles it.** Absorbed and recycled amino acids can be assembled into new proteins according to cellular instructions. Their side groups influence interactions and folding; a particular mixture alone does not specify a protein.

**Methionine (Met) — Indispensable amino acid.** An indispensable amino acid used in proteins. [Reference 1](https://medlineplus.gov/ency/article/002222.htm) · [Reference 2](https://www.ncbi.nlm.nih.gov/books/NBK26830/)

**What the name includes.** Met (M) has the side group CH₂–CH₂–S–CH₃. Contains sulfur in a thioether. This is different from cysteine’s thiol. Side-group chemistry is a separate classification from whether dietary supply is indispensable.

**Where it appears.** This amino acid occurs within food proteins, alongside other amino acids. Protein-rich foods supply mixtures; no single food belongs exclusively to one amino acid.

**How the body handles it.** Absorbed and recycled amino acids can be assembled into new proteins according to cellular instructions. Their side groups influence interactions and folding; a particular mixture alone does not specify a protein.

**Phenylalanine (Phe) — Indispensable amino acid.** An indispensable amino acid used in proteins. [Reference 1](https://medlineplus.gov/ency/article/002222.htm) · [Reference 2](https://www.ncbi.nlm.nih.gov/books/NBK26830/)

**What the name includes.** Phe (F) has the side group CH₂–phenyl. An aromatic side chain that is largely hydrophobic. Side-group chemistry is a separate classification from whether dietary supply is indispensable.

**Where it appears.** This amino acid occurs within food proteins, alongside other amino acids. Protein-rich foods supply mixtures; no single food belongs exclusively to one amino acid.

**How the body handles it.** Absorbed and recycled amino acids can be assembled into new proteins according to cellular instructions. Their side groups influence interactions and folding; a particular mixture alone does not specify a protein.

**Threonine (Thr) — Indispensable amino acid.** An indispensable amino acid used in proteins. [Reference 1](https://medlineplus.gov/ency/article/002222.htm) · [Reference 2](https://www.ncbi.nlm.nih.gov/books/NBK26830/)

**What the name includes.** Thr (T) has the side group CH(OH)–CH₃. Its side chain includes a hydroxyl group. Side-group chemistry is a separate classification from whether dietary supply is indispensable.

**Where it appears.** This amino acid occurs within food proteins, alongside other amino acids. Protein-rich foods supply mixtures; no single food belongs exclusively to one amino acid.

**How the body handles it.** Absorbed and recycled amino acids can be assembled into new proteins according to cellular instructions. Their side groups influence interactions and folding; a particular mixture alone does not specify a protein.

**Tryptophan (Trp) — Indispensable amino acid.** An indispensable amino acid used in proteins. [Reference 1](https://medlineplus.gov/ency/article/002222.htm) · [Reference 2](https://www.ncbi.nlm.nih.gov/books/NBK26830/)

**What the name includes.** Trp (W) has the side group CH₂–indole. A bulky aromatic side chain, usually classed as largely nonpolar. Side-group chemistry is a separate classification from whether dietary supply is indispensable.

**Where it appears.** This amino acid occurs within food proteins, alongside other amino acids. Protein-rich foods supply mixtures; no single food belongs exclusively to one amino acid.

**How the body handles it.** Absorbed and recycled amino acids can be assembled into new proteins according to cellular instructions. Their side groups influence interactions and folding; a particular mixture alone does not specify a protein.

**Valine (Val) — Indispensable amino acid.** An indispensable amino acid used in proteins. [Reference 1](https://medlineplus.gov/ency/article/002222.htm) · [Reference 2](https://www.ncbi.nlm.nih.gov/books/NBK26830/)

**What the name includes.** Val (V) has the side group CH(CH₃)₂. A branched nonpolar side chain. Side-group chemistry is a separate classification from whether dietary supply is indispensable.

**Where it appears.** This amino acid occurs within food proteins, alongside other amino acids. Protein-rich foods supply mixtures; no single food belongs exclusively to one amino acid.

**How the body handles it.** Absorbed and recycled amino acids can be assembled into new proteins according to cellular instructions. Their side groups influence interactions and folding; a particular mixture alone does not specify a protein.

#### Eleven the body can usually make

**Alanine (Ala) — Standard amino acid · usually synthesized.** Alanine is one of the twenty standard protein amino acids. Usually synthesized by the body. See the conditional-needs entry for the nutritional qualification. [Reference 1](https://www.ncbi.nlm.nih.gov/books/NBK26830/) · [Reference 2](https://medlineplus.gov/ency/article/002222.htm)

**What the name includes.** Ala (A) has the side group CH₃. A small nonpolar methyl side chain. Side-group chemistry is a separate classification from whether dietary supply is indispensable.

**Where it appears.** This amino acid occurs within food proteins, alongside other amino acids. Protein-rich foods supply mixtures; no single food belongs exclusively to one amino acid.

**How the body handles it.** Absorbed and recycled amino acids can be assembled into new proteins according to cellular instructions. Their side groups influence interactions and folding; a particular mixture alone does not specify a protein.

**Arginine (Arg) — Standard amino acid · usually synthesized.** Arginine is one of the twenty standard protein amino acids. Usually synthesized by the body. See the conditional-needs entry for the nutritional qualification. [Reference 1](https://www.ncbi.nlm.nih.gov/books/NBK26830/) · [Reference 2](https://medlineplus.gov/ency/article/002222.htm)

**What the name includes.** Arg (R) has the side group (CH₂)₃–guanidinium. Usually positively charged near physiological pH. Side-group chemistry is a separate classification from whether dietary supply is indispensable.

**Where it appears.** This amino acid occurs within food proteins, alongside other amino acids. Protein-rich foods supply mixtures; no single food belongs exclusively to one amino acid.

**How the body handles it.** Absorbed and recycled amino acids can be assembled into new proteins according to cellular instructions. Their side groups influence interactions and folding; a particular mixture alone does not specify a protein.

**Asparagine (Asn) — Standard amino acid · usually synthesized.** Asparagine is one of the twenty standard protein amino acids. Usually synthesized by the body. See the conditional-needs entry for the nutritional qualification. [Reference 1](https://www.ncbi.nlm.nih.gov/books/NBK26830/) · [Reference 2](https://medlineplus.gov/ency/article/002222.htm)

**What the name includes.** Asn (N) has the side group CH₂–CONH₂. An uncharged amide side chain in the usual physiological context. Side-group chemistry is a separate classification from whether dietary supply is indispensable.

**Where it appears.** This amino acid occurs within food proteins, alongside other amino acids. Protein-rich foods supply mixtures; no single food belongs exclusively to one amino acid.

**How the body handles it.** Absorbed and recycled amino acids can be assembled into new proteins according to cellular instructions. Their side groups influence interactions and folding; a particular mixture alone does not specify a protein.

**Aspartic acid (Asp) — Standard amino acid · usually synthesized.** Aspartic acid is one of the twenty standard protein amino acids. Usually synthesized by the body. See the conditional-needs entry for the nutritional qualification. [Reference 1](https://www.ncbi.nlm.nih.gov/books/NBK26830/) · [Reference 2](https://medlineplus.gov/ency/article/002222.htm)

**What the name includes.** Asp (D) has the side group CH₂–COO⁻. Usually negatively charged near physiological pH. Side-group chemistry is a separate classification from whether dietary supply is indispensable.

**Where it appears.** This amino acid occurs within food proteins, alongside other amino acids. Protein-rich foods supply mixtures; no single food belongs exclusively to one amino acid.

**How the body handles it.** Absorbed and recycled amino acids can be assembled into new proteins according to cellular instructions. Their side groups influence interactions and folding; a particular mixture alone does not specify a protein.

**Cysteine (Cys) — Standard amino acid · usually synthesized.** Cysteine is one of the twenty standard protein amino acids. Usually synthesized by the body. See the conditional-needs entry for the nutritional qualification. [Reference 1](https://www.ncbi.nlm.nih.gov/books/NBK26830/) · [Reference 2](https://medlineplus.gov/ency/article/002222.htm)

**What the name includes.** Cys (C) has the side group CH₂–SH. Its thiol can form a disulfide bond with another cysteine under suitable conditions. Side-group chemistry is a separate classification from whether dietary supply is indispensable.

**Where it appears.** This amino acid occurs within food proteins, alongside other amino acids. Protein-rich foods supply mixtures; no single food belongs exclusively to one amino acid.

**How the body handles it.** Absorbed and recycled amino acids can be assembled into new proteins according to cellular instructions. Their side groups influence interactions and folding; a particular mixture alone does not specify a protein.

**Glutamic acid (Glu) — Standard amino acid · usually synthesized.** Glutamic acid is one of the twenty standard protein amino acids. Usually synthesized by the body. See the conditional-needs entry for the nutritional qualification. [Reference 1](https://www.ncbi.nlm.nih.gov/books/NBK26830/) · [Reference 2](https://medlineplus.gov/ency/article/002222.htm)

**What the name includes.** Glu (E) has the side group CH₂–CH₂–COO⁻. Usually negatively charged near physiological pH. Side-group chemistry is a separate classification from whether dietary supply is indispensable.

**Where it appears.** This amino acid occurs within food proteins, alongside other amino acids. Protein-rich foods supply mixtures; no single food belongs exclusively to one amino acid.

**How the body handles it.** Absorbed and recycled amino acids can be assembled into new proteins according to cellular instructions. Their side groups influence interactions and folding; a particular mixture alone does not specify a protein.

**Glutamine (Gln) — Standard amino acid · usually synthesized.** Glutamine is one of the twenty standard protein amino acids. Usually synthesized by the body. See the conditional-needs entry for the nutritional qualification. [Reference 1](https://www.ncbi.nlm.nih.gov/books/NBK26830/) · [Reference 2](https://medlineplus.gov/ency/article/002222.htm)

**What the name includes.** Gln (Q) has the side group CH₂–CH₂–CONH₂. An amide side chain with one more methylene group than asparagine. Side-group chemistry is a separate classification from whether dietary supply is indispensable.

**Where it appears.** This amino acid occurs within food proteins, alongside other amino acids. Protein-rich foods supply mixtures; no single food belongs exclusively to one amino acid.

**How the body handles it.** Absorbed and recycled amino acids can be assembled into new proteins according to cellular instructions. Their side groups influence interactions and folding; a particular mixture alone does not specify a protein.

**Glycine (Gly) — Standard amino acid · usually synthesized.** Glycine is one of the twenty standard protein amino acids. Usually synthesized by the body. See the conditional-needs entry for the nutritional qualification. [Reference 1](https://www.ncbi.nlm.nih.gov/books/NBK26830/) · [Reference 2](https://medlineplus.gov/ency/article/002222.htm)

**What the name includes.** Gly (G) has the side group H. The smallest side group. Glycine has no chiral alpha carbon and gives the backbone unusual flexibility. Side-group chemistry is a separate classification from whether dietary supply is indispensable.

**Where it appears.** This amino acid occurs within food proteins, alongside other amino acids. Protein-rich foods supply mixtures; no single food belongs exclusively to one amino acid.

**How the body handles it.** Absorbed and recycled amino acids can be assembled into new proteins according to cellular instructions. Their side groups influence interactions and folding; a particular mixture alone does not specify a protein.

**Proline (Pro) — Standard amino acid · usually synthesized.** Proline is one of the twenty standard protein amino acids. Usually synthesized by the body. See the conditional-needs entry for the nutritional qualification. [Reference 1](https://www.ncbi.nlm.nih.gov/books/NBK26830/) · [Reference 2](https://medlineplus.gov/ency/article/002222.htm)

**What the name includes.** Pro (P) has the side group Ring to backbone N. The side chain reconnects to the backbone nitrogen, restricting its geometry. Side-group chemistry is a separate classification from whether dietary supply is indispensable.

**Where it appears.** This amino acid occurs within food proteins, alongside other amino acids. Protein-rich foods supply mixtures; no single food belongs exclusively to one amino acid.

**How the body handles it.** Absorbed and recycled amino acids can be assembled into new proteins according to cellular instructions. Their side groups influence interactions and folding; a particular mixture alone does not specify a protein.

**Serine (Ser) — Standard amino acid · usually synthesized.** Serine is one of the twenty standard protein amino acids. Usually synthesized by the body. See the conditional-needs entry for the nutritional qualification. [Reference 1](https://www.ncbi.nlm.nih.gov/books/NBK26830/) · [Reference 2](https://medlineplus.gov/ency/article/002222.htm)

**What the name includes.** Ser (S) has the side group CH₂–OH. Its hydroxyl group can participate in hydrogen bonding. Side-group chemistry is a separate classification from whether dietary supply is indispensable.

**Where it appears.** This amino acid occurs within food proteins, alongside other amino acids. Protein-rich foods supply mixtures; no single food belongs exclusively to one amino acid.

**How the body handles it.** Absorbed and recycled amino acids can be assembled into new proteins according to cellular instructions. Their side groups influence interactions and folding; a particular mixture alone does not specify a protein.

**Tyrosine (Tyr) — Standard amino acid · usually synthesized.** Tyrosine is one of the twenty standard protein amino acids. Usually synthesized by the body. See the conditional-needs entry for the nutritional qualification. [Reference 1](https://www.ncbi.nlm.nih.gov/books/NBK26830/) · [Reference 2](https://medlineplus.gov/ency/article/002222.htm)

**What the name includes.** Tyr (Y) has the side group CH₂–phenyl–OH. An aromatic side chain with a polar hydroxyl group. Categories describe tendencies, not absolute behavior. Side-group chemistry is a separate classification from whether dietary supply is indispensable.

**Where it appears.** This amino acid occurs within food proteins, alongside other amino acids. Protein-rich foods supply mixtures; no single food belongs exclusively to one amino acid.

**How the body handles it.** Absorbed and recycled amino acids can be assembled into new proteins according to cellular instructions. Their side groups influence interactions and folding; a particular mixture alone does not specify a protein.

#### The qualification

**Conditionally essential amino acids (CAA) — Depends on physiological context.** Ordinarily synthesizable amino acids can require a dietary source in particular physiological or illness states. Examples include arginine, cysteine, glutamine and tyrosine. This is not a fixed extra requirement for every healthy adult, or a label for everyday psychological stress. [Reference](https://medlineplus.gov/ency/article/002222.htm)

### Vitamins

Thirteen vitamin entries, grouped by solubility.

#### Fat-soluble · A, D, E, K

**Vitamin A (A) — Essential vitamin.** Retinoids support vision, immune function and the growth and differentiation of cells. Retinal participates in rhodopsin, the light-sensitive protein in the retina. [Reference 1](https://ods.od.nih.gov/factsheets/VitaminA-HealthProfessional/) · [Reference 2](https://ods.od.nih.gov/factsheets/VitaminA-Consumer/) · [Reference 3](https://www.ncbi.nlm.nih.gov/books/NBK222318/?report=reader)

**What the name includes.** Vitamin A includes preformed retinoids and provitamin A carotenoids, such as beta-carotene, which the body can convert into vitamin A.

**Where it appears.** Eggs, dairy and fish supply preformed vitamin A; carrots, sweet potatoes and leafy greens supply provitamin A carotenoids.

**How the body handles it.** Much of the body's vitamin A reserve is stored in the liver as retinyl esters for later use.

**Vitamin D (D) — Essential vitamin.** After activation, supports calcium absorption and calcium–phosphate balance for bone mineralization. Calcitriol is the active form, produced mainly through liver and kidney processing. Vitamin D can also be synthesized in skin after ultraviolet exposure; it is still classified among the 13 vitamins. Its role is not evidence that extra intake always helps. [Reference](https://ods.od.nih.gov/factsheets/VitaminD-HealthProfessional/)

**What the name includes.** Vitamin D has two main dietary forms, D2 (ergocalciferol) and D3 (cholecalciferol), which follow the body's activation pathway.

**Where it appears.** Fatty fish provide vitamin D, with smaller amounts in egg yolks; some milks, plant drinks and cereals are fortified.

**How the body handles it.** The liver first converts vitamin D into calcidiol; a second conversion, mainly in the kidneys, produces the active hormone calcitriol.

**Vitamin E (E) — Essential vitamin.** A fat-soluble antioxidant that helps protect cells from oxidative damage. Alpha-tocopherol is the form recognized to meet human vitamin E needs. [Reference](https://ods.od.nih.gov/factsheets/VitaminE-HealthProfessional/)

**What the name includes.** Natural vitamin E comprises eight tocopherol and tocotrienol forms; alpha-tocopherol is the form recognized as meeting human vitamin E requirements.

**Where it appears.** Nuts, seeds and vegetable oils supply vitamin E, with additional contributions from leafy greens and some fortified cereals.

**How the body handles it.** After intestinal absorption, the liver preferentially returns alpha-tocopherol to circulation while metabolizing and excreting other vitamin E forms.

**Vitamin K (K) — Essential vitamin.** Required for modifying proteins involved in blood clotting and bone metabolism. Vitamin K supports gamma-carboxylation of selected glutamate residues. [Reference](https://ods.od.nih.gov/factsheets/VitaminK-HealthProfessional/)

**What the name includes.** Vitamin K includes K1 (phylloquinone) and a family of K2 forms called menaquinones, which differ in their side chains.

**Where it appears.** Leafy greens and some vegetable oils supply K1; certain fermented foods and animal foods contribute differing amounts of K2.

**How the body handles it.** Following intestinal absorption, vitamin K travels in lipoproteins; its tissue reserves are relatively small compared with other fat-soluble vitamins.

