24.1 — Protein, Fat and Carbohydrate

Everything you have ever eaten was built from three things.

Protein, fat and carbohydrate. Plus water, alcohol, and the tiny quantities of vitamins and minerals that come along with them.

Every diet argument you have ever heard is an argument about the ratio between those three, and almost all of them ignore the fact that the body can run perfectly well on a wide range of ratios — which is why populations eating wildly different diets have all managed to be healthy.

This page is what each one actually does.

Energy

A calorie is a unit of energy — strictly, the energy needed to raise 1 gram of water by 1 degree Celsius. What food labels call a calorie is actually a kilocalorie, a thousand of those. A kilojoule is the metric unit; 1 kcal is about 4.2 kJ.

The energy in the three macronutrients:

Protein: 4 kcal per gram.Carbohydrate: 4 kcal per gram.Fat: 9 kcal per gram.Alcohol: 7 kcal per gram.Fibre: around 2 kcal per gram, from bacterial fermentation in the colon.

Fat carries more than twice the energy per gram of the other two, which is the reason the body stores energy as fat rather than as anything else. Storing the same energy as glycogen with its attached water would weigh several times more. A person carrying 15 kilograms of fat is carrying around 135,000 kcal — enough to walk for weeks.

And where the energy goes:

Basal metabolic rate — 60 to 75 percent. What it costs to be alive: keeping your heart beating, your brain running, your kidneys filtering and your temperature stable. The brain alone uses around 20 percent of it (Chapter 11.2).

Physical activity — 15 to 30 percent.

The thermic effect of food — around 10 percent. Digesting and processing food costs energy, and protein costs the most: around 20 to 30 percent of its energy is spent processing it, against 5 to 10 percent for carbohydrate and 0 to 3 percent for fat. Which is one real reason protein is more satiating than the calorie count suggests.

Protein

What it is for

Protein is the only macronutrient the body cannot store as a reserve.

Every gram of protein in your body is doing a job — enzymes, muscle, antibodies, hormones, transport molecules, the collagen holding your skin and joints together (Chapter 1.3).

Which means when protein intake is inadequate, the body dismantles working tissue to get amino acids. Muscle goes first.

Amino acids: 20 of them. Nine are essential — the body cannot make them, so they must come from food: histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan and valine.

Complete proteins contain all nine in useful amounts — meat, fish, eggs, dairy, soy and quinoa.

Most plant proteins are lower in one or more — grains are low in lysine, legumes low in methionine.

And the old advice about carefully combining proteins at every meal is unnecessary. The body maintains a pool of amino acids across the day, so eating a variety of plant proteins over the course of a day covers it perfectly well.

How much

The official minimum is around 0.8 grams per kilogram of body weight per day — that is the amount that prevents deficiency in most adults, which is a different question from the amount that is optimal.

The evidence for more, in specific situations:

Older adults: 1.0 to 1.2 g/kg. Because of anabolic resistance — ageing muscle responds less to the same amount of protein, so more is needed to trigger muscle building. This matters enormously, because muscle loss is one of the strongest predictors of losing independence (Chapter 24.8).

People doing resistance training: 1.4 to 2.0 g/kg, with benefit plateauing around 1.6 in most studies.

During weight loss: higher, around 1.2 to 1.6 g/kg, to preserve muscle while losing fat.

Distribution matters too: spreading protein across meals — roughly 25 to 40 grams per meal — stimulates muscle protein synthesis better than one large serving, because the response saturates.

Concerns about high protein and kidney damage apply to people who already have kidney disease. In people with healthy kidneys, higher protein intakes have not been shown to cause harm.

Where to get it

Around 20 to 30 grams of protein is in: 100 g of chicken or fish; 3 to 4 eggs; 150 g of Greek yoghurt; 100 g of hard cheese; 250 g of cooked lentils; 150 g of tofu or tempeh; or 400 ml of milk.

