The Molecules of Life

TL;DR. Six classes of molecule do everything. Water is the medium. Proteins are the machines and the structure, built from 20 amino acids of which 9 must come from food. Carbohydrates are the fast fuel. Fats are the dense fuel, the membranes, and the raw material for several hormones. Nucleic acids hold the instructions. Vitamins and minerals are the small parts that let the machines work, needed in milligram quantities and catastrophic when absent. Nearly every nutrition argument you have ever heard is an argument about the middle three, and most of them make more sense once you know what the molecules actually are.

Key takeaways

• A protein is a chain of amino acids folded into a shape, and the shape is the function. Change the shape and you change the job, which is what a mutation does.
• Nine amino acids are essential, meaning your body cannot make them and they must come from food. The other eleven you can build yourself.
• You recycle roughly 250 to 300 grams of protein a day internally, far more than you eat, which is why protein needs are lower than the supplement industry implies and higher than the official minimum for older and active people.
• ATP is the body's energy currency, and you make and spend roughly your own body weight in it every day by recycling the same molecules thousands of times.
• Enzymes are proteins that speed up specific reactions, often by factors of millions. Almost every drug that ends in "-inhibitor" is blocking one.
• Vitamins are not fuel. They are parts, mostly enzyme helpers, which is why more than enough gives no extra benefit and several are toxic in excess.

Water

In short: About 60 percent of you, and it is the reason biochemistry works at all rather than a passive filler.

Water is the solvent every reaction in your body happens in, and its unusual properties are load-bearing:

• It dissolves charged and polar things (salts, sugars, most proteins) and repels greasy things (fats). That single split is what makes cell membranes possible: a membrane forms spontaneously because fat molecules cluster away from water. Life is organised by what water will and will not mix with.
• It carries an enormous amount of heat per degree, so your temperature moves slowly rather than swinging with every meal or effort. Sweat exploits the same property: evaporating water removes a lot of heat.
• It transports. Blood is mostly water, and so is the fluid between cells, urine, and the contents of the gut.

Water balance is tightly controlled. You lose roughly 2 to 2.5 litres a day through urine, skin, breath, and stool, and you replace it from drinks, food (fruit and vegetables are mostly water), and the water your own metabolism produces. Thirst is triggered by a rise in blood concentration of about 1 to 2 percent, which is sensitive, and it becomes less reliable with age, which is why dehydration is common in older people.

The electrolytes dissolved in that water are as important as the water itself. Sodium, potassium, chloride, calcium, magnesium, and phosphate carry electrical charge, and nerves and muscle work by moving them across membranes. Get them wrong and the consequences are immediate: low sodium causes confusion and seizures, high or low potassium stops the heart.

Don't be confused: drinking more water is not automatically better. Kidneys excrete excess readily, so extra water mostly produces extra urine. Drinking a very large volume quickly can dilute blood sodium dangerously, which has killed people in endurance events and in water drinking contests. The useful rule is to drink to thirst plus a little more in heat, illness, or hard exercise, and to check urine colour rather than counting glasses.

Proteins and amino acids

In short: Chains of amino acids folded into shapes, and they do nearly every active job in the body.

What an amino acid is

An amino acid is a small molecule with three parts: an amino group at one end, an acid group at the other, and a side chain in the middle that makes it different from the other 19. The side chain is everything: it decides whether that position on a protein is greasy or water-loving, positively or negatively charged, bulky or small, reactive or inert.

Amino acids link end to end by peptide bonds. A few linked together is a peptide; dozens to thousands is a protein. GLP-1 (Chapter 66) is 30 amino acids, so it is a peptide. Haemoglobin is 574, so it is a protein.

The twenty, and the nine that matter for your diet

Some of the second column are conditionally essential: during illness, injury, rapid growth, or in premature infants, the body cannot make enough and they must be supplied. Glutamine and arginine are the usual examples.

Protein quality means how well a food's amino acid profile matches what you need. Animal proteins (eggs, dairy, meat, fish) contain all nine essential amino acids in useful proportions. Most single plant foods are low in one or two: grains are low in lysine, legumes are low in methionine. Eating a variety across the day solves this completely, and the old advice to carefully combine proteins within a single meal turned out to be unnecessary, because the body maintains a free amino acid pool that lasts hours.

From chain to shape to job

A protein chain folds, spontaneously and reproducibly, into a specific three-dimensional shape driven mainly by which side chains want to avoid water. That shape creates surfaces and pockets that fit other molecules. The shape is the function.

That is why heat, extreme pH, and some chemicals denature proteins, unfolding them irreversibly: a cooked egg white cannot be uncooked. It is also why a single amino acid substitution can be catastrophic, as in sickle cell disease (Chapter 48), and why misfolding causes disease directly in Alzheimer's, Parkinson's, and the prion diseases.