#### Water-soluble · eight B vitamins + C

**Thiamin (B1) — Essential vitamin.** Its active form helps enzymes handle glucose, fats and amino acids. Thiamin diphosphate (TDP), also called thiamin pyrophosphate (TPP). [Reference](https://ods.od.nih.gov/factsheets/Thiamin-HealthProfessional/)

**What the name includes.** Thiamin occurs free or with phosphate groups attached; thiamin diphosphate, also called TPP, is its main active cofactor form.

**Where it appears.** Whole grains, pork, fish and other meats contain thiamin; enriched or fortified breads and cereals can also contribute.

**How the body handles it.** Digestion releases free thiamin before small-intestinal uptake; the body keeps only small reserves, principally in the liver.

**Riboflavin (B2) — Essential vitamin.** Provides the vitamin component of FAD and FMN, cofactors used in energy metabolism. FAD and FMN participate in oxidation–reduction reactions. [Reference](https://ods.od.nih.gov/factsheets/Riboflavin-HealthProfessional/)

**What the name includes.** Riboflavin occurs free or within FMN and FAD, the vitamin-containing cofactors that help enzymes transfer electrons during metabolism.

**Where it appears.** Milk, eggs and lean meats provide riboflavin, along with some vegetables and grain products enriched or fortified with it.

**How the body handles it.** Riboflavin is absorbed mainly in the first part of the small intestine, with small reserves in the liver, heart and kidneys.

**Niacin (B3) — Essential vitamin.** Precursor to NAD and NADP, used in fuel breakdown, biosynthesis and cellular maintenance. NAD⁺/NADH and NADP⁺/NADPH transfer reducing equivalents. [Reference](https://ods.od.nih.gov/factsheets/Niacin-HealthProfessional/)

**What the name includes.** Niacin is a collective name for nicotinic acid, nicotinamide and related compounds that can supply the vitamin's activity.

**Where it appears.** Poultry, beef, fish, nuts, legumes and grains supply niacin; enriched and fortified grain products are additional sources.

**How the body handles it.** Absorbed niacin is rebuilt into NAD and NADP; the liver can also make NAD from the amino acid tryptophan.

**Pantothenic acid (B5) — Essential vitamin.** Needed to make coenzyme A and acyl carrier protein, which carry carbon groups during metabolism. The CoA in acetyl-CoA depends on vitamin B5. [Reference](https://ods.od.nih.gov/factsheets/PantothenicAcid-HealthProfessional/)

**What the name includes.** Much food vitamin B5 is embedded in coenzyme A and related compounds; digestion releases pantothenic acid for reuse.

**Where it appears.** Mushrooms, poultry, eggs and whole grains are among the many plant and animal foods that supply pantothenic acid.

**How the body handles it.** After intestinal absorption, tissues use pantothenic acid to rebuild coenzyme A, supporting both fatty-acid breakdown and fatty-acid synthesis.

**Vitamin B6 (B6) — Essential vitamin.** Active B6 cofactors support amino-acid reactions, glycogen breakdown and neurotransmitter production. Pyridoxal phosphate (PLP) supports amino-group transfer. [Reference](https://ods.od.nih.gov/factsheets/VitaminB6-HealthProfessional/)

**What the name includes.** Vitamin B6 is a family of six related forms, including pyridoxine; PLP and PMP are its active enzyme helpers.

**Where it appears.** Fish, poultry, chickpeas, potatoes and noncitrus fruits provide vitamin B6, and some breakfast cereals have it added.

**How the body handles it.** Phosphate groups are removed before uptake in the jejunum; cells use B6 cofactors for amino-acid reactions and glycogen breakdown.

**Biotin (B7) — Essential vitamin.** Cofactor for carboxylases in glucose, fatty-acid and amino-acid metabolism. Pyruvate carboxylase and acetyl-CoA carboxylase use biotin. [Reference](https://ods.od.nih.gov/factsheets/Biotin-HealthProfessional/)

**What the name includes.** Biotin occurs as free vitamin or attached to food proteins; digestion must release the bound vitamin before absorption.

**Where it appears.** Cooked eggs, fish, seeds, nuts and sweet potatoes are examples of foods contributing biotin to a mixed diet.

**How the body handles it.** Digestive enzymes release protein-bound biotin before small-intestinal absorption; the liver holds much of the body's stored biotin.

**Folate (B9) — Essential vitamin.** Folate cofactors carry single-carbon units needed for DNA production and amino-acid metabolism. Tetrahydrofolate derivatives support nucleotide synthesis. [Reference](https://ods.od.nih.gov/factsheets/Folate-HealthProfessional/)

**What the name includes.** Folate describes a family of related compounds; natural food folates differ chemically from folic acid commonly added during fortification.

**Where it appears.** Leafy greens, beans, peas and asparagus supply natural folates, while some breads and cereals contain added folic acid.

**How the body handles it.** Food folates are trimmed before intestinal absorption; circulating folate is mainly 5-MTHF, and the liver holds a substantial reserve.

**Vitamin B12 (B12) — Essential vitamin.** Supports nervous-system function, red-blood-cell formation and DNA synthesis. Cofactor for methionine synthase and methylmalonyl-CoA mutase. [Reference](https://ods.od.nih.gov/factsheets/VitaminB12-HealthProfessional/)

**What the name includes.** Vitamin B12 comprises cobalt-containing cobalamins; methylcobalamin and adenosylcobalamin are the two forms cells use as enzyme cofactors.

**Where it appears.** Fish, meat, eggs and dairy contain B12; some breakfast cereals and nutritional yeasts are fortified with it.

**How the body handles it.** Intrinsic factor normally enables uptake in the terminal ileum; despite being water-soluble, B12 can have body stores lasting years.

**Vitamin C (C) — Essential vitamin.** Required for collagen and carnitine synthesis; acts as an antioxidant and helps absorb nonheme iron. Ascorbate supports collagen-building enzyme reactions. [Reference](https://ods.od.nih.gov/factsheets/VitaminC-HealthProfessional/)

**What the name includes.** Vitamin C is ascorbic acid, also called ascorbate in its ionized form; it is a water-soluble organic nutrient.

**Where it appears.** Citrus fruits, bell peppers, kiwifruit, strawberries and broccoli are familiar examples of foods that contribute vitamin C.

**How the body handles it.** The body controls vitamin C through intestinal absorption and kidney excretion; vitamin C also improves absorption of nonheme iron.

### Minerals & electrolytes

“Major” and “trace” refer to quantities, not importance.

#### Major dietary minerals

**Calcium (Ca) — Essential mineral.** A major part of bones and teeth; calcium ions also help muscles contract and cells communicate. Intracellular calcium is a signal, while most body calcium is stored in the skeleton. [Reference 1](https://ods.od.nih.gov/factsheets/Calcium-HealthProfessional/) · [Reference 2](https://medlineplus.gov/fluidandelectrolytebalance.html)

**What the name includes.** Calcium occurs in mineral compounds, including bone's calcium phosphate, and as dissolved ions involved in signaling and muscle contraction.

**Where it appears.** Milk, yogurt, calcium-set tofu, fish with edible bones and some fortified plant drinks are examples of calcium sources.

**How the body handles it.** Vitamin D supports active calcium absorption in the intestine; bones and teeth hold almost all of the body's calcium.

**Phosphorus (P) — Essential mineral.** Present in bones, DNA, RNA, membranes and ATP. Phosphate transfer helps regulate enzymes. ATP is adenosine triphosphate: its name includes its three phosphate groups. [Reference 1](https://ods.od.nih.gov/factsheets/Phosphorus-HealthProfessional/) · [Reference 2](https://medlineplus.gov/fluidandelectrolytebalance.html)

**What the name includes.** Food phosphorus occurs in phosphates and organic compounds; plant seeds also hold some in phytate, which people digest less effectively.

**Where it appears.** Dairy, eggs, meat, fish, beans, nuts and whole grains are among the many foods that provide phosphorus.

**How the body handles it.** Phosphorus is absorbed in the small intestine; the kidneys, intestines and bones coordinate phosphate balance across the body.

**Magnesium (Mg) — Essential mineral.** Supports enzyme systems in glycolysis, oxidative phosphorylation and protein synthesis. Many reactions using ATP require magnesium. [Reference 1](https://ods.od.nih.gov/factsheets/Magnesium-HealthProfessional/) · [Reference 2](https://medlineplus.gov/fluidandelectrolytebalance.html)

**What the name includes.** Magnesium, symbol Mg, is a mineral used as charged ions in enzyme chemistry, including reactions involving ATP.

**Where it appears.** Leafy greens, beans, lentils, nuts, seeds and whole grains supply magnesium, with variable contributions from drinking water.

**How the body handles it.** Bone holds much of the body's magnesium, with most of the remainder in soft tissues; kidneys regulate urinary losses.

**Sodium (Na) — Essential mineral.** Helps maintain body-fluid balance and normal nerve and muscle function. A major positively charged ion outside cells. [Reference 1](https://medlineplus.gov/sodium.html) · [Reference 2](https://ods.od.nih.gov/factsheets/Potassium-HealthProfessional/) · [Reference 3](https://medlineplus.gov/fluidandelectrolytebalance.html) · [Reference 4](https://medlineplus.gov/ency/article/002415.htm)

**What the name includes.** Sodium occurs in several food compounds; familiar table salt is sodium chloride, while baking soda supplies sodium as bicarbonate.

**Where it appears.** Table salt, soy sauce, breads, cheeses and prepared soups can supply sodium, with smaller natural contributions from foods such as milk.

**How the body handles it.** The kidneys adjust how much sodium leaves in urine, helping control fluid balance alongside sodium's roles in nerve and muscle function.

**Potassium (K) — Essential mineral.** Helps maintain fluid inside cells and the gradients used by nerves and muscles. The sodium–potassium pump uses ATP to maintain ion gradients. [Reference 1](https://ods.od.nih.gov/factsheets/Potassium-HealthProfessional/) · [Reference 2](https://medlineplus.gov/fluidandelectrolytebalance.html)

**What the name includes.** Potassium, symbol K, is a mineral electrolyte supplied by different food compounds; it is distinct from vitamin K.

**Where it appears.** Beans, lentils, potatoes, fruits, vegetables, milk and yogurt are examples of foods contributing potassium to a varied diet.

**How the body handles it.** Most body potassium is inside cells; intestinal absorption and kidney-controlled urinary losses help maintain the balance needed for electrical signaling.

**Chloride (Cl) — Essential mineral.** Helps maintain body-fluid balance and is a component of stomach digestive juices. Table salt provides sodium and chloride. [Reference 1](https://medlineplus.gov/ency/article/002417.htm) · [Reference 2](https://medlineplus.gov/fluidandelectrolytebalance.html) · [Reference 3](https://medlineplus.gov/lab-tests/chloride-blood-test/)

**What the name includes.** Chloride is an electrolyte supplied by salts such as sodium chloride and potassium chloride, with the salts separating into dissolved ions.

**Where it appears.** Table salt and foods made with it contribute chloride, which also occurs in foods such as tomatoes, celery and olives.

**How the body handles it.** Chloride helps balance body fluids and forms part of stomach acid; the body can remove excess chloride in urine.

#### Established trace minerals

**Iron (Fe) — Essential mineral.** Iron in hemoglobin carries oxygen from lungs to tissues; myoglobin supports oxygen use in muscle. Iron is part of oxygen-handling proteins. [Reference](https://ods.od.nih.gov/factsheets/Iron-HealthProfessional/)

**What the name includes.** Dietary iron comes as heme and nonheme iron; plant and fortified foods supply nonheme, while animal foods can supply both.

**Where it appears.** Meat, seafood, beans, lentils, nuts and iron-fortified grain products provide iron in different amounts and chemical forms.

**How the body handles it.** Transferrin carries iron through blood; ferritin provides storage, while hepcidin helps regulate how much iron enters the circulation.

**Zinc (Zn) — Essential mineral.** Participates in many enzymes, DNA and protein synthesis, cell division and wound repair. A catalytic and structural participant in cell chemistry. [Reference](https://ods.od.nih.gov/factsheets/Zinc-HealthProfessional/)

**What the name includes.** Zinc, symbol Zn, is the mineral element incorporated into zinc-containing enzymes and structural proteins throughout the body.

**Where it appears.** Meat, seafood and dairy supply zinc, as do beans, nuts and whole grains, although the amount absorbed differs.

**How the body handles it.** Phytate in some plant foods can bind zinc and reduce absorption; intestinal uptake and losses help regulate the body's zinc balance.

**Copper (Cu) — Essential mineral.** Copper-containing enzymes help energy production, iron handling and connective-tissue synthesis. Ceruloplasmin is a copper-containing protein involved in iron metabolism. [Reference](https://ods.od.nih.gov/factsheets/Copper-HealthProfessional/)

**What the name includes.** Copper, symbol Cu, is the trace element incorporated into cuproenzymes, the copper-containing proteins that carry out particular chemical reactions.

**Where it appears.** Shellfish, nuts, seeds, whole grains and chocolate are examples of copper sources, along with organ meats such as liver.

**How the body handles it.** Copper is absorbed in the upper small intestine; the liver helps regulate it by sending excess copper into bile.

**Iodine (I) — Essential mineral.** A constituent of thyroid hormones, which help regulate metabolic activity and development. Thyroxine (T4) and triiodothyronine (T3) contain iodine. [Reference](https://ods.od.nih.gov/factsheets/Iodine-HealthProfessional/)

**What the name includes.** Iodine occurs in food as iodide, iodate and other forms; iodate is converted to iodide before absorption.

**Where it appears.** Iodized salt, fish, other seafood, eggs and dairy can supply iodine, while amounts in seaweed vary widely.

**How the body handles it.** Absorbed iodide circulates to the thyroid, which concentrates it to make thyroid hormones; much of the remainder leaves in urine.

**Selenium (Se) — Essential mineral.** Selenoproteins participate in thyroid-hormone metabolism and protection from oxidative damage. Glutathione peroxidases are selenium-containing enzymes. [Reference](https://ods.od.nih.gov/factsheets/Selenium-HealthProfessional/)

**What the name includes.** Food selenium is commonly incorporated into the amino acids selenomethionine and selenocysteine, rather than present as a separate free element.

**Where it appears.** Seafood, meat, eggs and grains provide selenium; Brazil nuts can be especially rich, and plant-food content varies with soil.

**How the body handles it.** The body processes absorbed selenium forms into intermediates used to build selenoproteins, while urinary excretion helps regulate selenium balance.

**Manganese (Mn) — Essential mineral.** A cofactor in enzymes handling nutrients, reactive oxygen species and tissue formation. Manganese superoxide dismutase and arginase use manganese. [Reference](https://ods.od.nih.gov/factsheets/Manganese-HealthProfessional/)

**What the name includes.** Manganese, symbol Mn, is a trace mineral used by selected enzymes; magnesium, symbol Mg, is a different mineral.

**Where it appears.** Whole grains, nuts, legumes, leafy vegetables and tea are examples of foods and drinks that contribute manganese.

**How the body handles it.** After intestinal uptake, manganese travels bound to blood proteins; the body regulates its balance largely through absorption and biliary excretion.

**Molybdenum (Mo) — Essential mineral.** Part of a cofactor required by enzymes that process sulfur-containing compounds and other molecules. Sulfite oxidase and xanthine oxidase require a molybdenum cofactor. [Reference](https://ods.od.nih.gov/factsheets/Molybdenum-HealthProfessional/)

**What the name includes.** Molybdenum is a trace element assembled into the molybdenum cofactor, a reusable helper for a small group of enzymes.

**Where it appears.** Beans, lentils and other legumes supply molybdenum, as do whole grains, nuts, milk and organ meats such as liver.

**How the body handles it.** After uptake from the digestive tract, molybdenum enters tissue cofactors; the kidneys regulate its levels mainly through urinary excretion.

**Where do salts and electrolytes fit?** Salt is a compound; an electrolyte carries charge in solution. Sodium, potassium, chloride, calcium, magnesium and phosphate have electrolyte roles. Phosphorus is the nutrient label; phosphate is an ionic form. Bicarbonate is another body electrolyte, not an extra essential mineral. [Reference](https://medlineplus.gov/fluidandelectrolytebalance.html)

### Plant compounds

Selected families in food, with overlapping chemistry.

**A family can cross the map.** Phytosterols are also lipids. Some carotenoids supply vitamin A. Polyphenols include flavonoids, stilbenes and lignans. These are selected families, not a list of universal dietary essentials.

#### Phytochemical families

**Polyphenols (Pph) — Selected phytochemical family.** A broad group of plant-associated compounds that includes flavonoids, stilbenes and lignans. Flavonoids include anthocyanin pigments. Polyphenols are not one essential nutrient. Chemical activity in a laboratory does not establish a dietary health benefit. [Reference 1](https://www.cancer.gov/publications/dictionaries/cancer-terms/def/polyphenol) · [Reference 2](https://www.ars.usda.gov/research/publications/publication/?seqNo115=285399)

**Carotenoids (Car) — Some supply vitamin A.** Some carotenoids can be converted into vitamin A; others cannot. Only provitamin A carotenoids contribute to vitamin A supply. Beta-carotene can supply vitamin A. Lycopene, lutein and zeaxanthin cannot. [Reference](https://ods.od.nih.gov/factsheets/VitaminA-HealthProfessional/)

**Glucosinolates (Gsl) — Selected phytochemical family.** Preparation, chewing and digestion can break glucosinolates into compounds such as indoles and isothiocyanates. Found in plants such as broccoli and cabbage. This chemistry is not proof that a particular food or isolated compound prevents cancer. [Reference](https://www.cancer.gov/about-cancer/causes-prevention/risk/diet/cruciferous-vegetables-fact-sheet)

**Allium sulfur compounds (All) — Selected phytochemical group.** Garlic contains sulfur compounds whose forms change when its tissue is cut or crushed. Allicin is one familiar example. This is a food-source grouping, not every organosulfur compound. Glucosinolates are also sulfur-containing compounds. [Reference](https://www.govinfo.gov/content/pkg/GOVPUB-HE20_3000-PURL-LPS99722/pdf/GOVPUB-HE20_3000-PURL-LPS99722.pdf)

**Phytosterols (Pst) — Plant-sterol family.** Phytosterols are plant sterols found in plant-cell structures and plant foods. They overlap the sterol family. Their presence in food does not make them an established essential human nutrient. [Reference](https://www.ars.usda.gov/research/publications/publication/?seqNo115=349582)

### Special cases & body-made compounds

Useful connections, with their qualifications kept visible.

