Macronutrients Explained: Protein, Carbs, Fat & Alcohol

Nutrition basicsMacronutrientsEvidence based

Macronutrients ("macros") are the nutrients your body needs in large amounts every day. Three of them — protein, carbohydrate and fat — are the foundation of every meal and the only components of food that supply energy, measured in calories (kcal). A fourth source of energy, alcohol, also belongs on this page: it provides calories, but unlike the others your body has no actual need for it. Everything else you need — vitamins and minerals — comes in tiny amounts and are called micronutrients (covered in their own section).

Understanding macros isn't about counting every gram. It's about knowing what each one is for, which foods deliver it, and the difference between a higher-quality and a lower-quality version of the same macro. That single idea — quality, not just quantity — runs through this whole page.

The four energy sources at a glance

Each gram of a macronutrient releases a fixed amount of energy. These conversion factors (the "Atwater factors") are how every nutrition label is calculated.¹

Notice fat carries more than twice the energy of protein or carbohydrate per gram — which is why fatty foods are so calorie-dense, and why alcohol (closer to fat than to sugar in energy) adds up faster than people expect.

What it is & how it's built

Protein is built from small building blocks called amino acids, strung together like beads on a chain and then folded into a working shape. There are 20 amino acids in human nutrition. Your body can make most of them itself, but 9 are "essential" — meaning you must get them from food: histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan and valine.⁴ When you eat protein, you digest it back down into those amino acids and reassemble them into the proteins you need.

What it does

Protein is the body's construction material. It builds and repairs muscle, skin, hair and organs, and it forms enzymes, many hormones, antibodies for your immune system and the carriers that move things around in your blood. It also keeps you feeling full longer than carbs or fat, which is why higher-protein meals help with appetite.

Where you find it

"Complete" vs "incomplete" protein: a complete protein contains all 9 essential amino acids in good amounts. Animal foods and soy are complete; most single plant foods are lower in one or two amino acids. This is easy to solve — eating a variety of plants across the day (for example beans with grains) covers the full set, so a well-planned vegetarian or vegan diet reaches all essentials.⁴

How much you need

The official baseline (RDA) to prevent deficiency in healthy adults is 0.8 g of protein per kg of body weight per day⁴ — about 56 g for a 70 kg person. That is a floor, not an optimum: people who are older, very active, building muscle, or losing weight generally do better with more, and many researchers now point to roughly 1.2–1.6 g/kg for those groups.⁴

What it is & how it's built

Carbohydrates are made of sugar units. One sugar unit on its own (like glucose or fructose) is a simple carbohydrate; many units linked into long chains (like starch) are a complex carbohydrate. During digestion your body breaks most carbohydrates back down into glucose, which travels in your blood as your main fuel. Fibre is a special kind of carbohydrate: the chains are bonded in a way humans can't digest, so it passes through largely intact — and that is exactly what makes it valuable.

What it does

Glucose is the body's preferred quick energy, especially for the brain and for hard-working muscles. Carbohydrate not needed right away is stored in the liver and muscles as glycogen. Fibre, meanwhile, feeds your gut bacteria, slows the rise in blood sugar after a meal, helps lower cholesterol and keeps digestion regular.

Where you find it — and the quality ladder

The single most useful distinction in carbs isn't "carbs vs no carbs" — it's fibre-rich and whole vs refined and sugary. Whole-food carbohydrates come packaged with fibre, vitamins and minerals; refined ones have had most of that stripped away.

How much you need

Carbohydrate typically makes up the largest share of energy — about 45–60% of daily calories.² For fibre, aim for at least 25–35 g per day, an amount most people fall short of.² The clearest target to cut back on is free/added sugar: the WHO recommends keeping it under 10% of energy, and ideally below 5%.⁵

What it is & how it's built

Dietary fat is built from fatty acids — chains of carbon atoms — attached in threes to a small backbone (a "triglyceride"). The shape of those chains decides everything about how a fat behaves and how it affects your health:

• Saturated — straight, tightly packed chains; usually solid at room temperature (butter, fatty meat, coconut).
• Unsaturated — chains with one or more "kinks"; usually liquid (olive oil, rapeseed oil, nuts, fish). Split into mono- and polyunsaturated.
• Trans — mostly artificial unsaturated fats reshaped to act like saturated; the worst type for the heart.

What it does

Fat is concentrated energy and far more than a fuel. It builds the membrane around every cell, is the raw material for many hormones, cushions organs, and is required to absorb the fat-soluble vitamins A, D, E and K. Two polyunsaturated fats — omega-3 and omega-6 — are essential: your body can't make them, so they must come from food.

Where you find it — the types matter most

How much you need

Total fat of around 25–40% of energy fits a healthy diet — what matters most is the type.² The WHO recommends keeping saturated fat under 10% of energy and trans fat under 1%, replacing them with unsaturated fats from plants and fish.⁶

What it is & how it's built

The alcohol in drinks is a single small molecule, ethanol, made when yeast ferments sugar. It is not a nutrient — your body has no requirement for it — but it does carry energy: 7 kcal per gram, almost as much as fat, and these are "empty" calories with essentially no vitamins, minerals or benefit attached.¹

What it does in the body

Your body treats ethanol as a toxin and prioritises clearing it, mainly in the liver — which is why fat-burning and normal metabolism pause while alcohol is being processed. Beyond the calories, the more important issue is health risk.