What proteins actually do

How much protein you need, honestly

The official minimum (RDA) is 0.8 g per kg of body weight per day, which is set as the amount that prevents deficiency in almost everyone, not the amount that is optimal.

The evidence for higher intakes is strongest in two groups:

Two practical points. Distribution matters more than people expect: roughly 25 to 40 g in each of three or four meals stimulates muscle protein synthesis better than the same total eaten mostly at dinner. And you recycle far more protein than you eat: about 250 to 300 g a day is broken down and rebuilt internally, against a dietary intake of perhaps 60 to 100 g. Your body is a recycling operation with a top-up, not a throughput system.

Carbohydrates

In short: The fast fuel, in three sizes, and the only class of nutrient with no essential requirement.

Carbohydrates are sugars, in chains of different lengths:

• Monosaccharides, single sugars: glucose (the universal fuel), fructose (fruit sugar, handled mainly by the liver), galactose.
• Disaccharides, two joined: sucrose (table sugar: glucose plus fructose), lactose (milk sugar: glucose plus galactose, and the one most adults worldwide cannot digest, Chapter 62), maltose.
• Polysaccharides, long chains: starch in plants, glycogen in your liver and muscle, and fibre, which you cannot digest at all.

Glucose is the reference fuel. Every cell can burn it, the brain relies on it almost exclusively under normal conditions (it can switch partly to ketones during fasting), and red blood cells can use nothing else because they have no mitochondria. This is why blood glucose is defended so tightly and why Chapter 18 exists.

Fibre is a carbohydrate you cannot digest, and it matters anyway.

Fermentable fibre is eaten by colon bacteria, which produce short-chain fatty acids that feed the cells lining the colon directly. Most people eat around half the recommended 25 to 30 g a day, and low fibre intake is among the most consistent dietary associations with colorectal cancer, cardiovascular disease, and mortality.

There is no essential carbohydrate. The body can make all the glucose it needs from amino acids and from the glycerol backbone of fats. This is the physiological basis for low-carbohydrate diets being survivable, and it is not an argument that they are superior, only that carbohydrate, uniquely among the three macronutrients, has no minimum requirement.

Fats

In short: Dense fuel, every cell membrane, several hormones, and the transport system for four vitamins.

Fatty acids are chains of carbon atoms with an acid group at one end. Three or so attached to a glycerol backbone make a triglyceride, which is what dietary fat and body fat mostly are.

The classification everyone argues about is just chemistry:

Two fatty acids are essential: linoleic acid (omega-6) and alpha-linolenic acid (omega-3). Your body cannot make either. From them it builds the longer-chain omega-3s EPA and DHA, though the conversion is inefficient, which is why oily fish is the reliable source. DHA is a major structural component of brain and retinal membranes.

Fats do four things beyond fuel:

1. Membranes. Every cell is wrapped in a double layer of phospholipids, fat molecules with a water-loving head and water-fearing tails. Cholesterol sits among them controlling fluidity.
2. Hormone precursor. Cholesterol is the raw material for cortisol, aldosterone, oestrogen, testosterone, and vitamin D. This is why cholesterol is essential and why the goal is controlling the particles that deposit it in artery walls (Chapter 21), not eliminating it.
3. Vitamin transport. Vitamins A, D, E, and K dissolve only in fat, so a fat-free meal absorbs them poorly, and conditions that block fat absorption cause deficiency in all four.
4. Signalling. Prostaglandins and related molecules, built from fatty acids, mediate inflammation, pain, fever, and clotting. This is exactly what aspirin and the other NSAIDs block (Chapter 16).

Nucleic acids

In short: DNA is the archive, RNA is the working copy, and the whole system exists to specify proteins.

DNA is a four-letter code (A, C, G, T) in a double helix, about 3 billion letters long, packed into 46 chromosomes in nearly every cell. Roughly 20,000 stretches of it are genes.

RNA is the single-stranded working copy. Messenger RNA carries a gene's instructions out of the nucleus, transfer RNA brings the matching amino acid, and ribosomal RNA forms part of the machine that links them.

The flow is: DNA to RNA to protein. That is the reason the previous section exists: nearly all of your genetic information is instructions for building proteins, and this book's genetic diseases (Chapter 15, Chapter 49) are all cases of one protein being wrong or missing.

Two practical consequences appear repeatedly. mRNA vaccines (Chapter 65) simply deliver the middle step, letting your own cells make one protein for the immune system to learn. And antibiotics and antivirals frequently work by attacking the bacterial or viral version of this machinery, which is different enough from ours to target.

ATP: the energy currency

In short: The molecule that carries energy from fuel to work, made and spent so fast that you turn over roughly your own body weight in it daily.