**The label matters.** Sulfur and cobalt occur within other required nutrients. Fluoride has a dental benefit; chromium’s essentiality is disputed. A body-made molecule can be important without being a separate dietary requirement.

#### Elemental context

**Sulfur (S) — Required within other nutrients.** Sulfur occurs in methionine and cysteine. Their breakdown supplies sulfate for building sulfated compounds. There is no separate dietary sulfate requirement when sulfur-amino-acid needs are met. Sulfur is often listed as a major mineral. Methionine and cysteine supply much of the sulfate used in metabolism; this is not a separate elemental sulfur target. [Reference](https://www.nationalacademies.org/read/10925/chapter/9)

**Cobalt (Co) — Required within vitamin B12.** Cobalt is a constituent of vitamin B12. Its essential nutritional role is covered by the requirement for that vitamin. See the vitamin B12 card. Cobalt in vitamin B12 is not interchangeable with consuming elemental cobalt or other cobalt compounds. [Reference](https://ods.od.nih.gov/factsheets/VitaminB12-HealthProfessional/)

**Fluoride (F) — Beneficial; not essential.** Fluoride helps prevent tooth decay. This benefit is distinct from being essential for human growth or having a recognized deficiency disease. It can appear in nutrient reference tables even though essentiality has not been established. A nutrient can have an Adequate Intake reference value for a benefit without being classified as essential. [Reference 1](https://ods.od.nih.gov/factsheets/Fluoride-HealthProfessional/) · [Reference 2](https://efsa.onlinelibrary.wiley.com/doi/abs/10.2903/j.efsa.2013.3332)

**Chromium (Cr) — Essentiality disputed.** The US Food and Nutrition Board classified chromium as essential in 2001 and has not re-evaluated it. EFSA found no convincing evidence of an essential human requirement. The US classification dates to 2001; EFSA reassessed essentiality in 2014. Essentiality remains unsettled despite its appearance in some nutrient-reference tables. [Reference](https://ods.od.nih.gov/factsheets/Chromium-HealthProfessional/)

#### Made in the body

**Carnitine (Carn) — Usually made; conditional need.** Carnitine helps transport long-chain fatty acids into mitochondria. Healthy adults generally synthesize enough. Dietary need can become important in specific conditions. A role in fatty-acid transport does not by itself establish a fat-loss benefit. [Reference](https://ods.od.nih.gov/factsheets/Carnitine-HealthProfessional/)

**ATP (ATP) — Body-made energy carrier.** ATP helps couple energy-releasing reactions to cellular work. Cells continually use and regenerate it. ATP is central to metabolism, but it is not an essential dietary nutrient or a long-term fuel store. [Reference](https://openstax.org/books/biology-2e/pages/6-4-atp-adenosine-triphosphate)

**Creatine (Crtn) — Body-made; also found in food.** The body makes creatine from amino-acid precursors. Phosphocreatine helps regenerate ATP, particularly during brief, intense demands. Creatine is not one of the nine indispensable amino acids. Creatinine is a different molecule: a breakdown product excreted in urine. [Reference](https://ods.od.nih.gov/factsheets/ExerciseAndAthleticPerformance-HealthProfessional/)

Original chart by Shreyam Adhikari / Carbon Atlas. Learning framework inspired by [Dr. Andy Galpin’s nutrient overview](https://www.youtube.com/watch?v=E3lis9G8zoU); classifications refined from the references attached to each entry.

<a id="big-picture"></a>

## 03 · The whole journey

Metabolism includes the reactions that break molecules down and the reactions that build them up. **Catabolism** dismantles fuel and supplies usable energy and smaller molecules. **Anabolism** uses energy and building blocks to make things such as proteins, glycogen and lipids. Both run continuously. A body that is rebuilding tissue is also spending energy to remain alive.

A food Calorie is a kilocalorie, a unit of energy, equal to 4.184 kilojoules. The familiar estimates—about 4 kcal per gram of digestible carbohydrate or protein and 9 per gram of fat—describe average metabolizable food energy, not the number of ATP molecules manufactured from every gram. Digestion and processing matter too. [3]

ATP, adenosine triphosphate, is a small molecule cells repeatedly regenerate. Its conversion to ADP and phosphate can be coupled to processes such as moving ions, contracting muscle and building molecules. The complete reaction releases free energy; breaking a chemical bond alone costs energy. Calling ATP a “battery” is useful if we remember that cells keep rebuilding it. ATP is a working supply, while glycogen and triglycerides hold much larger fuel reserves. [4]

<!-- DRAFT3 FOOD JOURNEY START -->

### One meal, different journeys

Follow each component from its form in food through digestion and absorption to transport and possible destinations. These pathways overlap; they are not a queue or a prediction for an individual meal.

### Follow the starch in a meal.

**1. Joined glucose units.** Starch contains glucose chains. Rice, potatoes and many other foods contain it alongside other molecules.

**2. Chains become sugars.** Digestive enzymes break digestible starch down, ultimately releasing glucose units ready for absorption.

**3. Across the intestinal lining.** Glucose crosses cells of the small intestine and reaches intestinal blood. Digestion and absorption are different steps.

**Transport.** Portal blood takes absorbed glucose to the liver. Some is handled there; glucose also reaches the wider circulation and other tissues.

- **Use and build:** Cells can use glucose to regenerate ATP and supply material for synthesis.

- **Liver glycogen:** The liver can store glucose units and later help support blood glucose.

- **Muscle glycogen:** Muscle stores glucose units for its own work. This store has a different job from liver glycogen.

This follows digestible starch. Fructose and galactose need other processing steps; fiber takes a different branch. Use, building and storage can happen together. [6], [7], [9]

### Follow a long-chain dietary fat.

**1. A glycerol backbone, three tails.** Much of the fat we eat is triglyceride: fatty acids attached to glycerol.

**2. Disperse, then split.** Bile helps disperse fats. Lipases break triglycerides into products including fatty acids and monoacylglycerols.

**3. Rebuild and package.** Fatty acids and monoacylglycerols enter intestinal cells. There, triglycerides are rebuilt and packaged into chylomicrons.

**Transport.** Chylomicrons enter lymph, then the wider blood circulation. Lipoprotein lipase releases fatty acids for tissues; chylomicron remnants are taken up by the liver later.

- **Release fuel to tissues:** Released fatty acids can enter cells and feed mitochondrial oxidation.

- **Build or store lipids:** Fatty acids can join membrane lipids or be stored in triglycerides.

- **Remnants reach the liver:** The liver handles the particles left after much of their triglyceride has been removed.

This is the main long-chain route. Shorter fatty acids can take portal blood more directly. Food triglyceride does not travel intact into a patch of body fat. [6], [17], [42], [76]

### Follow protein into new possibilities.

**1. Amino-acid chains.** Food proteins have ordered chains and varied structures. Eating a protein does not preserve its original job inside your body.

**2. Cut the peptide bonds.** Stomach and intestinal digestion break proteins into smaller peptides and amino acids.

**3. Amino acids reach blood.** Intestinal cells absorb amino acids and small peptides, breaking peptides down further. Some amino acids are used by the intestine itself.

**Transport.** Amino acids largely enter portal blood. The liver handles part of the arriving supply; others reach tissues throughout the body, including muscle.

- **Assemble new proteins:** Cells combine absorbed and recycled amino acids in sequences specified by their instructions.

- **Make other molecules:** Amino acids also supply material for compounds beyond proteins.

- **Handle carbon and nitrogen:** Breakdown sends carbon into metabolic pathways; much of the nitrogen leaves through urea in urine.

There is no dedicated surplus-protein tank. Protein synthesis and breakdown continue together; a protein-rich meal is not the same amount of new muscle. [6], [21], [22]

### Follow the microbial branch.

**1. Fiber in the food matrix.** Different fibers have different structures. This route follows the portion that microbes can ferment.

**2. A different kind of breakdown.** Fiber escapes digestion in the small intestine. Microbes in the colon can transform some of it into short-chain fatty acids and gases.

**3. Microbial products cross.** Short-chain fatty acids can be absorbed across the colon lining. Unfermented material and microbial biomass also contribute to stool.

**Separate unabsorbed branch.** Material that remains in the gut contributes to stool. This separate branch does not cross the lining or enter either circulation route.

**Transport.** Some microbial products fuel colon cells locally. Others enter portal blood and reach the liver and wider metabolism.

- **Support colon cells:** Butyrate is an important fuel for the cells lining the colon.

- **Join host metabolism:** Absorbed microbial products can be processed by the liver and other tissues.

Not all fiber ferments equally. Solubility, viscosity and fermentability describe different, overlapping properties. [5], [8], [65], [68]

<!-- DRAFT3 FOOD JOURNEY END -->

Atoms do not vanish into ATP or heat. During ordinary metabolic chemistry, their arrangement changes. Energy can be transferred through coupled reactions while the material remains accounted for in products. Following carbon and following energy are related tasks, but they answer different questions.

---

<a id="digestion"></a>

## 04 · Digestion & absorption

<!-- DRAFT3 DIGESTION START -->

Eating puts food in the digestive tract. Absorption moves its products across the intestinal wall into the body’s transport systems. Bile from the liver, stored in the gallbladder, disperses fats; pancreatic and intestinal enzymes break molecules down. [5], [6]

### Mouth

Chewing makes smaller pieces. Salivary amylase begins starch digestion. [5]

### Stomach

Mixing, acid and pepsin begin substantial protein digestion. The meal leaves gradually. [5]

### Small intestine

Pancreatic enzymes and enzymes at the intestinal surface finish much of digestion. Bile disperses fat. Most nutrient absorption happens here. [5]

### Colon

Microbes ferment some remaining fiber. Water and electrolytes are absorbed; unabsorbed material leaves in stool. [5]

### Does everything go to the liver first?

**Sugars — Small intestine.** Glucose, fructose, galactose.

Intestinal blood → Portal vein → Liver → Wider circulation

The liver processes part of this supply; glucose also passes on to other tissues. Fructose and galactose enter different processing steps before joining shared pathways. [6], [9]

**Protein — Small intestine.** Amino acids after peptide breakdown.

Intestinal blood → Portal vein → Liver & other tissues

Intestinal cells use some amino acids themselves. The liver handles much of the arriving supply; others, including much of the branched-chain amino-acid supply, reach tissues such as muscle. [6], [22], [73]

**Most dietary fat — Small intestine.** Long-chain fatty acids + monoacylglycerols.

Rebuilt triglycerides → Chylomicrons → Lymph → blood → Muscle & fat tissue

Intestinal cells repackage these fats. Lipoprotein lipase releases fatty acids for tissues; the remaining chylomicron particles are later taken up by the liver. This route does not visit the liver first. [6], [17]

**Fat-soluble vitamins — Small intestine.** Vitamins A, D, E and K.

Absorbed with lipids → Chylomicrons → Lymph → blood

They travel with the dietary lipid route initially, then have vitamin-specific transport and storage. These are helpers and materials, not ATP-producing fuels. [6]

**Water-soluble vitamins & minerals — Mainly small intestine; sites vary.** Individual vitamins and mineral ions.

Intestinal absorption → Portal blood → Liver & tissues

There is no single vitamin or mineral transporter. B12 needs intrinsic factor and is absorbed in the terminal ileum. Iron is mainly absorbed near the start of the small intestine. Calcium absorption occurs along it, with mechanisms that vary by region. [6], [30], [75], [77]

**Fiber & water — Small intestine and colon.** Microbial products; water stays water.

Some fiber → microbes → Short-chain fatty acids → Colon cells & portal blood

Butyrate is important fuel for colon cells; other microbial products reach the liver and circulation. Water needs no digestion and is absorbed mostly in the small intestine, with more recovered in the colon. [5], [8]

Short-chain and many medium-chain fatty acids can reach portal blood more directly; lipid properties and meal conditions matter. Water-soluble choline forms commonly reach portal blood, while lipid-associated forms can accompany chylomicrons. Plant compounds have varied intestinal, hepatic and microbial transformations. [76], [37], [78]

<!-- DRAFT3 DIGESTION END -->

<a id="carbohydrates"></a>

## 05 · Carbohydrate’s route

<!-- DRAFT3 CARBOHYDRATES START -->

Absorbed sugars → Portal blood → liver → Glucose in blood → Cells / glycogen

After a starchy meal, absorbed glucose joins the blood supply. Cells can use it to regenerate ATP or to make other molecules. The liver handles much of the absorbed fructose and galactose through different chemical steps. Fiber follows the microbial branch instead.

Glucose stored as glycogen has two main homes: the liver can release glucose to support the blood supply; muscle keeps its glycogen for its own work. Carbon can also enter fat synthesis, but there is no universal ‘glycogen full, now make fat’ switch. Use, synthesis and storage run together.

[8], [9], [19], [20]

<!-- DRAFT3 CARBOHYDRATES END -->

### Further exploration

### Try the building-block experiment

Start with glucose, fructose or galactose: each is one monosaccharide with the formula C₆H₁₂O₆, but their atoms are arranged differently. The article's one-click pills move from one sugar to a pair and then to longer glucose chains. Fructose occurs in fruit among other sugars; a food is not a pure sugar type. [7]

In the interactive article, join or hydrolyse two sugars, then compare the connection while holding the sugar identities fixed. Maltose and cellobiose both contain two glucose residues: maltose has an α(1→4) linkage, while cellobiose has a β(1→4) linkage. Sucrose joins glucose with fructose; lactose joins galactose with glucose. A formal condensation of two C₆H₁₂O₆ sugars gives C₁₂H₂₂O₁₁ plus water. This equation is bookkeeping, not a recipe for cellular synthesis. [7], [70]

The long-chain comparison uses glucose throughout. Amylose is mostly unbranched; amylopectin and glycogen include α(1→6) branch points along α(1→4) chains, with glycogen more frequently branched. Cellulose uses β(1→4) links. Drawn branch spacing is illustrative, not measured. [7]

Glucose, fructose and galactose are **monosaccharides**: individual sugar units. Join two units and you have a **disaccharide**. Sucrose contains glucose and fructose; lactose contains glucose and galactose. Starch is a much longer glucose polymer. Plants store it in forms that include amylose and branched amylopectin. We store glucose in a different branched polymer, glycogen. [7]

“Simple” and “complex” describe molecular structure. They do not reliably tell you how quickly an entire food will affect blood glucose. Processing, cooking, the food's physical structure, serving size and the other foods eaten alongside it also matter. A structural diagram is not a universal ranking of foods. [56]

**Fiber** includes carbohydrates that escape digestion by human enzymes; lignin is also included in common dietary-fiber definitions. Solubility, viscosity and fermentability are different properties. Some fiber reaches the colon, where microbes convert it into short-chain fatty acids such as acetate, propionate and butyrate. Human isotope studies show that these microbial products can enter host metabolism. Fiber is therefore neither identical to starch nor necessarily metabolically inert. [8]

Fructose and galactose also connect to central carbohydrate pathways after additional processing. They do not need to be portrayed as taking an inevitable one-way route into body fat.

### A sugar has a shape in space

The formula of glucose is **C₆H₁₂O₆**. In its alpha-D-glucopyranose form, the ring contains five carbons and one oxygen; the sixth carbon sits outside the ring. The six-carbon formula does not mean six carbon atoms occupy the ring. Glucose is one example of a sugar, not a template for every carbohydrate. [RCSB PDB: GLC](https://www.rcsb.org/ligand/GLC).

Joining sugar units creates different structures. Starch and cellulose both contain glucose units, yet their linkages differ, and human digestive enzymes handle them differently. A drawn sugar unit represents a whole molecule or residue, not a single carbon atom. [Cooper: molecular composition of cells](https://www.ncbi.nlm.nih.gov/books/NBK9879/).

### The storage branch: glycogen

Glucose can be used now, used in biosynthesis or linked into glycogen. Liver glycogen helps maintain blood glucose between meals. Muscle glycogen serves the muscle's own activity. Muscle cannot export its glycogen as free glucose in the way the liver can. These are different stores with different jobs—not one tank that must reach a universal fill line before anything else happens. Their amounts change with body size, nutrition and activity. [9]

### Can carbohydrate become fat?

Yes. **De novo lipogenesis** builds new fatty acids from smaller precursors. Carbohydrate can supply pyruvate, then mitochondrial acetyl-CoA. Carbon can be exported as citrate, which is cleaved to supply cytosolic acetyl-CoA. Acetyl-CoA carboxylase makes malonyl-CoA; fatty acid synthase builds the chain using energy and NADPH. Fatty acids can then be joined to a glycerol backbone. This is a biosynthetic branch, not the purpose of the Krebs cycle. [51], [52]

How much carbohydrate follows that route depends on the situation. In one short overfeeding study, carbohydrate intake substantially increased carbohydrate oxidation and suppressed fat oxidation while the absolute amount of newly synthesized liver fat remained modest. Another controlled experiment found substantial fatty-acid synthesis with an energy-balanced, very-low-fat, high-carbohydrate formula diet. Different conditions produced different fluxes. [19], [20]

Consequently, “carbs never become fat” and “every extra carb immediately becomes fat” are both poor descriptions. New fat synthesis and reduced use of existing fat are distinct ways to change the balance of storage.

