🥗 Nutrition · 4 min read

What macronutrients are

The three groups and their energy content

“Macro” here refers to quantity: the body takes in these nutrients in gram amounts, unlike vitamins and minerals, which are a matter of milligrams or less. For calculating energy, the so-called Atwater factors are commonly used: around 4 kilocalories per gram of protein, around 9 per gram of fat and around 4 per gram of carbohydrate.

These values are calculation figures, not exact measurements. They rest on average digestibility and vary by food — sometimes markedly for fibre and for heavily versus minimally processed foods. Nutrition labels on packaging are based on the same factors and carry the same imprecision.

Protein

Proteins are chains of amino acids. During digestion the body breaks down ingested protein into these building blocks and reassembles them — for musculature, enzymes, transport molecules and much else. Nine of the twenty protein-forming amino acids are considered essential: the body cannot produce them itself and has to take them in through food.

Protein occurs in animal and plant foods alike, in differing amounts and with differing amino-acid patterns. Unlike fat and carbohydrate, the body does not lay down protein as a store to any appreciable extent — what is not used is broken down and the energy it contains used otherwise.

Fat

Dietary fats consist mainly of triglycerides, that is, fatty acids on a glycerol backbone. Saturated, monounsaturated and polyunsaturated fatty acids are distinguished; most foods contain mixtures of them. Two polyunsaturated fatty acids are considered essential and have to be taken in through food.

At around 9 kilocalories per gram, fat is the most energy-dense of the three nutrients. It is also the carrier medium for the fat-soluble vitamins A, D, E and K — without fat in a meal these are absorbed less well. The body stores surplus energy, regardless of its origin, very largely as fat tissue.

Carbohydrates

Carbohydrates range from a single sugar molecule to long chains such as starch. The body breaks most of them down to glucose. Part is stored as glycogen in the liver and musculature; these stores are limited and are drawn on more heavily during intense than during easy exertion.

Fibre is also a carbohydrate but is broken down little or not at all in the small intestine. It therefore provides less usable energy than its gram figure suggests — one reason nutrition labels sometimes list fibre separately. Unlike protein and fat, no carbohydrate is considered essential: the body can produce glucose itself when needed.

Alcohol as a fourth energy source

Alcohol provides around 7 kilocalories per gram and is often overlooked in nutrition tables because it is not a nutrient in the proper sense — the body does not need it and cannot store it. It is broken down preferentially, which temporarily alters how the other energy sources are handled.

For energy balance this simply means: alcoholic drinks contribute energy that is missing from many records. That is not a judgement but a figure worth knowing when estimating your intake.

How the body switches between energy sources

The body does not use fat and carbohydrate on an either-or basis but always in a mixture. Which share dominates shifts with intensity: at rest and during easy movement a larger part of the energy comes from fat, and as intensity rises the carbohydrate share increases — simply because it is available faster.

This is occasionally taken to mean that easy exertion is better for reducing fat. That falls short: a percentage share is not the same as an absolute amount, and for changes in body composition over weeks the overall energy balance is what counts, not the fuel mix during a single session.

Protein plays a special role in this equation. It too is drawn on for energy, but under ordinary conditions to a lesser extent — the body preferentially uses the other two for that.

What a macro split says — and what it does not

A split such as “30 percent protein, 30 percent fat, 40 percent carbohydrate” describes quantity ratios, nothing more. It says nothing about which foods those quantities come from, how satiating they are, how well they are tolerated, or which vitamins and minerals come with them.

Two ways of eating with an identical macro split can differ markedly in everything else. A split is therefore a starting point for planning — not a yardstick for the quality of a diet.

Last reviewed: 14.08.2026