Burning fuel does not directly power anything. Energy from food is used to attach a third phosphate group to a molecule called ADP, making ATP (adenosine triphosphate). Snapping that phosphate off releases energy exactly where it is needed: to contract a muscle fibre, pump an ion across a membrane, or build a protein.

Three ways to make it, and knowing them explains a great deal about exercise (Chapter 9):

You hold only about 250 g of ATP at any moment and use roughly 50 to 75 kg of it a day, meaning each molecule is recharged hundreds or thousands of times daily. Mitochondria do most of that work, which is why mitochondrial density in muscle is one of the main things that improves with endurance training, and why mitochondrial dysfunction appears in the ageing chapter.

Don't be confused: lactate is not what makes your muscles sore. Lactate rises during hard exercise and clears within an hour, and it is a fuel that other tissues burn rather than a waste product. The soreness that peaks a day or two later is microscopic muscle damage and inflammation, which is a different process entirely.

Vitamins

In short: Thirteen small molecules you cannot make, needed in tiny amounts, mostly acting as helpers that let enzymes work.

A vitamin is an organic compound required in small amounts that the body cannot synthesise adequately. Most act as coenzymes: small helper molecules an enzyme needs in order to function. That is why they are needed in milligram or microgram quantities, and why having more than enough does nothing extra.

Fat-soluble (A, D, E, K) versus water-soluble (B group and C) is the distinction that matters practically. Water-soluble vitamins are excreted when in excess, so toxicity is rare (B6 is the notable exception). Fat-soluble vitamins accumulate, so excess can be genuinely harmful: too much vitamin A causes liver damage and birth defects, and very high vitamin D causes dangerous blood calcium.

The honest position on supplements: correcting a genuine deficiency works and matters enormously. Taking vitamins without a deficiency has repeatedly failed to show benefit in large trials, and several have caused harm (Chapter 63). The common real deficiencies worth knowing about are iron, vitamin D, B12, iodine, and folate.

Minerals

In short: Inorganic elements you must eat, some in grams and some in micrograms, and the ones you have heard least about are often the ones with the largest population effect.

Putting it together: what a meal becomes

In short: Protein becomes amino acids for rebuilding, carbohydrate becomes glucose for immediate fuel, fat becomes fatty acids for storage and membranes, and nothing is stored as what it arrived as.

A plate of chicken, rice, olive oil, and broccoli, followed through the body:

1. Chicken protein is cut by stomach acid and enzymes into amino acids, absorbed in the small intestine, and enters a shared pool. Some rebuilds muscle and enzymes; the rest is stripped of its nitrogen (excreted as urea by the kidneys) and burned or stored.
2. Rice starch is cut into glucose, absorbed, and raises blood glucose, which triggers insulin. Glucose is burned immediately, stored as glycogen until those stores are full, and converted to fat beyond that.
3. Olive oil is emulsified by bile, cut by lipase, absorbed, reassembled into triglycerides, packaged into particles, and distributed for burning, storage, or membrane building.
4. Broccoli fibre is not digested at all. It slows the absorption of everything above, adds bulk, and is fermented in the colon.
5. The vitamins and minerals in all of it are absorbed separately and used as parts, not fuel.

Two things follow. Your body does not store food as food; everything is broken to components and rebuilt, which is why "you are what you eat" is chemically true and nutritionally misleading. And the same calories behave differently depending on what they arrive with: fibre, protein, and fat all slow the glucose rise from the same amount of starch, which is most of why whole foods and refined ones produce different responses (Chapter 8).

Sources and notes

Standard biochemistry (Lehninger, Principles of Biochemistry; Berg, Tymoczko, and Stryer, Biochemistry). Essential amino acid list and protein RDA of 0.8 g/kg/day: WHO/FAO/UNU protein and amino acid requirements report and national dietary reference intakes. Higher protein targets for older and training adults: Bauer et al., JAMDA, 2013 (PROT-AGE), and Morton et al., British Journal of Sports Medicine, 2018, which found benefit plateauing around 1.6 g/kg/day. Whole-body protein turnover of roughly 250 to 300 g/day is a standard tracer-study figure. ATP turnover of 50 to 75 kg/day is a commonly cited calculation from total daily energy expenditure. Lactate as fuel rather than waste: Brooks, Cell Metabolism, 2018. Fibre intake and outcomes: Reynolds et al., The Lancet, 2019. Trans fat elimination outcomes: national policy evaluations. Vitamin and mineral functions and deficiency syndromes are standard nutrition references.

Open questions. Optimal protein intake across the lifespan is still argued, particularly the balance between muscle preservation and the possibility that high protein intake affects longevity pathways. The health effects of specific saturated fatty acids, and whether the food matrix matters more than the fatty acid profile, remain actively debated.

Next: how these molecules are organised into the smallest living unit, and how often each part of you is replaced. 👉