The names in that synthesis route will become clearer in the energy chapter. For now, the key distinction is between storing an existing molecule, oxidizing fuel, and using smaller precursors to build something new.

---

<!-- FIBER GUIDE START -->

<a id="fiber-world"></a>

### Fiber: one word, three different questions

**Soluble / insoluble:** How readily a fiber dissolves in water.

**Viscous / nonviscous:** Whether it thickens the surrounding fluid under the conditions present.

**Fermentability:** How readily gut microbes break it down. This is a spectrum.

Soluble does not automatically mean viscous, and insoluble does not mean unfermentable. These properties overlap. [64], [65]

| Fiber example | In water | Thickening | Microbial use |
| --- | --- | --- | --- |
| Cellulose | Insoluble | Nonviscous | Generally limited; variable |
| Pectins | Usually soluble | Can thicken or gel | Readily fermented |
| Oat / barley β-glucans | Soluble fraction varies | Can be viscous; depends on form | Fermentable |
| Inulin / fructans | Generally soluble | Usually low viscosity | Readily fermented |
| Resistant starch | Often insoluble (RS2 / RS3) | Usually low (RS2 / RS3) | Often substantial; varies by type |
| Psyllium husk | Soluble + insoluble fractions | Viscous; gel-forming | Relatively limited |



**Cellulose.** A structural part of plant cell walls. Cellulose and starch both contain glucose units. Their linkages differ, and human digestive enzymes do not break down cellulose. [7], [64], [65]

**Pectins.** Found in the cell walls of fruits and other plants. Pectin is a family, not one uniform substance. Its chemistry and the surrounding conditions affect gel formation. [64], [65]

**Oat / barley β-glucans.** Mixed-linkage beta-glucans in cereal cell walls. Processing can change how much dissolves and the size of its chains, changing viscosity. These descriptions do not apply to every fungal or yeast beta-glucan. [64], [66]

**Inulin / fructans.** Examples occur in chicory and onions. This is the useful counterexample: a soluble fiber can feed microbial fermentation without forming a thick gel. [64], [65], [67]

**Resistant starch.** Starch that escapes small-intestinal digestion. Resistance describes digestion. Starch structure, food processing and the microbial community all influence what happens next. [64], [65]

**Psyllium husk.** A gel-forming, fiber-rich plant husk. A second counterexample: forming a thick gel does not tell you how readily a fiber will be fermented. [64]

Typical properties, not fixed scores. Fiber form, chain size, food structure, processing and the gut community can change behavior. Whole foods contain mixtures; these examples do not predict an individual response.

Microbes can ferment some fiber into short-chain fatty acids and gases. Unfermented material and microbial biomass contribute to stool. [8], [68] The amber fiber artwork is conceptual; it is not molecular geometry or a measured microbial community.

<!-- FIBER GUIDE END -->

<a id="fats"></a>

## 06 · Dietary fat’s route

<!-- DRAFT3 FATS START -->

Food triglyceride → Digested & rebuilt → Chylomicron in blood → Fatty acids enter cells

Fat in food does not travel intact into a patch of body fat. Most dietary triglycerides are digested, absorbed and rebuilt before transport. Their fatty acids can still become stored body triglyceride. The chemistry is related; the food, transport particle and living tissue are different things.

Cells can oxidize fatty acids for ATP, store them, or incorporate them into lipids such as membrane phospholipids. Cholesterol helps build membranes and is a precursor for steroid hormones and bile acids. Fat does not replace the amino acids required to build protein.

[6], [15], [17], [21]

### Which kind of fat? Four questions

**What is the whole structure?** Lipids are a broad family. A triglyceride, a phospholipid and cholesterol have different structures and jobs.

- **Triglycerides.** Three fatty acids are attached to glycerol. This is a major form of dietary fat and the main lipid stored in fat cells. Digestion breaks the package apart. Intestinal cells rebuild much of it for transport in chylomicrons.

- **Phospholipids.** These lipids contain a phosphate-bearing, water-interacting region and water-avoiding regions. Their arrangement helps make membranes. The builder shows phosphatidylcholine, one example. Not every phospholipid has the same backbone or head group.

- **Cholesterol.** Cholesterol has a ring-based structure. It participates in membranes and supplies material for steroid hormones, bile acids and vitamin D. It is neither a triglyceride nor a fatty acid. The body makes it as well as obtaining some from food.

[15], [17], [42]

**Which features does a tail have?** These labels overlap. One ALA chain is simultaneously 18 carbons long, polyunsaturated, cis and omega-3.

- **Saturation.** Saturated: none. Monounsaturated: one. Polyunsaturated: two or more. This counts carbon–carbon double bonds, not the number of fatty-acid tails. A triglyceride can have three different tails. An oil or a food usually contains a mixture of fatty acids.

- **Cis and trans.** Cis and trans describe arrangements around a double bond. A trans fatty acid is still unsaturated; the geometry is a separate feature. Compare oleic and elaidic acid in the builder: both are 18:1, but their double-bond geometry differs.

- **Omega position.** Omega-3 and omega-6 identify the first double bond counted from the methyl end. They are families, not single molecules. LA is an essential omega-6 fatty acid; ALA is an essential omega-3. EPA and DHA are other omega-3s, with longer chains.

- **Chain length.** Fatty acids also differ in length. Chain length influences properties, handling and transport. The main dietary-fat route describes long-chain fats. Short-chain fatty acids from microbial fermentation can enter portal blood more directly. The absorption chapter explains this branch.

[15], [16], [8], [76]

**How does lipid travel through blood?** Lipoproteins are transport particles made of lipids and proteins. Albumin carries much of the fatty acid released from stores.

- **Chylomicrons.** Intestinal cells package dietary lipids into these particles. They enter lymph before the wider blood circulation. Lipoprotein lipase releases fatty acids to tissues; the liver later takes up chylomicron remnants.

- **VLDL and LDL.** VLDL exports triglyceride from the liver. As triglyceride is removed, the particles are remodeled; LDL is relatively enriched in cholesterol. LDL is a particle, not a special chemical kind of cholesterol or a saturation category.

- **HDL.** HDL participates in cholesterol movement between tissues and the liver. The particles and their jobs are more complicated than a single good/bad label. A blood HDL-cholesterol measurement describes carried cholesterol; it is not a direct score for every HDL function.

- **Albumin.** Many non-esterified fatty acids released from adipose tissue travel bound to albumin, a blood protein. Albumin is not ALA. One is a carrier protein; the other is a particular omega-3 fatty acid.

[42], [73], [79]

**What does body fat describe?** Cell type and location answer different questions again. Neither tells you whether a fatty-acid tail is saturated or omega-3.

- **White adipose tissue.** White fat cells store triglyceride, release fuel and participate in hormone signaling. They are living tissue, not passive bags of excess food. Storage and release are regulated flows. A meal does not have a predetermined destination in one specific depot.

- **Brown and beige cells.** Brown fat has abundant mitochondria. UCP1 can divert the proton gradient toward heat production. Beige thermogenic cells can occur within some white-fat depots. This changes how energy is handled; it does not turn one dietary fatty-acid category into a body-fat color.

- **Subcutaneous and visceral.** Subcutaneous adipose tissue lies under the skin. Visceral adipose tissue lies around abdominal organs. These are anatomical locations, not synonyms for saturated and unsaturated fat. The storage chapter follows release back into circulation.

[46], [47], [74]

### Four fatty-acid names to keep separate

**LA (Linoleic acid), 18:2 Omega-6.** Dietary essential; found in many seeds, nuts and vegetable oils.

**ALA (Alpha-linolenic acid), 18:3 Omega-3.** Dietary essential; examples include flax, chia and walnuts.

**EPA (Eicosapentaenoic acid), 20:5 Omega-3.** Present in fish and some algal sources; formed from ALA only to a limited extent.

**DHA (Docosahexaenoic acid), 22:6 Omega-3.** An important membrane fatty acid, including in retina and brain; fish and algal sources can supply it.

LA and ALA are dietary essentials. Conversion of ALA to EPA and DHA is limited. [16]

<!-- DRAFT3 FATS END -->

### Further exploration

### Try the attachment experiment

Attach one, two or three fatty acids to glycerol in the schematic to distinguish monoacylglycerol, diacylglycerol and triacylglycerol. Each fatty-acid ester hydrolysis uses water and removes one attachment. Compare phosphatidylcholine: two fatty-acyl tails and a phosphate–choline head on a glycerol backbone. Its phosphate ester linkages are separate from its two fatty-acid ester links. These are structural comparisons, not a direct cellular reaction sequence. [15], [71]

Hold the fatty-acid chain length at 18 carbons. Stearic acid has no C=C bonds; oleic and elaidic acids each have one, with cis and trans geometry respectively; linoleic acid has two cis double bonds; α-linolenic acid (ALA) has three. Changing geometry does not make trans fat saturated. The on-page tail icons show this distinction without counting individual carbon atoms. [15], [16]

The labels answer separate questions. Saturation counts carbon–carbon double bonds; cis/trans describes their geometry; omega counts to the first double bond from the methyl end. Linoleic acid is both polyunsaturated and omega-6; ALA is both polyunsaturated and omega-3. Triglyceride names a whole molecule that can contain different fatty-acid chains. Cholesterol belongs to the sterol family, while LDL and HDL are lipoprotein particles carrying lipids. [15], [16], [42]

### Different kinds of fat mean different things

A **triglyceride** contains a glycerol backbone joined to three fatty acids. It is the main storage form discussed here. **Phospholipids** are important membrane components. **Cholesterol** is a sterol used in membranes and as a precursor for bile acids and steroid hormones. Cholesterol is not another name for triglyceride. [15]

A fatty acid is **saturated** when it has no carbon–carbon double bonds, **monounsaturated** with one, and **polyunsaturated** with more than one. Cis and trans describe geometry around a double bond. Omega-3 and omega-6 identify the location of the first double bond counted from the methyl end. These labels describe chemistry; white, brown and beige describe kinds of fat cells. [15]

Linoleic acid and alpha-linolenic acid are essential fatty acids: humans need a dietary source. EPA and DHA are other omega-3s; conversion from alpha-linolenic acid is limited. The chemistry does not justify the blanket slogan that all omega-6 fats cause inflammation or that one exact omega ratio suits everyone. [16]

### Mobilizing fat is only the beginning

**Lipolysis** splits stored triglyceride into fatty acids and glycerol. Released fatty acids can circulate, be taken up by tissues, be stored again, or be oxidized. Release from a fat cell is not proof that the released fat has already been burned. [17]

Fatty acids are activated to fatty acyl-CoA. Long-chain fatty acids use the carnitine shuttle to reach the mitochondrial matrix. **Beta-oxidation** repeatedly shortens the chain, usually two carbons at a time, supplying acetyl-CoA and reducing equivalents for energy metabolism. The acetyl-CoA joins the Krebs cycle. Glycerol follows its own route into carbohydrate metabolism. Fat oxidation therefore connects to oxidative phosphorylation through several steps; fat does not leap directly into ATP. [18], [50]

Most common even-chain fatty acids cannot supply a net gain of glucose through the usual acetyl-CoA/Krebs route in humans. The glycerol portion of a triglyceride can contribute to glucose production, and odd-chain fatty acids have a different entry point through propionyl-CoA. “Fat can become glucose” needs those qualifications. [18], [57]

### One backbone, three chains

A triglyceride's three-carbon glycerol backbone is attached to three fatty-acid chains through ester bonds. The chains do not have to be identical. **Tripalmitin** is one particular example: all three chains are saturated palmitate chains, each containing 16 carbons. Its formula is **C₅₁H₉₈O₆**. That is one molecule, not the universal composition of dietary fat or adipose tissue. [RCSB PDB: tripalmitoylglycerol](https://www.rcsb.org/ligand/4RF), [OpenStax: fats and oils](https://openstax.org/books/organic-chemistry/pages/27-1-waxes-fats-and-oils).

Separating the backbone and chains explains the molecule's parts. Digestion and stored-fat lipolysis have their own sequences of reactions; neither should be confused with the later oxidation that releases carbon as CO₂.

### The transport particles have different routes

Lipid transport also has separate routes. Chylomicrons carry much of the fat arriving from the intestine. Liver-derived VLDL can be remodeled through IDL into LDL. HDL participates in cholesterol transport and exchange. These particles do not form a single chain running chylomicron → LDL → HDL. [42]

---

<a id="proteins"></a>

## 07 · Protein’s route

<!-- DRAFT3 PROTEINS START -->

Dietary protein → Amino acids → New proteins / other molecules → Breakdown when needed

The body assembles absorbed and recycled amino acids into its own proteins: enzymes, muscle machinery, transporters and much more. A protein-rich meal does not become the same amount of new muscle. Synthesis and breakdown continue throughout the day.

There is no dedicated surplus-protein tank. When amino acids are broken down, nitrogen is handled largely through liver urea production and urinary excretion. Carbon skeletons can join fuel pathways or glucose synthesis, depending on the amino acid. Protein can contribute energy while carbohydrate and fat are still available.

[21], [22]

<!-- DRAFT3 PROTEINS END -->

<!-- PROTEIN STORY START -->

### The order. The fold. The work.

Eating protein supplies material. Your cells decide what to build with it. Mixing amino acids provides ingredients; a cell needs an instruction to assemble a particular protein. [3], [69]

**1. Amino acids — The ingredients.** Digested food and recycled body proteins supply amino acids. Cells can also make many types. The familiar set has 20 standard types, each with a different side group. [3], [21]

**2. An ordered chain — The instruction.** A gene is copied into messenger RNA. A ribosome reads that instruction; transfer RNAs deliver amino acids in the specified order. Peptide bonds link them into a growing chain. This assembly uses energy. [69]

**3. A shape emerges — The structure.** Interactions within the chain and with its surroundings guide folding. Parts can begin folding during assembly. Helper proteins called chaperones can reduce unwanted tangling and aggregation. [69], [82]

**4. Structure enables a job — The working molecule.** The arrangement of chemical groups lets proteins bind, catalyze, carry or pull. Some need other chains, added groups or further processing to work. Different sequences and structures support different jobs. [21], [69]

Gly–Ala–Gly and Ala–Gly–Gly have the same amino-acid counts but different orders. These short chains illustrate sequence; they do not predict a fold or function.

| Protein | An example of its work |
| --- | --- |
| Amylase | Breaks down starch |
| Hemoglobin | Carries oxygen |
| Collagen | Supports tissues |
| Myosin | Helps muscles contract |

### Meet Trp-cage: a separate measured example

The designed miniprotein TC5b has 20 residue positions and 12 amino-acid types. Read this actual sequence from N to C terminus: **NLYIQWLKDGGPSSGRPPPS**. Repeated letters mean repeated types.

The chart’s image and companion 3D viewer use 304 deposited atoms from model 1 of the 38-model solution-NMR ensemble. A ribbon traces the backbone fold; other views show the same atomic coordinates, with hydrogens initially hidden. Sticks represent inferred connectivity, not resolved bond orders. This is a structure example, not a protein from the meal or a prediction from the builder. [RCSB PDB: 1L2Y](https://www.rcsb.org/structure/1L2Y).

### What makes it fold?

For many proteins in water, water-avoiding side groups cluster inside. Hydrogen bonds and other interactions help stabilize the structure. The sequence matters, and so do the surroundings. A working protein can still move and change shape. [82]

### What if the shape changes?

Heat or changes in acidity can disrupt a protein’s usual structure: denaturation. Its activity may change or disappear. Unfolding a chain and cutting its peptide bonds are different changes; digestive enzymes do the cutting. [21], [6]

### Four names you may come across

- **Primary:** The amino-acid order.

- **Secondary:** Local patterns, including helices and sheets.

- **Tertiary:** The overall 3D arrangement of one chain.

- **Quaternary:** Several chains assembled together, when present.