Fat

Fat was blamed for heart disease for forty years, and the story turned out to be considerably more complicated than the one everybody was told.

What it does

Energy storage.

Cell membranes — every membrane in your body is built from lipids (Chapter 1.4).

Hormones — all the steroid hormones are made from cholesterol (Chapter 12.1).

Absorption of vitamins A, D, E and K, which are fat-soluble and cannot be absorbed without fat present.

Insulation, and physical protection of organs.

And the brain, which is around 60 percent fat by dry weight.

The types

Saturated fat — no double bonds, solid at room temperature. Butter, cheese, fatty meat, coconut oil, palm oil.

Raises LDL cholesterol on average, and the size of that effect varies by the specific fatty acid and by the food it comes in.

And the honest current position: replacing saturated fat with unsaturated fat reduces cardiovascular events, which is well supported. Replacing it with refined carbohydrate does not. Which explains much of the confusion — the 1980s advice to cut fat led people to eat more refined starch and sugar, and the outcome was not what was intended.

Dairy fat behaves differently from the fat in processed meat, which is part of the argument for thinking about foods rather than isolated nutrients.

Monounsaturated fat — one double bond. Olive oil, avocado, most nuts.

Consistently associated with good outcomes, and the backbone of the Mediterranean dietary pattern.

Polyunsaturated fat — several double bonds. Two families that are genuinely essential:

Omega-6 — linoleic acid, in vegetable oils, nuts and seeds. Plentiful in modern diets.

Omega-3 — alpha-linolenic acid in flaxseed, walnuts and rapeseed oil; and EPA and DHA in oily fish.

The body converts ALA to EPA and DHA very inefficiently — a few percent — which is why oily fish, or an algae-based supplement for vegetarians, is the reliable source.

Omega-3s are structural components of the brain and retina, and they reduce triglycerides and inflammation. The evidence that fish oil supplements prevent heart disease in the general population is weaker than once believed; eating fish is more consistently associated with benefit.

Trans fat — the one with no defensible role.

Industrially produced by partially hydrogenating vegetable oil to make it solid and shelf-stable.

It raises LDL and lowers HDL simultaneously, which is the worst combination available, and it is associated with a markedly increased cardiovascular risk per gram than any other fat.

Which is why it has been banned or effectively eliminated in a large number of countries — one of the cleanest public health wins in nutrition, with measurable falls in cardiovascular events afterwards.

Small amounts occur naturally in dairy and meat and appear not to carry the same risk.

Cholesterol in food

And this deserves clearing up, because the advice reversed.

Dietary cholesterol has a much smaller effect on blood cholesterol than was once thought, because the liver makes most of the body's cholesterol and adjusts production in response to intake.

Which is why eggs were removed from the restricted list in most dietary guidelines. For most people, an egg a day has little effect on cardiovascular risk.

Saturated and trans fats affect blood cholesterol far more than dietary cholesterol does.

Carbohydrate

The body's preferred fuel, and the one that is genuinely not essential — the liver can make glucose from protein and from the glycerol backbone of fat (Chapter 9.4), which is why very low carbohydrate diets are survivable.

The types

Simple sugars: glucose, fructose, galactose (single units); sucrose — table sugar, glucose plus fructose; lactose — milk sugar; maltose.

Starches: long chains of glucose. Bread, rice, pasta, potatoes, grains.

Fibre: carbohydrate we cannot digest, discussed below.

Glycaemic index and load

Glycaemic index measures how fast a food raises blood glucose compared with pure glucose.

And it is more limited than it is usually presented as, for two reasons:

It says nothing about portion size. Watermelon has a high GI and very little carbohydrate per serving, so it barely moves blood glucose.

Glycaemic load corrects for this — GI multiplied by the grams of carbohydrate in a realistic portion — and is the more useful number.

And individual responses vary substantially. Studies measuring continuous glucose in many people found that the same food produces very different responses in different individuals, which limits how universal any GI table can be.