This doesn't have to read as a lecture — but on a site about health, honesty matters: there is no amount of alcohol that improves your health, and from a nutrition standpoint it adds significant calories while displacing better food. Less is genuinely better.

How your cells turn food into energy

You eat protein, carbohydrate and fat. But your cells don't run on steak or bread — they run on a single molecule called ATP (adenosine triphosphate). Every movement, every thought, every heartbeat is powered by ATP. The story of how food becomes ATP is one of the most elegant systems in biology — and it all happens inside your cells, continuously, billions of times per second.

ATP — the cell's energy currency

Think of ATP as a rechargeable battery. It has three phosphate groups linked together, and the bond holding the last one stores energy. When a cell needs energy — to contract a muscle, send a nerve signal or build a protein — it snaps off that third phosphate (ATP → ADP + Pᵢ), releasing energy the cell can immediately use. The spent ADP is then recharged back to ATP using energy from the food you ate. Your body recycles its own weight in ATP every day — roughly 50–75 kg of it — even though you only carry about 250 grams at any moment.¹¹

The four stages of energy extraction

All three energy-yielding macronutrients — carbohydrate, fat and protein — eventually feed into the same cellular machinery. The path from food to ATP runs through four main stages. Carbohydrate takes the most direct route; fat and protein join the party a little further down the line.

1. Glycolysis — splitting sugar

Glycolysis happens in the cell's cytoplasm and doesn't need oxygen. A single glucose molecule (6 carbons) is split into two molecules of pyruvate (3 carbons each), yielding a net 2 ATP plus some high-energy electron carriers (NADH). This is the oldest energy pathway in evolution — every living organism on Earth does some version of it. It's fast but not very efficient: only about 5% of the energy in glucose is captured at this stage.¹²

2. The citric acid cycle (Krebs cycle)

If oxygen is available, pyruvate enters the mitochondria — the cell's power plants — and is converted to acetyl-CoA. This feeds into the citric acid cycle, a circular series of reactions where acetyl-CoA is broken down completely to CO₂. The cycle itself only makes a small amount of ATP directly (1 ATP-equivalent per turn), but its real job is stripping electrons from the fuel and loading them onto carrier molecules (NADH and FADH₂). Those electrons are what drive the next — and by far most productive — stage.¹³

Fatty acids also enter here as acetyl-CoA (via beta-oxidation), and amino acids can enter at various points in the cycle depending on their structure. This is why the citric acid cycle is the universal crossroads of energy metabolism.

3. The electron transport chain

Embedded in the inner membrane of the mitochondria are four large protein complexes (I through IV) that work like a relay race. NADH and FADH₂ drop off their electrons at Complex I or II, and the electrons are passed down the chain — Complex I → II → III → IV — each handoff releasing a little energy. That energy is used to pump protons (H⁺) from the mitochondrial matrix into the space between the two mitochondrial membranes, creating an electrochemical gradient — like charging a dam before opening the gates.¹⁴

4. Oxidative phosphorylation

The protons that have been pumped out now flow back through a remarkable enzyme called ATP synthase (Complex V). Like water turning a turbine in a hydroelectric dam, the flow of protons back through ATP synthase drives the recharging of ADP into ATP. This is where most of your energy comes from: oxidative phosphorylation produces roughly 28–34 ATP per glucose molecule — more than 90% of the total yield. The electrons that entered the chain are finally passed to oxygen (O₂), which combines with protons to form water — which is why you need oxygen to live.¹⁵

This is the biochemical foundation underneath everything on this page. When you eat, you're not just getting calories — you're fuelling a system that every day builds, recharges and spends billions of ATP molecules to keep you alive. And it all depends on the vitamins and minerals covered in the Micronutrients guide: B vitamins run the citric acid cycle, iron carries the oxygen for the electron transport chain, magnesium stabilises ATP itself.¹⁶

What it is

Water is not a macronutrient in the energy sense — it supplies zero calories — but it is a nutrient in every practical sense. Your body is roughly 60% water, and every chemical reaction you depend on happens in it: digestion, circulation, temperature control, waste removal, joint lubrication and nerve signalling. You can survive weeks without food, but only days without water.⁸

How much you need

There is no single correct number — water needs depend on your size, activity level, climate and diet. A commonly cited baseline is roughly 2–3 litres per day for adults, but thirst is a reliable guide for most healthy people. Food supplies about 20% of your daily water (fruits and vegetables are mostly water), and all non-alcoholic drinks count.⁹

Water is the forgotten fundamental: it carries every macro- and micronutrient to where they're needed. Even mild dehydration — 1–2% of body weight — can impair concentration, physical performance and mood.¹⁰

Putting it together

You don't need to weigh your food or memorise percentages. The principles on this page do most of the work:

• Include protein at each meal — from a mix of animal and/or plant sources.
• Make carbohydrates mostly whole and fibre-rich — vegetables, fruit, whole grains, legumes — and keep added sugar low.
• Choose unsaturated fats — olive/rapeseed oil, nuts, oily fish — over saturated and trans fats.
• Keep alcohol to a minimum, knowing there is no health benefit to it.
• Stay hydrated — water is the medium every other nutrient works in.
• Favour variety and whole foods — the quality of each macro matters as much as the amount.

From here, the natural next step is our Micronutrients guide — the 30 vitamins and minerals that work alongside these macros. Or head to Supplements for deep dives on individual nutrients, with the same evidence-first approach.