These describe levels of organization; quaternary structure applies when several chains assemble. [21], [82]

<!-- PROTEIN STORY END -->

### Try the sequence experiment

Build Gly–Ala–Gly: three amino-acid positions, two peptide bonds and only two different amino-acid types. Replace the middle alanine with serine. The side group changes from a methyl to a hydroxymethyl group; chain length and bond count do not change. Hydrolysing one peptide bond uses one water molecule and produces two fragments, while retaining all three amino-acid units. [21]

A peptide bond connects a carbonyl carbon to the next residue's nitrogen, C(=O)–N. In cells, ribosomes use activated amino acids carried on tRNAs; direct condensation of free amino acids is not the mechanism being simulated here. The builder's eight-position limit is a display limit, not a boundary between peptide and protein. It does not predict a folded structure or function. The separate deposited protein model appears above the builder for comparison. [21], [69]

Proteins are amino-acid chains whose structures enable many different jobs. Enzymes, transporters, antibodies and contractile proteins all belong here. Digestion breaks food proteins down; the body uses the resulting amino acids to assemble its own proteins. Nine amino acids are essential in adult nutrition because we cannot make enough of them. “Nonessential” means the body can synthesize the amino acid; it does not mean the molecule is unimportant. [21], [3]

Your proteins are continually being built and broken down. This **protein turnover** allows renewal, repair and adaptation. Amino acids released during breakdown can be reused. There is no separate, inert protein storage depot equivalent to a glycogen granule or triglyceride droplet: a muscle is active tissue with work to do. [22]

Nutrients also participate in signaling. Systems including mTOR help regulate protein synthesis, while ribosomes assemble amino acids into proteins. Muscle growth depends on the balance of synthesis and breakdown over time. A short-term increase in measured synthesis does not directly predict the same percentage increase in muscle size. [58]

When amino acids are broken down, their nitrogen and carbon need different handling. Amino groups can be transferred between molecules, and nitrogen is ultimately disposed of largely through liver production of **urea**, which the kidneys excrete. The remaining carbon skeletons enter metabolism at different places, including pyruvate, acetyl-CoA or Krebs-cycle intermediates. Some can support glucose production; others are ketogenic, and some are both. [22]

Protein is therefore both structural material and potential fuel. It does not have to wait until all carbohydrate and fat are exhausted before any amino acids are oxidized. Nor does eating an amino acid tell us that it will become muscle. The body distributes material according to demand and available machinery.

The nine essential amino acids are **histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan and valine**. Examples whose nutritional essentiality can depend on context include arginine, cysteine, glutamine and tyrosine. This is a description of synthesis and supply, not a supplement prescription. [MedlinePlus amino acids](https://medlineplus.gov/ency/article/002222.htm).

We now have the ingredients and their possible destinations. The next step is to separate the journey of their atoms from the transfer of their chemical energy.

---

<a id="storage"></a>

## 08 · Build, store & release

<!-- DRAFT3 STORAGE START -->

Use, synthesis and storage occur together. Hormones and demand change their rates. A meal has no predetermined visceral or subcutaneous destination. Insulin and nutrient availability after eating generally favour storage while restraining fat release. [39], [74]

### After a meal

Incoming nutrients support current work and replenish reserves. Insulin generally favours storage and restrains fat release.

- **Liver: Glucose → glycogen.** Some arriving glucose can be stored; the liver also processes and shares nutrients.
- **Muscle: Glucose → local glycogen.** Muscle can replenish its own working reserve while continuing to use ATP.
- **Adipose tissue: Fatty acids → triglyceride.** Storage is favoured; fat release is generally reduced, rather than switched off everywhere. [9], [39], [74]

### Between meals

With less arriving from the gut, internal supplies help maintain the blood and keep tissues working.

- **Liver: Glycogen → blood glucose.** Glycogen breakdown helps support blood glucose alongside glucose production from other precursors.
- **Muscle: Local reserve → local work.** Muscle glycogen stays available for that muscle; it does not directly refill blood glucose.
- **Adipose tissue: Triglyceride → circulating fatty acids.** Released fatty acids travel with albumin. Tissues can use them or store them again. [9], [57], [73], [74]

### During movement

Working muscle draws on several supplies together. Intensity and duration change their contributions; there is no universal order in which tanks empty.

- **Liver: Glucose supply → blood.** Liver glucose output helps support working tissues and the rest of the body.
- **Muscle: Local glycogen + blood fuels → ATP.** Glycogen and arriving glucose contribute alongside fatty acids and other fuels; harder efforts often rely more on carbohydrate.
- **Adipose tissue: Released fatty acids → tissue uptake.** The circulating supply can contribute fuel. A working muscle does not identify the body-fat depot supplying it. [9], [39], [73]

### Liver glycogen

Glucose units stored in liver cells can be released as blood glucose between meals. The liver also makes glucose from precursors such as lactate and glycerol. [9], [57]

### Muscle glycogen

Each muscle fiber uses its own glycogen. Muscle cannot directly export a useful supply of free glucose from that store; it can exchange other carbon-containing products, including lactate. [9], [73]

### Adipose triglyceride

Fat cells store fatty acids joined to glycerol. Subcutaneous fat lies under the skin; visceral fat lies around abdominal organs. These are locations, not saturated or unsaturated fat types. [17], [74]

### Working tissue & nutrient stores

Protein is active tissue, continually renewed. Vitamins and minerals have different stores: examples include B12 in the liver, calcium in bone and iron in ferritin. They are not all immediately discarded when unused. [22], [30], [75], [77]

### Stored fat returns to circulation

Adipose triglyceride → lipolysis → released fatty acids carried with albumin in blood → tissues. Albumin is a carrier protein; ALA is alpha-linolenic acid, a particular omega-3 fatty acid. They are different things. [16], [79]

The liver exports triglyceride in VLDL; muscle also has some local triglyceride. Regional fat release differs. A working muscle does not identify the adipose depot supplying its fuel, and release can be followed by oxidation or storage again. [42], [73], [80]

<!-- DRAFT3 STORAGE END -->

<a id="energy"></a>

## 09 · How cells make ATP

<!-- DRAFT3 ENERGY START -->

The cycle loads carriers; the membrane makes most ATP during aerobic glucose oxidation. Glycolysis occurs outside mitochondria. The following compartments are schematic, not a map of every reaction.

### Split glucose: Glycolysis

**Where:** Cytosol · outside mitochondria.

**In:** Glucose. **Out:** Pyruvate + a little ATP + NADH.

This pathway can regenerate ATP without directly using oxygen. Pyruvate can enter mitochondria or become lactate. [10], [60]

### Prepare the entrance: Pyruvate oxidation / beta-oxidation

**Where:** Mitochondrial matrix.

**In:** Pyruvate or fatty-acid breakdown products. **Out:** Acetyl-CoA + electron carriers.

Pyruvate processing releases CO2. Fatty-acid breakdown supplies acetyl-CoA by a different route. Amino-acid carbon can enter at several points. [12], [18], [50]

### Turn the cycle: Krebs / citric acid / TCA cycle

**Where:** Mitochondrial matrix.

**In:** Acetyl-CoA joins a regenerated acceptor. **Out:** CO2 + NADH / FADH2 + a little ATP equivalent.

A sequence of reactions returns to its starting acceptor. Much of the captured energy leaves on electron carriers. The cycle also supplies ingredients for synthesis. [12], [13]

### Use the gradient: Electron transport + ATP synthase

**Where:** Inner mitochondrial membrane.

**In:** Electrons from carriers + oxygen. **Out:** Most ATP in aerobic glucose oxidation + water.

Electron transport pumps protons across the membrane. Their return through ATP synthase helps make ATP from ADP and phosphate. Oxygen accepts electrons at the chain’s end and is reduced to water. [14]

Oxygen is reduced to water at the respiratory chain. CO2 is released in carbon-removing reactions, including pyruvate oxidation and the Krebs cycle. Carbon does not simply mix with oxygen in a final disposal step. [12], [14]

The cycle regenerates oxaloacetate, its starting acceptor. NADH and FADH2 carry electrons. B2 contributes to FAD, B3 to NAD and B5 to coenzyme A; these nutrients support reactions without themselves supplying calories. [13], [24], [25], [26]

### Turn the cycle: four useful moments

Krebs cycle, citric acid cycle and TCA cycle are three names for the same pathway. These snapshots show one turn per acetyl group, not the complete yield from one glucose molecule.

**Ready.** A four-carbon acceptor is waiting. Acetyl-CoA brings a two-carbon acetyl group; the acceptor is part of the working cycle, not a fresh dietary input each turn.

**Join.** Four carbons plus two make a six-carbon intermediate. Acetyl-CoA has delivered its acetyl group. The point is the combination, not memorizing every enzyme name.

**Release & load.** Two carbon-removing reactions release two CO₂ and load two NADH. The intermediate has four carbons again, but the starting acceptor has not yet been regenerated.

**Regenerate.** The remaining reactions restore oxaloacetate and complete the turn’s output: three NADH, one FADH₂ equivalent and one GTP/ATP equivalent. The restored acceptor can receive another acetyl group.

This is a carbon-count view, not isotope tracing. The entering acetyl carbons are not generally the first two carbons released as CO2. [12], [13]

### A reversible lactate branch

**Make lactate: Pyruvate + NADH + H⁺ → Lactate + NAD⁺.** Lactate dehydrogenase transfers reducing power from NADH to pyruvate. NAD⁺ is available again for glycolysis. The three-carbon skeleton stays; this reaction adds no ATP. Glycolysis can keep supplying ATP when NAD⁺ is regenerated. [60], [61]

**Use lactate: Lactate + NAD⁺ → Pyruvate + NADH + H⁺.** The same reversible reaction can convert lactate to pyruvate while reducing NAD⁺ to NADH. Pyruvate can then enter other pathways, depending on the tissue and conditions. Lactate is a circulating metabolite and fuel, not an automatic dead end. [60], [61]

Lactate formation regenerates NAD+ for glycolysis, including when oxygen is available. Oxygen is not a reactant in the LDH step. [60], [61], [62]

**Local muscle:** Lactate → Pyruvate → Mitochondrial oxidation. Muscle can form lactate and also use it. After conversion to pyruvate, its carbon can enter oxidative pathways. Production and use can overlap across cells. [60], [61], [73]

**Another tissue:** Lactate in blood → Uptake, e.g. by the heart → Pyruvate → oxidation. Blood carries lactate between tissues. The heart and other tissues can take it up and use its carbon for oxidative metabolism; the contribution changes with conditions. [83], [61]

**Liver:** Lactate in blood → Liver → new glucose → Glucose returns to blood. Liver glucose production can reuse lactate carbon. This is part of the Cori cycle: recycling carbon costs energy in the liver. It is not a free extra ATP-producing loop. [57], [61]

Teaching structure informed by Andy Galpin’s [energy lesson, 38:24](https://www.youtube.com/watch?v=L-hO3937Cv8&t=2304s) and [lactate introduction, 1:44](https://www.youtube.com/watch?v=QtOcIZqkieA&t=104s). Reaction details follow the scientific references; these are original explanations and diagrams.

<!-- DRAFT3 ENERGY END -->

### Further exploration

The Krebs cycle is a meeting point, not the whole of metabolism. Follow one **free glucose** through the connected reactions, keeping a carbon account and an electron-carrier account alongside each other.

### Glycolysis: six carbons become two threes

In the **cytosol**, outside the mitochondria, glycolysis converts a six-carbon glucose into two three-carbon pyruvate molecules. It spends two ATP and makes four, leaving a net gain of **two ATP per free glucose**. It also produces two NADH molecules, which carry electrons into subsequent energy-transfer reactions. No carbon dioxide is released in glycolysis itself. [10]

Glycolysis does not directly require oxygen. Converting pyruvate to lactate regenerates NAD⁺, allowing glycolysis to continue. [60] Lactate formation can occur even when oxygen is present; human exercise measurements do not support treating lactate release as a simple sign of absent oxygen. [62] Lactate is a transferable fuel and precursor. The human brain can take up and oxidize circulating lactate, and lactate carbon can return to glucose through gluconeogenesis. [11], [57], [61]

### Pyruvate oxidation: the first CO₂ exit

Pyruvate entering the mitochondrial matrix can be converted by the pyruvate dehydrogenase complex into **acetyl-CoA**. Each three-carbon pyruvate yields a two-carbon acetyl group, one CO₂ and one NADH. Because glycolysis made two pyruvates, this happens twice per glucose. Acetyl-CoA is a junction shared with fat and amino-acid metabolism. [12]

### The Krebs cycle: a loop that transfers electrons

The Krebs cycle, citric acid cycle and TCA cycle are three names for the same pathway. A two-carbon acetyl group combines with four-carbon oxaloacetate to form six-carbon citrate. Reactions return the pathway to oxaloacetate so another turn can begin.

Per acetyl-CoA entering the cycle, the usual accounting is **two CO₂, three NADH, one FADH₂ and one GTP or ATP equivalent**. Much of the useful output is carried by NADH and reduced flavin chemistry, rather than produced immediately as ATP. The cycle also supplies intermediates for building other molecules. [12], [13]

The carbon accounting is a net result across continued cycling. The two carbon atoms that just entered as acetyl-CoA are not generally the same two atoms expelled during that first turn. A net carbon total should therefore not be mistaken for the identity of individual atoms in one turn.

### Oxidative phosphorylation: where oxygen finishes the relay

At the inner mitochondrial membrane, electrons pass through respiratory-chain components. This transfers energy into a proton gradient. Protons flowing back through ATP synthase help drive ATP production from ADP and phosphate.

**Oxygen is the final electron acceptor and is reduced to water.** CO₂ is released in other reactions, including pyruvate oxidation and the Krebs cycle; it is not the exhaust product of ATP synthase. Oxygen sustains the electron-transfer system that lets oxidative fuel processing continue. [14]

Using conventional estimates for the electron carriers gives roughly **30–32 ATP per glucose** in human cells. It is a teaching estimate, not an exact biological invoice: shuttle pathways, transport costs, proton leak and other uses of intermediates affect the accounting. The useful lesson is the connection: glycolysis and the cycle supply carriers, the respiratory chain builds a gradient, and ATP synthase uses that gradient. [13], [14], [50]

### The net glucose account

The following is reaction accounting for the specified pathway, not a measurement of a person's metabolism.

| Stage, per free glucose | Carbon account | Reduced carriers produced at this stage | Direct ATP/GTP gain |
| --- | --- | --- | --- |
| Glycolysis | 1 × 6-carbon glucose → 2 × 3-carbon pyruvate; no CO₂ | 2 NADH | 2 ATP net |
| Pyruvate oxidation | 2 × 3-carbon pyruvate → 2 × 2-carbon acetyl groups + 2 CO₂ | 2 NADH | 0 |
| Two Krebs turns | Net release of another 4 CO₂ while the cycle pool is regenerated | 6 NADH + 2 FADH₂ equivalents | 2 GTP/ATP equivalents |
| Before oxidative phosphorylation | 6 carbons accounted for as a net 6 CO₂ across continued oxidation | Total: 10 NADH + 2 FADH₂ equivalents | Total: 4 ATP/GTP equivalents |

“Two-carbon acetyl group” does not mean the whole acetyl-CoA molecule has only two carbons: CoA is the carrier attached to that group. The reduced-carrier totals are produced across different compartments. Their electrons reach the respiratory chain through appropriate pathways; cytosolic NADH does not simply cross the inner mitochondrial membrane as the same molecule. [10], [12], [14], [50]

### What a single cycle turn does

After acetyl-CoA joins oxaloacetate, the intermediate sequence is citrate → isocitrate → alpha-ketoglutarate → succinyl-CoA → succinate → fumarate → malate → oxaloacetate.

| Point in the cycle | Carbons in the named intermediate | Cumulative CO₂ released | Cumulative NADH | Cumulative FADH₂ equivalents | Cumulative GTP/ATP equivalents |
| --- | ---: | ---: | ---: | ---: | ---: |
| Oxaloacetate, before entry | 4 | 0 | 0 | 0 | 0 |
| Citrate, after the 2-carbon acetyl group joins | 6 | 0 | 0 | 0 | 0 |
| Isocitrate | 6 | 0 | 0 | 0 | 0 |
| Alpha-ketoglutarate | 5 | 1 | 1 | 0 | 0 |
| Succinyl-CoA | 4 | 2 | 2 | 0 | 0 |
| Succinate | 4 | 2 | 2 | 0 | 1 |
| Fumarate | 4 | 2 | 2 | 1 | 1 |
| Malate | 4 | 2 | 2 | 1 | 1 |
| Oxaloacetate regenerated | 4 | 2 | 3 | 1 | 1 |

These are intermediate carbon counts; carbon in attached CoA is excluded. Enzyme-bound flavin chemistry is summarized as an FADH₂ equivalent. The bookkeeping deliberately does not label the incoming acetyl carbons as the two CO₂ molecules leaving on that first turn. [12], [13]

### Why recycling NAD matters

NAD⁺ accepts electrons in reactions including glycolysis; its reduced form is NADH. Continued operation needs a way to regenerate NAD⁺. Lactate dehydrogenase provides one connection:

**Pyruvate + NADH + H⁺ ⇌ lactate + NAD⁺.** [60], [61]

This reaction does not directly make ATP. It enables continued ATP production through glycolysis by regenerating an electron acceptor. Carbon is retained: a three-carbon pyruvate becomes a three-carbon lactate.

Consider a deliberately tiny, closed bookkeeping example with three glucose molecules and four available NAD⁺ molecules, while all other required ingredients are assumed available. Processing two glucose molecules through glycolysis uses those four NAD⁺ and leaves four NADH, four pyruvates and four net ATP. A third round cannot proceed in this example until NAD⁺ is regenerated. Converting two of the pyruvates to lactate restores two NAD⁺, allowing the third glucose to pass through glycolysis and bringing the net ATP total to six.

Those starting quantities are chosen to expose the dependency; they are not cellular concentrations, reaction speeds or a prediction of fatigue. Cells have connected routes that regenerate carriers, including transfer of cytosolic reducing equivalents into mitochondrial oxidation. Recycling a carrier and oxidizing a carbon skeleton are distinct accounting steps. [10], [14], [60]

The whole pathway now explains two outputs often confused with one another: ATP-supported work and heat concern energy; CO₂ and water concern matter.