The practical version, which survives all the caveats: whole foods with their fibre intact — whole grains, legumes, vegetables, whole fruit — produce slower, smaller glucose rises than refined ones. That is reliable, and it does not require a table.

Fibre

And this is the part of the carbohydrate story with the strongest evidence of the lot.

Soluble fibre — dissolves in water, forming a gel. Oats, beans, apples, psyllium. Slows digestion, lowers LDL cholesterol, and improves glucose control.

Insoluble fibre — adds bulk and speeds transit. Wholegrains, vegetable skins, nuts.

Resistant starch — starch that escapes digestion and is fermented in the colon. In cooled cooked potato and rice, green bananas, and legumes. Cooking and cooling starch increases it, which is a genuinely useful and slightly odd fact.

What fibre does:

Feeds the gut bacteria (Chapter 9.6), which ferment it into short-chain fatty acids that nourish the cells lining the colon and have anti-inflammatory effects throughout the body.

Lowers cholesterol, improves glucose control, adds bulk and satiety, and keeps bowels regular.

And the outcome data is strong. A large systematic review found that people with the highest fibre intakes had 15 to 30 percent lower rates of death from all causes, cardiovascular disease, stroke, type 2 diabetes and colorectal cancer — with benefit continuing to rise with intake.

Target: 25 to 30 grams a day, and most people in Western countries get around half that.

Increase gradually and with fluid, because a sudden jump causes bloating and gas.

Sugar

The World Health Organization recommends free sugars — added sugars plus those in honey, syrups and fruit juice — below 10 percent of energy, and ideally below 5 percent.

For an adult eating 2,000 kcal, 5 percent is about 25 grams, or six teaspoons.

A single can of ordinary fizzy drink contains around 35 grams.

Sugar in whole fruit is not counted, because it comes packaged with fibre, water and micronutrients, and the evidence associates whole fruit consumption with good outcomes.

Fruit juice is counted, because removing the fibre removes the difference.

The clearest harms of high free sugar intake: tooth decay, which is direct and dose-related; excess energy intake, because sugary drinks are poorly compensated for at later meals; and, in the case of sugar-sweetened beverages specifically, consistent associations with type 2 diabetes and weight gain.

And on fructose: the claims that it is uniquely toxic run ahead of the evidence at normal intakes. Large amounts, particularly in liquid form, do promote liver fat. Fructose from whole fruit does not appear to be a problem.

Alcohol

7 kcal per gram, with no nutritional value.

And the evidence has shifted decisively.

The old J-shaped curve — showing that light drinkers lived longer than abstainers — is now largely attributed to a methodological problem: the abstainer group included people who had stopped drinking because they were ill, the so-called sick quitters.

Studies using genetic methods, which avoid that confounding, find no protective level.

And alcohol is a group 1 carcinogen, causally linked to cancers of the mouth, throat, oesophagus, liver, colon and breast — with breast cancer risk rising from low levels of intake.

The honest summary: there is no amount that is established as beneficial, risk rises with amount, and the risk at low levels is small. That is a different message from either "a glass of wine is good for you" or "any amount is dangerous", and it is what the data supports.

The ratio question

And here is the answer nobody selling a diet book wants to give: the ratio matters far less than the food quality.

Trials comparing low-fat with low-carbohydrate diets, matched for calories and protein, consistently find similar average weight loss with enormous individual variation.

Populations eating 70 percent carbohydrate and populations eating 70 percent fat have both been healthy.

What is consistently associated with good outcomes, across every dietary pattern studied:

Plenty of vegetables, fruit, legumes, wholegrains and nuts.Adequate protein.Unsaturated fats predominating.Little ultra-processed food.Little free sugar.Enough fibre.And a total energy intake that matches what you spend.

Everything else is a detail, and the arguments about the details have absorbed a great deal of attention that the list above deserved.

What the next page fixes

Chapter 24.2 covers vitamins and minerals — what each one does, the deficiencies that still occur, and which supplements are worth taking and which are not.