---

<a id="context"></a>

## 10 · Tomorrow’s energy

<!-- DRAFT3 CONTEXT START -->

Tomorrow you climb a hill. Stored ATP is a small working supply. Phosphocreatine rapidly helps regenerate it; glycolysis and oxidative metabolism contribute from the start. These are overlapping systems, not tanks emptied in order. [72], [73]

| Situation | Phosphocreatine | Glycolysis | Oxidative metabolism |
| --- | --- | --- | --- |

| A sudden hard effort: Lift, jump, accelerate | Rapid ATP buffering | Rises quickly | Already contributing; ramps up |

| Sustained hard work: A hard climb or interval | Small reserve; must be rebuilt | Large contribution from carbohydrate | Substantial contribution alongside glycolysis |

| Steady easier movement: Walk or comfortable ride | Buffers changes in demand | Continues supplying pyruvate | Often supplies most ATP; mixes carbohydrate and fat |

| Recovery & the next meal: Rest, eat, replenish | Replenished using ATP | Continues at a lower demand | Supports recovery; fuel mixture keeps changing |

These descriptions are qualitative, not measured percentages. Easier movement can use fat before glycogen is depleted; harder work generally relies more on carbohydrate. Blood glucose and local muscle glycogen are two sources feeding shared pathways. Circulating fatty acids and local muscle lipids can both contribute. [9], [72], [73]

The burn is not a direct measure of local fat loss or glycogen use. Sensory nerves respond to combinations of chemical changes; lactate alone does not explain it. [81]

During recovery, ATP helps restore phosphocreatine, meals help replenish glycogen and tissue renewal continues. [72], [73]

<!-- DRAFT3 CONTEXT END -->

### Further exploration

The same molecule can have different destinations in different tissues. The **liver** processes incoming nutrients, stores and releases glucose, makes ketones and handles much of nitrogen disposal. **Muscle** spends ATP to contract and can use circulating fuels and local stores. **Adipose tissue** stores triglyceride and releases fuel, while also participating in hormonal communication. [9], [22], [39]

The **brain** usually relies heavily on glucose but can also use lactate and, when available in sufficient amounts, ketone bodies. Classic fasting research demonstrated a substantial shift toward ketones; it did not establish a universal hour when everyone's brain “switches over.” Red blood cells lack mitochondria, so they remain dependent on glycolysis for ATP. [10], [11], [40]

The **kidneys** remove urea, regulate fluids and acid–base balance, and contribute to glucose production. The **heart and blood vessels** move oxygen and substrates between tissues. The **lungs** exchange gases. The **gut and its microbes** add another route into the network through digestion, absorption and microbial metabolites. No single organ is “the metabolism.” [2], [5], [8], [41], [55]

### After a mixed meal

Nutrients arrive while organs keep using energy. Insulin often rises, supports nutrient uptake and storage, and suppresses adipose lipolysis and liver glucose output. In muscle and adipose tissue, it helps bring GLUT4 transporters to the cell surface. Many other cells take up glucose through transporters that do not depend on this same insulin-controlled step. “Insulin is the key to every cell” would misrepresent the map. [39], [43]

### Between meals

As absorption subsides, stored fuel contributes more. Liver glycogen breakdown and gluconeogenesis help sustain circulating glucose; fat mobilization and oxidation become more prominent. Glucagon acts chiefly on the liver in this account, rather than directly telling skeletal muscle to release glucose. Gluconeogenesis is a regulated, ongoing process whose contribution changes—not a pathway that suddenly appears at one fasting deadline. [39], [9]

With prolonged low carbohydrate availability, the liver can export more **ketone bodies**, made from acetyl-CoA. Other tissues can convert them back toward acetyl-CoA for oxidation. Ketones distribute fuel; they do not demonstrate that a particular diet has produced a particular amount of net body-fat loss. [17], [40]

### During movement

ATP demand rises. Phosphocreatine can rapidly help regenerate ATP, glycolysis can supply ATP quickly, and oxidative metabolism contributes too. The systems overlap from the beginning; they are not three engines that switch on at exactly ten seconds and two minutes. [44]

As effort becomes more intense, carbohydrate use generally becomes more prominent. Exercise duration, nutrition, training and the particular muscles recruited change the mixture. Tracer experiments show that fuel use changes across intensity and time, but their results should not be turned into exact universal percentages for every reader. Muscle contraction can also stimulate glucose uptake through pathways distinct from insulin signaling. [45], [43], [53]

A larger **percentage** of energy coming from fat is not the same thing as a larger total amount of fat oxidized. Neither measurement alone tells us how body-fat stores change across days. What is entering storage and what is leaving it both matter.

### Heat and changing energy needs

Metabolism supports heat production as well as ATP-dependent work. Brown and beige fat cells contain machinery that can let the mitochondrial proton gradient dissipate through UCP1, releasing energy as heat. Active brown fat has been demonstrated in adults, but its presence is not evidence for a guaranteed fat-loss effect from a cold-exposure routine. These cell types should sit on a different visual layer from saturated and unsaturated fatty acids. [46], [47]

Resting expenditure, movement and food processing all contribute to energy use. A lower requirement after weight loss can reflect a smaller body and physiological adaptation. Aging also deserves care: a large 2021 analysis found broadly stable expenditure adjusted for body composition from about age twenty to sixty, followed by a later decline. That does not mean every person's unadjusted expenditure remains fixed for forty years. [48], [49]

---

<a id="breath"></a>

## 11 · Breath & metabolism

The surprising answer is that much of it leaves through your lungs.

Stored fat contains carbon, hydrogen and oxygen. When cells oxidize that fuel, its atoms are rearranged into carbon dioxide and water. Its chemical energy is transferred through reactions that support cellular work and release heat. The material and the energy have different destinations. Fat does not simply become “energy” and disappear. This distinction is the central point of Meerman and Brown's account of fat loss. [1]

Carbon dioxide made in your tissues travels in your blood, much of it temporarily carried as bicarbonate. In the lungs, it becomes CO₂ gas again, crosses into the air sacs and is exhaled. Metabolic water joins the body's water pool and can eventually leave through urine, breath, sweat and other routes. [2]

Breathing faster by itself does not make your cells oxidize proportionally more fat. Breathing removes CO₂ produced by metabolism; it is not a command telling fat cells to empty. A hard effort can raise both fuel use and ventilation, but the association is not a breathing trick.

The earlier chapters supplied the missing middle: fuel processing, pyruvate oxidation, continued cycling, electron transfer and transport between tissues.

### A representative fat-mass account

A rounded model used by Meerman and Brown is:

**10 kg triglyceride + 29 kg oxygen → 28 kg carbon dioxide + 11 kg water.**

Approximately 8.4 kg of the **original triglyceride mass** leaves in CO₂ and 1.6 kg becomes water. The full product masses are larger because inhaled oxygen also contributes matter. These are two different accounts: the destination of the original fat mass, and the total mass of all products. [1]

This is a representative triglyceride model, not the tripalmitin example from the structure chapter, the composition of someone's adipose tissue, or a calculation of personal weight loss. The published correction fixes an oxygen-to-water label in the original figure to 9.4 kg. [BMJ correction](https://www.bmj.com/content/349/bmj.g7782).

Water has several possible exit routes. The lungs are the main exit for carbon in completely oxidized fat, not a device for bypassing the preceding chemistry.

---

### This whole exchange is metabolism

A meal supplies molecules. Digestion and absorption make them available; organs distribute and transform them. Cells build, store, release and oxidize material while continually regenerating ATP. Tomorrow’s movement draws on that changing supply—and some of the carbon eventually leaves in your breath.

Metabolism includes all of this: the building as well as the breakdown, the work as well as the heat, the stored material as well as the material leaving.

<a id="quiz"></a>

## 12 · Check your understanding

The interactive Recall Lab contains **100 questions across 10 levels**, with **10 questions per round**. Fresh mix draws one question from each level and prioritizes the least-answered questions. Level practice covers all ten questions in that level in shuffled order. Answer choices are shuffled in both modes.

Correct answers can advance after four seconds; missed answers wait for you. Turn Quick flow off to advance every question yourself. The same browser can retain an unfinished round, per-question progress and the latest 20 completed rounds, including answers and explanations. No account or score uploads are used.

The full question-and-answer collection follows for offline reading. These are learning checks, not a health assessment.

### Level 1 · Foundations

Name the main ideas.

**1. What does metabolism include?**

**Answer: The body's network of building and breakdown reactions.** Metabolism includes synthesis as well as breakdown, across cells and tissues. [3], [18]

**2. Which group supplies amino acids?**

**Answer: Proteins.** Digestion of protein supplies amino acids used in the body's own molecules. [21], [22]

**3. In nutrition, what does essential mean?**

**Answer: A dietary supply is needed because synthesis is insufficient.** Essentiality describes how a requirement is supplied, not which molecule matters most. [3]

**4. Which family includes sugars and starch?**

**Answer: Carbohydrates.** Carbohydrates include single sugars, joined sugars and larger structures. [7]

**5. Which nutrient supplies no calories?**

**Answer: Water.** Water provides the medium for much of the body's chemistry without being a caloric fuel. [3], [31]

**6. What is a calorie a unit of?**

**Answer: Energy.** Food calories describe energy, not the mass or atom count of a nutrient. [3]

**7. What kind of molecule is ATP?**

**Answer: A reusable carrier that helps couple energy to cellular work.** Cells continually regenerate ATP to support processes such as contraction and transport. [4]

**8. Much of the carbon from oxidized body fat eventually leaves as what?**

**Answer: Carbon dioxide in exhaled air.** Carbon atoms remain matter and can leave the body in CO₂ after oxidation. [1], [2]

**9. What do macro and micro describe in macronutrients and micronutrients?**

**Answer: The amounts needed.** These nutrition labels concern quantities, not molecular dimensions or importance. [3], [31]

**10. Which statement about a mixed meal fits the article?**

**Answer: It contains several nutrient families at once.** A food is a mixture: fuel, structural components and supporting nutrients can occur together. [5], [3]

### Level 2 · Molecular families

Distinguish structures and labels.

**11. Which is a monosaccharide?**

**Answer: Fructose.** Fructose is a single sugar; sucrose joins two units and starch contains many. [7]

**12. Which two sugars form sucrose?**

**Answer: Glucose and fructose.** Sucrose combines glucose with fructose; lactose contains glucose and galactose. [7]

**13. Which starch component is highly branched?**

**Answer: Amylopectin.** Amylopectin has branch points, whereas amylose is mostly unbranched. [7]

**14. What is the basic structure of a triglyceride?**

**Answer: Glycerol joined to three fatty acids.** Triglycerides esterify three fatty acids to a glycerol backbone. [15]

**15. A saturated fatty acid has how many carbon–carbon double bonds?**

**Answer: None.** Saturation describes double bonds along a fatty-acid chain. [15]

**16. What does monounsaturated mean?**

**Answer: One carbon–carbon double bond in the fatty-acid chain.** The prefix counts double bonds, not carbons or triglyceride tails. [15]

**17. Omega-3 locates the first double bond from which end?**

**Answer: The methyl end of the fatty acid.** Omega numbering starts at the terminal methyl end. [16]

**18. What do cis and trans distinguish?**

**Answer: Geometry around a double bond.** Bond geometry is a separate classification from the number of double bonds. [15]

**19. Cholesterol belongs to which lipid family?**

**Answer: Sterols.** Cholesterol has a ring-based sterol structure, not a glycerol-and-three-tails structure. [15], [42]

**20. What links amino-acid residues in a protein chain?**

**Answer: Peptide bonds.** Peptide bonds form the protein backbone; the resulting sequence can fold into a working shape. [21]

### Level 3 · Digestion & delivery

Follow nutrients into the body.

**21. Where does most nutrient absorption take place?**

**Answer: The small intestine.** The small intestine absorbs most digested nutrients into transport routes. [5], [6]

**22. What is bile's role in fat digestion?**

**Answer: Helping disperse fat.** Bile helps emulsify fat; it is not itself a digestive enzyme. [5], [6]

**23. Digestible starch is broken down to supply which absorbable sugar?**

**Answer: Glucose.** Starch is built from glucose units that digestive enzymes can release. [5], [7]

**24. Which route carries most absorbed sugars first toward the liver?**

**Answer: Portal blood.** Most absorbed sugars enter portal circulation before reaching the liver. [6]

**25. Much long-chain dietary fat first enters circulation in which particles?**

**Answer: Chylomicrons.** Intestinal cells package much absorbed long-chain fat into chylomicrons, which enter lymph before blood. [6], [42]

**26. What are important products of triglyceride digestion?**

**Answer: Fatty acids and monoacylglycerols.** Digestion separates components that intestinal cells can absorb and reassemble. [6]

**27. How do enzymes affect digestion?**

**Answer: They catalyze the breakdown of food molecules.** Digestive enzymes accelerate reactions that produce smaller absorbable components. [5]

**28. Which organ makes bile?**

**Answer: The liver.** The liver produces bile; the gallbladder stores and releases it. [5]

**29. After amino acids are absorbed, must all of them become muscle?**

**Answer: No; they have several structural and metabolic fates.** Amino acids supply many proteins and other compounds, and can also be oxidized. [22]

**30. What makes the statement 'all dietary fat goes through lymph' too broad?**

**Answer: Transport differs with the form and chain length of the lipid.** The chylomicron route is especially important for long-chain fat; shorter fatty acids can enter portal blood. [6]

### Level 4 · The fiber world

Separate three overlapping properties.

**31. Which property asks how a fiber behaves in water?**

**Answer: Solubility.** Solubility concerns dissolution in water, a different question from microbial breakdown. [65]

**32. What does viscosity describe here?**

**Answer: How much a fiber thickens the surrounding fluid.** Viscosity depends on the fiber and the conditions, including its form and concentration. [64], [66]

**33. What is fermentability?**

**Answer: How readily gut microbes break a material down.** Microbial use is its own property; solubility does not determine it by itself. [64], [65]

**34. Which example is generally soluble, low-viscosity and readily fermented?**

**Answer: Inulin / fructans.** Inulin illustrates why soluble fiber need not form a thick gel. [64], [67]

**35. Which example forms a viscous gel but is relatively little fermented?**

**Answer: Psyllium husk.** Psyllium shows that thickening and fermentation can vary independently. [64]

**36. Which description best fits cellulose?**

**Answer: Insoluble and generally only partly or poorly fermented.** Cellulose contributes structure to plant cell walls and resists human digestive enzymes. [7], [64]

**37. Why can resistant starch belong in the fiber discussion?**

**Answer: It escapes digestion in the small intestine.** Starch describes structure; resistance describes digestion. The categories can overlap. [65]

**38. Which substance in common dietary-fiber definitions is not a carbohydrate?**

**Answer: Lignin.** Lignin is associated with plant cell walls but is chemically distinct from carbohydrate polymers. [65]

**39. What can microbial fermentation of fiber produce?**

**Answer: Short-chain fatty acids and gases.** Products such as acetate, propionate and butyrate can enter host metabolism. [8], [68]

**40. Why can two oat products have different beta-glucan viscosity?**

**Answer: Processing can change chain size and extractability.** The same fiber name does not specify its behavior in every processed food. [66]

### Level 5 · Energy language

Tell matter, energy and carriers apart.

**41. Which process is an example of anabolism?**

**Answer: Building a protein.** Anabolism assembles molecules; catabolism breaks them down. [18]

**42. Can building and breakdown reactions occur at the same time?**

**Answer: Yes, across the body's metabolic network.** A living body can synthesize one molecule while breaking down another. [18], [39]

**43. What happens to atoms during ordinary metabolic reactions?**

**Answer: They are rearranged into other molecules.** Following matter means accounting for atoms in substrates and products. [1], [13]

**44. What does NADH mainly carry between reactions?**

**Answer: Reducing equivalents, including electrons.** NADH links oxidation reactions to electron transfer; it is not the carbon-output ledger. [13], [14]

**45. What is true of breaking a chemical bond by itself?**

**Answer: It requires energy.** The net free-energy change depends on the entire reaction, including the products formed. [4]

**46. What distinguishes fat mobilization from fat oxidation?**

**Answer: Release from storage does not guarantee oxidation.** Released fatty acids may be used, transported or stored again. [17]

**47. Food's approximate 4, 4 and 9 kcal/g factors refer to what?**

**Answer: Average energy factors for carbohydrate, protein and fat.** Food-energy values are not molecular ATP yields. [3], [31]

**48. Which statement best describes ATP in cells?**

**Answer: It is continually used and regenerated.** The working ATP pool turns over rapidly while larger fuel stores supply energy. [4], [13]

**49. Which is the final electron acceptor in the respiratory chain?**

**Answer: Oxygen.** Oxygen accepts electrons at the end of the chain and is reduced to water. [14]

**50. Why does heat belong in the energy story?**

**Answer: Some transferred energy is released as heat.** Useful work and heat describe energy transfer; they do not replace the matter balance. [48]

### Level 6 · Inside the pathways

Connect the steps of fuel oxidation.

**51. Where does glycolysis take place?**

**Answer: The cytosol.** Glycolysis is the cytosolic pathway that converts glucose to pyruvate. [10]

**52. What is formed when pyruvate is prepared for entry into the Krebs cycle?**

**Answer: Acetyl-CoA, CO₂ and NADH.** Pyruvate oxidation removes a carbon as CO₂ and transfers the remaining acetyl group to CoA. [12]

**53. Which four-carbon molecule accepts an acetyl group at the start of the cycle?**

**Answer: Oxaloacetate.** The two-carbon acetyl group joins four-carbon oxaloacetate to form citrate. [12]

**54. Why is the citric acid pathway called a cycle?**

**Answer: Its starting acceptor is regenerated.** A cycle describes the reaction sequence, not the arrangement of enzymes in space. [12]

**55. Which statement about oxygen and the Krebs cycle is most accurate?**

**Answer: The cycle does not directly consume O₂, but depends on downstream carrier reoxidation.** The respiratory chain reoxidizes carriers and ultimately uses oxygen, linking the pathways. [12], [14]

**56. What powers ATP synthase during oxidative phosphorylation?**

**Answer: A proton electrochemical gradient.** Proton flow can drive rotation coupled to ATP formation. [13], [14]

**57. Where is the mitochondrial respiratory chain located?**

**Answer: The inner mitochondrial membrane.** Its membrane organization helps establish the gradient used for ATP synthesis. [14]

**58. What is regenerated when pyruvate is converted to lactate?**

**Answer: NAD⁺.** LDH transfers reducing equivalents from NADH to pyruvate, allowing glycolysis to keep using NAD⁺. [60]

**59. What does beta-oxidation usually remove from an even-chain fatty acyl group each round?**

**Answer: A two-carbon acetyl unit.** The chain is shortened through repeated reactions that yield acetyl-CoA and reducing equivalents. [50]

**60. What helps long-chain fatty acyl groups enter the mitochondrial matrix?**

**Answer: The carnitine shuttle.** Long-chain fatty-acid oxidation requires activation and transport before matrix beta-oxidation. [50]

### Level 7 · The supporting cast

Connect nutrients to their jobs.

**61. Which vitamin contributes to FAD and FMN?**

**Answer: Riboflavin (B2).** Riboflavin forms the flavin cofactors used by many redox enzymes. [24]

**62. Which vitamin contributes to NAD and NADP?**

**Answer: Niacin (B3).** Niacin contributes to these electron-transfer cofactors; their names are not interchangeable with FAD. [25]

**63. Which vitamin is a component of coenzyme A?**

**Answer: Pantothenic acid (B5).** Coenzyme A carries acyl groups, connecting vitamin chemistry to acetyl-CoA and other pathways. [26]

**64. Which vitamin's coenzyme forms are especially involved in amino-acid metabolism?**

**Answer: Vitamin B6.** Vitamin B6 coenzymes participate in many reactions, including amino-acid transformations. [27]

**65. Which set contains the four fat-soluble vitamins?**

**Answer: A, D, E and K.** Fat solubility groups these four vitamins; the B vitamins and vitamin C are water-soluble. [31]

**66. What distinguishes a major dietary mineral from a trace mineral?**

**Answer: The amount required.** Trace does not mean optional: some essential minerals are required in small quantities. [31], [36]

**67. Why do sodium, potassium and chloride appear in electrolyte discussions?**

**Answer: Their ions carry charge in body fluids.** Dissolved ions participate in fluid balance and electrical processes. [31]

**68. What is true of choline?**

**Answer: The body makes some, but a dietary contribution is still needed.** Some synthesis does not guarantee that the full requirement is met. [37]

**69. Which pair identifies the two essential fatty acids discussed in the article?**

**Answer: Linoleic acid and alpha-linolenic acid.** LA and ALA are the essential omega-6 and omega-3 fatty acids, respectively. [16], [36]

**70. How many amino acids are indispensable in adult human nutrition?**

**Answer: Nine.** The other standard amino acids can usually be synthesized, with qualifications in some physiological settings. [3]

### Level 8 · A body in context

Think across tissues and situations.

**71. Which glycogen store directly helps maintain blood glucose?**

**Answer: Liver glycogen.** Liver and muscle glycogen have different roles; muscle uses its store locally. [9]

**72. Why do mature red blood cells rely on glycolysis for ATP?**

**Answer: They have no mitochondria.** Without mitochondria, they cannot use mitochondrial oxidative phosphorylation. [10], [13]

**73. Which statement about the brain's fuels is most accurate?**

**Answer: Glucose is important, and lactate or ketones can also contribute.** Fuel use depends on availability and conditions; human studies demonstrate use beyond glucose alone. [11], [40]

**74. Where is much amino-acid nitrogen converted into urea?**

**Answer: The liver.** Urea production handles nitrogen; the kidneys then excrete much of it in urine. [22]

**75. Does insulin act as the glucose-entry key for every cell?**

**Answer: No; glucose transport mechanisms differ across tissues.** Insulin supports GLUT4-mediated uptake in muscle and adipose tissue, not every transport route. [43]

**76. What does UCP1 allow brown or beige fat mitochondria to do?**

**Answer: Dissipate part of the proton gradient as heat.** Uncoupling can direct energy toward thermogenesis rather than ATP synthesis. [47]

**77. After a mixed meal, which description is reasonable?**

**Answer: Use, building and storage pathways can all be active.** Fed-state changes alter pathway rates; they do not reduce the body to one exclusive pathway. [39]

**78. Can lactate be produced when oxygen is present?**

**Answer: Yes.** Lactate formation is linked to redox balance and flux, not an absolute on/off oxygen rule. [60], [62]

**79. What happens to much of the CO₂ carried in blood?**

**Answer: It is temporarily converted to bicarbonate.** Bicarbonate is a major transport form; it can be converted back to CO₂ at the lungs. [2]

**80. When amino acids are being oxidized, must all other fuels already be depleted?**

**Answer: No; fuel pathways overlap.** Amino-acid metabolism takes place alongside carbohydrate and lipid metabolism. [18], [22]

### Level 9 · Keep the accounts

Track carbon and pathway outputs.

**81. One six-carbon glucose becomes two pyruvates. How many carbons are in each pyruvate?**

**Answer: Three.** The split preserves all six carbons: 2 × 3 = 6. [10]

**82. For one free glucose, glycolysis spends 2 ATP and makes 4. What is the net gain?**

**Answer: 2 ATP.** Net gain subtracts the investment from gross production. [10]

**83. How many CO₂ molecules are directly released during glycolysis?**

**Answer: Zero.** Glycolysis retains the six glucose carbons in its two pyruvate products. [10]

**84. Both pyruvates from one glucose become acetyl-CoA. How many CO₂ are released in that preparation step?**

**Answer: Two.** Each three-carbon pyruvate yields one CO₂ and one two-carbon acetyl group. [12]

**85. A two-carbon acetyl group joins four-carbon oxaloacetate. How many carbons does citrate have?**

**Answer: Six.** The initial condensation combines the two carbon groups: 2 + 4 = 6. [12]

**86. How many NADH does one complete turn of the Krebs cycle produce?**

**Answer: Three.** The conventional per-turn ledger includes three NADH, one flavin-linked reducing equivalent and one GTP/ATP. [12], [13]

**87. For two turns of the cycle, how many direct GTP/ATP equivalents are made?**

**Answer: Two.** There is one substrate-level GTP/ATP equivalent per turn; downstream oxidative ATP is a separate count. [12], [13]

**88. In the article's mass example, 10 kg fat + 29 kg oxygen gives how much total product mass?**

**Answer: About 39 kg.** The rounded products are about 28 kg CO₂ plus 11 kg water. Inhaled oxygen contributes mass. [1]

**89. In that simplified example, about how much of the original 10 kg of fat contributes to CO₂ mass?**

**Answer: 8.4 kg.** The 8.4 kg tracks original fat mass only; the larger total CO₂ mass includes oxygen from outside the fat. [1]

**90. After complete oxidation of one glucose, what is its net carbon contribution to CO₂?**

**Answer: Six carbon atoms in six CO₂ molecules.** Complete oxidation ultimately accounts for all six carbons. This is not a claim about their timing in the first cycle turn. [13]

### Level 10 · Connect the whole map

Apply the distinctions to new situations.

**91. A fatty acid leaves adipose tissue and is later re-esterified. What can you conclude?**

**Answer: Mobilization occurred, but that molecule was stored again.** Release is one step. Subsequent storage means release alone cannot measure oxidation. [17]

**92. A meal raises carbohydrate oxidation and lowers fat oxidation. Must every extra carbohydrate gram have become fat?**

**Answer: No; changes in fuel use can also alter fat storage balance.** Direct fatty-acid synthesis and a shift in the fuels being oxidized are distinct contributions. [19], [20]

**93. A student says newly arriving acetyl carbons always leave in the cycle's first turn. What is missing?**

**Answer: Carbon counts do not identify which individual atoms leave.** Incoming acetyl carbons are generally retained in the first turn. A net ledger is not an atom-tracing experiment. [12]

**94. The glycolysis toy runs short of NAD⁺. What does making lactate directly help restore?**

**Answer: The oxidized carrier needed by glycolysis.** Reducing pyruvate to lactate oxidizes NADH back to NAD⁺; it does not reassemble glucose. [60]

**95. An illustration shows all nutrient carbon going straight to exhaled CO₂. What qualification is needed?**

**Answer: It shows one possible fate and omits storage, building and intermediate steps.** Metabolic pathways branch. A conceptual route is useful only when its scope is made clear. [18]

**96. A label says soluble fiber. Can that alone predict viscosity and fermentation?**

**Answer: No; the fiber's other properties and context are needed.** Inulin and psyllium illustrate different combinations within broad solubility categories. [64], [65]

**97. The lungs excrete CO₂. Does deliberately breathing faster therefore guarantee faster fat loss?**

**Answer: No; ventilation does not set fat oxidation by itself.** Exhalation is an exit after metabolism. Changing ventilation is not a direct command to oxidize stored fat. [1], [2], [41]

**98. Why can't common even-chain fatty acids provide net glucose through acetyl-CoA in the usual human pathway?**

**Answer: Acetyl-CoA entry does not create a net gain of gluconeogenic cycle carbon.** The pathway's carbon balance prevents that net conversion; glycerol and odd-chain fatty acids require separate discussion. [57]

**99. A nutrient participates in ATP-related chemistry. Does an extra supplement necessarily increase energy output?**

**Answer: No; being required does not mean extra supply is the limiting factor.** A biochemical role is not evidence that supplementation will improve a person's performance. [24], [25], [26]

**100. A protein example has 20 residue positions but repeats some amino acids. What does that imply?**

**Answer: It can contain fewer than 20 different amino-acid types.** Sequence length counts positions; the number of distinct amino-acid types is a different count. [21]

---

<a id="sources"></a>

## 13 · Sources and field notes

This primer grew from Shreyam Adhikari's two-year-old notes and Dr. Andy Galpin's physiology lessons. Galpin supplied an important teaching framework; this is an independent article, and he has not reviewed or endorsed it.

### The original notes and Galpin lessons

[Read the original Google Doc](https://docs.google.com/document/d/1Gm1ZX6PRb_QBfdjxeoIPJYOlRZJazi30kODUNmqtGpA/edit?usp=sharing).

- [The Physiology of Fat Loss](https://www.youtube.com/watch?v=GOdSilh7sxU)
- [Energy for Exercise, How We Make It](https://www.youtube.com/watch?v=L-hO3937Cv8)
- [New Science of Muscle Hypertrophy · Part 1: Physiology](https://www.youtube.com/watch?v=MyKrc-fheBw)
- [New Science of Muscle Hypertrophy · Part 2: Stimuli](https://www.youtube.com/watch?v=-FR5CQhsDg4)
- [New Science of Muscle Hypertrophy · Part 3: Eating & Training](https://www.youtube.com/watch?v=cw6XPWaEK20)
- [Every Macro and Micronutrient · 5 Min Phys](https://www.youtube.com/watch?v=E3lis9G8zoU)
- [“Nutrients”, What Does That Mean? · 25 Min Phys](https://www.youtube.com/watch?v=-5CN6UzYnaw)
- [What Lactate Is & What It Actually Does · 5 Min Phys](https://www.youtube.com/watch?v=QtOcIZqkieA)
- [What Lactate Is & What It Actually Does · 25 Min Phys](https://www.youtube.com/watch?v=imEsdKxtVyU)

The video titles sometimes describe a series format rather than the precise running time. The explanations here were checked against sources below; recovered automatic captions were used for source review, without inventing direct quotations from uncertain captions.

### About this edition

Made from two-year-old notes by Shreyam Adhikari, originally written around **2024** and kept in a personal Google Docs archive. Recovered on **9 September 2026**; last refined on **11 September 2026**. The original notes remain linked above.

The archive also touches sleep, circadian rhythms, immune-cell metabolism, mitochondrial maintenance and inherited pathway defects. This reader preserves the foundational metabolism explanation. A mechanism is not automatically a treatment; unsupported supplement stacks, fasting prescriptions, diagnostic cutoffs and longevity promises are outside this edition.

Molecular examples are drawn from the RCSB Protein Data Bank records linked in their chapters. Glucose and tripalmitin coordinates used by the companion site are computed ideal coordinates, while its complete protein structure follows model 1 from a solution-NMR ensemble. The interactive molecular views use [3Dmol.js](https://3dmol.org/) (BSD-3-Clause); source-rendered still images remain available without WebGL. Conceptual artwork is distinct from molecular data. The standalone reader does not require the site's controls to follow the chemistry.

### Scientific references

The numbered links support the nearby claims. Textbooks establish pathway foundations; official nutrient references support specific roles and classification details; original experiments support particular observations under their studied conditions. A small study's result is not a universal numerical prescription. Additional direct sources appear where the food-family and molecule descriptions are introduced.

1. [Meerman & Brown, BMJ, 16 Dec 2014](https://www.bmj.com/content/349/bmj.g7257)
2. [OpenStax, Anatomy & Physiology 2e §22.5, 20 Apr 2022](https://openstax.org/books/anatomy-and-physiology-2e/pages/22-5-transport-of-gases)
3. [OpenStax, Biology 2e §34.2, 28 Mar 2018](https://openstax.org/books/biology-2e/pages/34-2-nutrition-and-energy-production)
4. [OpenStax, Biology 2e §6.4, 28 Mar 2018](https://openstax.org/books/biology-2e/pages/6-4-atp-adenosine-triphosphate)
5. [NIDDK, Your Digestive System & How It Works](https://www.niddk.nih.gov/health-information/digestive-diseases/digestive-system-how-it-works)
6. [OpenStax, Anatomy & Physiology 2e §23.7, 20 Apr 2022](https://openstax.org/books/anatomy-and-physiology-2e/pages/23-7-chemical-digestion-and-absorption-a-closer-look)
7. [OpenStax, Biology 2e §3.2, 28 Mar 2018](https://openstax.org/books/biology-2e/pages/3-2-carbohydrates)
8. [Boets et al., J Physiol, 2017; online 2016](https://pubmed.ncbi.nlm.nih.gov/27510655/)
9. [OpenStax, Anatomy & Physiology 2e §24.2, 20 Apr 2022](https://openstax.org/books/anatomy-and-physiology-2e/pages/24-2-carbohydrate-metabolism)
10. [OpenStax, Biology 2e §7.2, 28 Mar 2018](https://openstax.org/books/biology-2e/pages/7-2-glycolysis)
11. [van Hall et al., J Cereb Blood Flow Metab, Jun 2009](https://pubmed.ncbi.nlm.nih.gov/19337275/)
12. [OpenStax, Biology 2e §7.3, 28 Mar 2018](https://openstax.org/books/biology-2e/pages/7-3-oxidation-of-pyruvate-and-the-citric-acid-cycle)
13. [Alberts et al., Molecular Biology of the Cell, 4e, 2002](https://www.ncbi.nlm.nih.gov/books/NBK26882/)
14. [OpenStax, Biology 2e §7.4, 28 Mar 2018](https://openstax.org/books/biology-2e/pages/7-4-oxidative-phosphorylation)
15. [OpenStax, Biology 2e §3.3, 28 Mar 2018](https://openstax.org/books/biology-2e/pages/3-3-lipids)
16. [NIH ODS, Omega-3 Fatty Acids](https://ods.od.nih.gov/factsheets/Omega3FattyAcids-HealthProfessional/)
17. [OpenStax, Anatomy & Physiology 2e §24.3, 20 Apr 2022](https://openstax.org/books/anatomy-and-physiology-2e/pages/24-3-lipid-metabolism)
18. [OpenStax, Biology 2e §7.6, 28 Mar 2018](https://openstax.org/books/biology-2e/pages/7-6-connections-of-carbohydrate-protein-and-lipid-metabolic-pathways)
19. [Schwarz et al., JCI, 1 Dec 1995](https://www.jci.org/articles/view/118342)
20. [Hudgins et al., JCI, 1 May 1996](https://www.jci.org/articles/view/118645)
21. [OpenStax, Biology 2e §3.4, 28 Mar 2018](https://openstax.org/books/biology-2e/pages/3-4-proteins)
22. [OpenStax, Anatomy & Physiology 2e §24.4, 20 Apr 2022](https://openstax.org/books/anatomy-and-physiology-2e/pages/24-4-protein-metabolism)
23. [NIH ODS, Thiamin](https://ods.od.nih.gov/factsheets/Thiamin-HealthProfessional/)
24. [NIH ODS, Riboflavin](https://ods.od.nih.gov/factsheets/Riboflavin-HealthProfessional/)
25. [NIH ODS, Niacin](https://ods.od.nih.gov/factsheets/Niacin-HealthProfessional/)
26. [NIH ODS, Pantothenic Acid](https://ods.od.nih.gov/factsheets/PantothenicAcid-HealthProfessional/)
27. [NIH ODS, Vitamin B6](https://ods.od.nih.gov/factsheets/VitaminB6-HealthProfessional/)
28. [NIH ODS, Biotin](https://ods.od.nih.gov/factsheets/Biotin-HealthProfessional/)
29. [NIH ODS, Folate](https://ods.od.nih.gov/factsheets/Folate-HealthProfessional/)
30. [NIH ODS, Vitamin B12](https://ods.od.nih.gov/factsheets/VitaminB12-HealthProfessional/)
31. [OpenStax, Anatomy & Physiology §24.7, 25 Apr 2013](https://openstax.org/books/anatomy-and-physiology/pages/24-7-nutrition-and-diet)
32. [NIH ODS, Magnesium](https://ods.od.nih.gov/factsheets/Magnesium-HealthProfessional/)
33. [NIH ODS, Phosphorus](https://ods.od.nih.gov/factsheets/Phosphorus-HealthProfessional/)
34. [NIH ODS, Manganese](https://ods.od.nih.gov/factsheets/Manganese-HealthProfessional/)
35. [NIH ODS, Molybdenum](https://ods.od.nih.gov/factsheets/Molybdenum-HealthProfessional/)
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37. [NIH ODS, Choline](https://ods.od.nih.gov/factsheets/Choline-HealthProfessional/)
38. [NIH ODS, Chromium](https://ods.od.nih.gov/factsheets/chromium/HealthProfessional/)
39. [OpenStax, Anatomy & Physiology 2e §24.5, 20 Apr 2022](https://openstax.org/books/anatomy-and-physiology-2e/pages/24-5-metabolic-states-of-the-body)
40. [Owen et al., JCI, Oct 1967](https://pubmed.ncbi.nlm.nih.gov/6061736/)
41. [OpenStax, Anatomy & Physiology 2e §26.4, 20 Apr 2022](https://openstax.org/books/anatomy-and-physiology-2e/pages/26-4-acid-base-balance)
42. [Endotext, Introduction to Lipids and Lipoproteins](https://www.ncbi.nlm.nih.gov/books/NBK305896/)
43. [Bradley et al., Physiological Reports, 2015](https://pmc.ncbi.nlm.nih.gov/articles/PMC4463815/)
44. [OpenStax, Anatomy & Physiology 2e §10.3, 20 Apr 2022](https://openstax.org/books/anatomy-and-physiology-2e/pages/10-3-muscle-fiber-contraction-and-relaxation)
45. [Romijn et al., Am J Physiol, Sep 1993](https://pubmed.ncbi.nlm.nih.gov/8214047/)
46. [Cypess et al., NEJM, 9 Apr 2009](https://pubmed.ncbi.nlm.nih.gov/19357406/)
47. [Human Brown Adipose Tissue Plasticity, NCBI Bookshelf, 2018](https://www.ncbi.nlm.nih.gov/books/NBK543783/)
48. [OpenStax, Anatomy & Physiology 2e §24.6, 20 Apr 2022](https://openstax.org/books/anatomy-and-physiology-2e/pages/24-6-energy-and-heat-balance)
49. [Pontzer et al., Science, 13 Aug 2021](https://pubmed.ncbi.nlm.nih.gov/34385400/)
50. [StatPearls, Biochemistry: Fatty Acid Oxidation](https://www.ncbi.nlm.nih.gov/books/NBK556002/)
51. [Reactome, ACLY tetramer transforms citrate to acetyl-CoA](https://reactome.org/content/detail/R-HSA-75848)
52. [Reactome, Conversion of malonyl-CoA and acetyl-CoA to palmitate](https://reactome.org/content/detail/R-HSA-75872)
53. [Lund et al., PNAS, 20 Jun 1995](https://pubmed.ncbi.nlm.nih.gov/7597034/)
54. [Zhang et al., PNAS, 31 Oct 2006](https://pubmed.ncbi.nlm.nih.gov/17050691/)
55. [Stumvoll et al., JCI, Nov 1995](https://pubmed.ncbi.nlm.nih.gov/7593645/)
56. [Cesbron-Lavau et al., Nutrients, 26 Jan 2021](https://pubmed.ncbi.nlm.nih.gov/33530525/)
57. [StatPearls, Biochemistry: Gluconeogenesis](https://www.ncbi.nlm.nih.gov/books/NBK544346/)
58. [Dr. Andy Galpin, New Science of Muscle Hypertrophy, Part 2: Stimuli](https://www.youtube.com/watch?v=-FR5CQhsDg4)
59. [NIH ODS, Fluoride](https://ods.od.nih.gov/factsheets/Fluoride-HealthProfessional/)
60. [Reactome: LDH tetramer reduces PYR to LACT](https://reactome.org/content/detail/R-HSA-71849)
61. [Reactome: LDH tetramer oxidises LACT to PYR](https://reactome.org/content/detail/R-HSA-70510)
62. [Richardson et al.: Lactate efflux from exercising human skeletal muscle, 1998](https://pubmed.ncbi.nlm.nih.gov/9688741/)
63. [Pollak et al.: Metabolite combinations and fatigue/pain sensations, 2014](https://pubmed.ncbi.nlm.nih.gov/24142455/)

[1]: https://www.bmj.com/content/349/bmj.g7257
[2]: https://openstax.org/books/anatomy-and-physiology-2e/pages/22-5-transport-of-gases
[3]: https://openstax.org/books/biology-2e/pages/34-2-nutrition-and-energy-production
[4]: https://openstax.org/books/biology-2e/pages/6-4-atp-adenosine-triphosphate
[5]: https://www.niddk.nih.gov/health-information/digestive-diseases/digestive-system-how-it-works
[6]: https://openstax.org/books/anatomy-and-physiology-2e/pages/23-7-chemical-digestion-and-absorption-a-closer-look
[7]: https://openstax.org/books/biology-2e/pages/3-2-carbohydrates
[8]: https://pubmed.ncbi.nlm.nih.gov/27510655/
[9]: https://openstax.org/books/anatomy-and-physiology-2e/pages/24-2-carbohydrate-metabolism
[10]: https://openstax.org/books/biology-2e/pages/7-2-glycolysis
[11]: https://pubmed.ncbi.nlm.nih.gov/19337275/
[12]: https://openstax.org/books/biology-2e/pages/7-3-oxidation-of-pyruvate-and-the-citric-acid-cycle
[13]: https://www.ncbi.nlm.nih.gov/books/NBK26882/
[14]: https://openstax.org/books/biology-2e/pages/7-4-oxidative-phosphorylation
[15]: https://openstax.org/books/biology-2e/pages/3-3-lipids
[16]: https://ods.od.nih.gov/factsheets/Omega3FattyAcids-HealthProfessional/
[17]: https://openstax.org/books/anatomy-and-physiology-2e/pages/24-3-lipid-metabolism
[18]: https://openstax.org/books/biology-2e/pages/7-6-connections-of-carbohydrate-protein-and-lipid-metabolic-pathways
[19]: https://www.jci.org/articles/view/118342
[20]: https://www.jci.org/articles/view/118645
[21]: https://openstax.org/books/biology-2e/pages/3-4-proteins
[22]: https://openstax.org/books/anatomy-and-physiology-2e/pages/24-4-protein-metabolism
[23]: https://ods.od.nih.gov/factsheets/Thiamin-HealthProfessional/
[24]: https://ods.od.nih.gov/factsheets/Riboflavin-HealthProfessional/
[25]: https://ods.od.nih.gov/factsheets/Niacin-HealthProfessional/
[26]: https://ods.od.nih.gov/factsheets/PantothenicAcid-HealthProfessional/
[27]: https://ods.od.nih.gov/factsheets/VitaminB6-HealthProfessional/
[28]: https://ods.od.nih.gov/factsheets/Biotin-HealthProfessional/
[29]: https://ods.od.nih.gov/factsheets/Folate-HealthProfessional/
[30]: https://ods.od.nih.gov/factsheets/VitaminB12-HealthProfessional/
[31]: https://openstax.org/books/anatomy-and-physiology/pages/24-7-nutrition-and-diet
[32]: https://ods.od.nih.gov/factsheets/Magnesium-HealthProfessional/
[33]: https://ods.od.nih.gov/factsheets/Phosphorus-HealthProfessional/
[34]: https://ods.od.nih.gov/factsheets/Manganese-HealthProfessional/
[35]: https://ods.od.nih.gov/factsheets/Molybdenum-HealthProfessional/
[36]: https://openstax.org/books/biology-2e/pages/34-2-nutrition-and-energy-production
[37]: https://ods.od.nih.gov/factsheets/Choline-HealthProfessional/
[38]: https://ods.od.nih.gov/factsheets/chromium/HealthProfessional/
[39]: https://openstax.org/books/anatomy-and-physiology-2e/pages/24-5-metabolic-states-of-the-body
[40]: https://pubmed.ncbi.nlm.nih.gov/6061736/
[41]: https://openstax.org/books/anatomy-and-physiology-2e/pages/26-4-acid-base-balance
[42]: https://www.ncbi.nlm.nih.gov/books/NBK305896/
[43]: https://pmc.ncbi.nlm.nih.gov/articles/PMC4463815/
[44]: https://openstax.org/books/anatomy-and-physiology-2e/pages/10-3-muscle-fiber-contraction-and-relaxation
[45]: https://pubmed.ncbi.nlm.nih.gov/8214047/
[46]: https://pubmed.ncbi.nlm.nih.gov/19357406/
[47]: https://www.ncbi.nlm.nih.gov/books/NBK543783/
[48]: https://openstax.org/books/anatomy-and-physiology-2e/pages/24-6-energy-and-heat-balance
[49]: https://pubmed.ncbi.nlm.nih.gov/34385400/
[50]: https://www.ncbi.nlm.nih.gov/books/NBK556002/
[51]: https://reactome.org/content/detail/R-HSA-75848
[52]: https://reactome.org/content/detail/R-HSA-75872
[53]: https://pubmed.ncbi.nlm.nih.gov/7597034/
[54]: https://pubmed.ncbi.nlm.nih.gov/17050691/
[55]: https://pubmed.ncbi.nlm.nih.gov/7593645/
[56]: https://pubmed.ncbi.nlm.nih.gov/33530525/
[57]: https://www.ncbi.nlm.nih.gov/books/NBK544346/
[58]: https://www.youtube.com/watch?v=-FR5CQhsDg4
[59]: https://ods.od.nih.gov/factsheets/Fluoride-HealthProfessional/
[60]: https://reactome.org/content/detail/R-HSA-71849
[61]: https://reactome.org/content/detail/R-HSA-70510
[62]: https://pubmed.ncbi.nlm.nih.gov/9688741/
[63]: https://pubmed.ncbi.nlm.nih.gov/24142455/

First published as an initial draft in [Shreyam’s log](https://shreyam1008.com.np/log/metabolism-101/) on **9 September 2026**. The underlying notes date to circa 2024.

[64]: https://www.ncbi.nlm.nih.gov/books/NBK619141/table/ch2.tab2/?report=objectonly

[65]: https://www.ncbi.nlm.nih.gov/books/NBK223591/

[66]: https://pubmed.ncbi.nlm.nih.gov/20527967/

[67]: https://pubmed.ncbi.nlm.nih.gov/20482283/

[68]: https://www.ncbi.nlm.nih.gov/books/NBK223590/

[69]: https://www.ncbi.nlm.nih.gov/books/NBK26829/
[70]: https://pubchem.ncbi.nlm.nih.gov/compound/Cellobiose
[71]: https://www.ncbi.nlm.nih.gov/books/NBK26883/

[72] [Baker et al., Interaction among Skeletal Muscle Metabolic Energy Systems during Intense Exercise, 2010](https://pmc.ncbi.nlm.nih.gov/articles/PMC3005844/). Simultaneous ATP systems, phosphocreatine buffering and replenishment; no exclusive sequential fuel switches.

[73] [Hargreaves & Spriet, Skeletal muscle energy metabolism during exercise, 2020](https://www.nature.com/articles/s42255-020-0251-4). Muscle carbohydrate and lipid sources and changing contributions with effort; qualitative explanations, no invented percentages. Author correction linked by publisher.

[74] [Endotext: Adipose Tissue — Physiology to Metabolic Dysfunction](https://www.ncbi.nlm.nih.gov/books/NBK555602/). Subcutaneous and visceral adipose, triglyceride turnover and regulation. No individual prediction of storage location.

[75] [NIH ODS: Iron](https://ods.od.nih.gov/factsheets/Iron-HealthProfessional/). Iron absorption and ferritin stores.

[76] [Intestinal lipid absorption and lipoprotein formation, 2014](https://pubmed.ncbi.nlm.nih.gov/24751933/). Intestinal lipid handling, chylomicrons and dependence of transport on lipid properties.

[77] [NIH ODS: Calcium](https://ods.od.nih.gov/factsheets/Calcium-HealthProfessional/). Bone stores and intestinal calcium absorption.

[78] [Williamson & Clifford, Role of the small intestine, colon and microbiota in determining the metabolic fate of polyphenols, 2017](https://pubmed.ncbi.nlm.nih.gov/28322745/). Different routes and intestinal, hepatic and microbial transformations; no promised health benefit.

[79] [Biochemistry, Lipolysis — NCBI Bookshelf](https://www.ncbi.nlm.nih.gov/books/NBK560564/). Triglyceride lipolysis and transport of released fatty acids bound to albumin.

[80] [Jensen et al., Effects of body fat distribution on regional lipolysis in obesity, 1991](https://www.jci.org/articles/view/115345). Human tracer evidence that regional contributions differ; does not predict an individual exercise session or spot reduction.

[81] [Pollak et al., Exogenously Applied Muscle Metabolites Synergistically Evoke Sensations of Muscle Fatigue and Pain in Human Subjects, 2014](https://pmc.ncbi.nlm.nih.gov/articles/PMC3946674/). Small human infusion study: mixtures of protons, ATP and lactate evoked sensations; individual metabolites did not. Not a complete account of exercise fatigue.

[82] [Alberts et al.: The Shape and Structure of Proteins, 2002](https://www.ncbi.nlm.nih.gov/books/NBK26830/). Sequence, side-group interactions, folding, chaperones and levels of protein organization.

[82]: https://www.ncbi.nlm.nih.gov/books/NBK26830/


[72] [Baker et al., Interaction among Skeletal Muscle Metabolic Energy Systems during Intense Exercise, 2010](https://pmc.ncbi.nlm.nih.gov/articles/PMC3005844/). Simultaneous ATP systems, phosphocreatine buffering and replenishment; no exclusive sequential fuel switches.

[72]: https://pmc.ncbi.nlm.nih.gov/articles/PMC3005844/


[73] [Hargreaves & Spriet, Skeletal muscle energy metabolism during exercise, 2020](https://www.nature.com/articles/s42255-020-0251-4). Muscle carbohydrate and lipid sources and changing contributions with effort; qualitative explanations, no invented percentages. Author correction linked by publisher.

[73]: https://www.nature.com/articles/s42255-020-0251-4


[74] [Endotext: Adipose Tissue — Physiology to Metabolic Dysfunction](https://www.ncbi.nlm.nih.gov/books/NBK555602/). Subcutaneous and visceral adipose, triglyceride turnover and regulation. No individual prediction of storage location.

[74]: https://www.ncbi.nlm.nih.gov/books/NBK555602/


[75] [NIH ODS: Iron](https://ods.od.nih.gov/factsheets/Iron-HealthProfessional/). Iron absorption and ferritin stores.

[75]: https://ods.od.nih.gov/factsheets/Iron-HealthProfessional/


[76] [Intestinal lipid absorption and lipoprotein formation, 2014](https://pubmed.ncbi.nlm.nih.gov/24751933/). Intestinal lipid handling, chylomicrons and dependence of transport on lipid properties.

[76]: https://pubmed.ncbi.nlm.nih.gov/24751933/


[77] [NIH ODS: Calcium](https://ods.od.nih.gov/factsheets/Calcium-HealthProfessional/). Bone stores and intestinal calcium absorption.

[77]: https://ods.od.nih.gov/factsheets/Calcium-HealthProfessional/


[78] [Williamson & Clifford, Role of the small intestine, colon and microbiota in determining the metabolic fate of polyphenols, 2017](https://pubmed.ncbi.nlm.nih.gov/28322745/). Different routes and intestinal, hepatic and microbial transformations; no promised health benefit.

[78]: https://pubmed.ncbi.nlm.nih.gov/28322745/


[79] [Biochemistry, Lipolysis — NCBI Bookshelf](https://www.ncbi.nlm.nih.gov/books/NBK560564/). Triglyceride lipolysis and transport of released fatty acids bound to albumin.

[79]: https://www.ncbi.nlm.nih.gov/books/NBK560564/


[80] [Jensen et al., Effects of body fat distribution on regional lipolysis in obesity, 1991](https://www.jci.org/articles/view/115345). Human tracer evidence that regional contributions differ; does not predict an individual exercise session or spot reduction.

[80]: https://www.jci.org/articles/view/115345


[81] [Pollak et al., Exogenously Applied Muscle Metabolites Synergistically Evoke Sensations of Muscle Fatigue and Pain in Human Subjects, 2014](https://pmc.ncbi.nlm.nih.gov/articles/PMC3946674/). Small human infusion study: mixtures of protons, ATP and lactate evoked sensations; individual metabolites did not. Not a complete account of exercise fatigue.

[81]: https://pmc.ncbi.nlm.nih.gov/articles/PMC3946674/


[83] [Gertz et al., Myocardial substrate utilization during exercise in humans, 1988](https://www.jci.org/articles/view/113822). Human isotope study of myocardial substrate use at rest and moderate exercise; supports cardiac lactate oxidation, not universal fuel percentages.

[83]: https://www.jci.org/articles/view/113822