🥗 Nutrition · 6 min read

Carbohydrates: structure, what they do in the body, and the most persistent misunderstandings

What carbohydrates are chemically

Carbohydrates are compounds of carbon, hydrogen and oxygen. Their smallest unit is the simple sugar — glucose or fructose, for example. Two of them form a disaccharide: table sugar consists of one molecule of glucose and one of fructose, milk sugar of glucose and galactose.

When many simple sugars are strung together, polysaccharides result. Starch, as found in grains, potatoes and pulses, is such a chain of glucose units. The human storage form, glycogen, is built the same way, only more highly branched.

Dietary fibre occupies a special position: it too consists of polysaccharides, but linked in ways that human digestive enzymes cannot cleave. It is therefore counted among the carbohydrates while behaving fundamentally differently in the body.

“Simple” and “complex” — a classification that carries little

The most common classification distinguishes simple and complex carbohydrates, where “simple” is meant to stand for short-chain and quickly available, “complex” for long-chain and slow. As a chemical description that is correct. As a prediction of how quickly a food raises blood glucose it is unreliable.

The reason is that chain length is only one of many factors. The structure of the food, its fat, protein and fibre content, preparation, degree of processing and even temperature all play a part. Starch that has been cooked and cooled behaves differently from freshly cooked starch. A fine white-flour product made of long starch chains is processed more quickly than a whole grain, although both are “complex”.

In practice: the question “simple or complex?” answers less than it promises. More informative is which food it is and in what state it is eaten.

Glycaemic index: what it measures and what it does not

The glycaemic index was developed to close exactly this gap. It measures how strongly blood glucose rises after eating a fixed amount of carbohydrate from a food, compared with a reference.

Its limits are considerable and well documented. The value refers to a standardised amount of carbohydrate, not to a usual portion — which is why glycaemic load, which includes portion size, was added later. Measurement is also of the food eaten alone; in a mixed meal with fat, protein and fibre the curve looks different. And values vary substantially between people and even within the same person on different days.

The glycaemic index is therefore a research instrument of limited reach, not a sorting system for shopping. How much it matters at all for healthy people in everyday life is disputed in the literature.

What happens in the body

Starch digestion begins in the mouth with an enzyme in saliva and continues in the small intestine. At the end of that chain stand simple sugars, absorbed through the intestinal wall into the blood. Blood glucose rises, the pancreas releases insulin, and cells take up glucose.

Part of it is used immediately for energy. Another part is stored as glycogen — in the liver and in muscle. These stores are limited; their size depends on body size, muscle mass and training status. Glycogen stored in a muscle is available only to that muscle, whereas the liver can release glucose into the blood and thereby keep blood glucose steady.

Glycogen is stored together with water. That explains much of the rapid weight change at the start of a low-carbohydrate diet: what disappears from the scale in the first days is predominantly water released along with depleted glycogen. On returning to the usual diet it comes back in the same way.

Carbohydrates and training

For short, intense efforts glucose is the preferred fuel, because it can supply energy even without sufficient oxygen. As intensity falls and duration grows, the share shifts towards fat; both pathways always run in parallel, only in different proportions.

In very long endurance efforts, depletion of glycogen stores can become performance-limiting — a well-studied phenomenon in exercise physiology. For a typical strength session of under an hour it plays a considerably smaller role than the prevalence of related advice suggests.

How many carbohydrates someone needs therefore depends on the volume and intensity of their training, not on a general rule. Training long and hard every day sets different conditions than three shorter sessions a week.

The myth of the fattening nutrient

The idea that carbohydrates in themselves cause weight gain has been proposed and investigated repeatedly over recent decades. In controlled studies holding energy intake and protein constant, no meaningful differences in fat mass appeared between low-carbohydrate and low-fat approaches.

The core therefore lies in energy balance, not in a single nutrient. Why low-carbohydrate approaches nevertheless often work well in everyday life has plausible reasons: they typically exclude many highly processed, energy-dense foods, frequently raise the protein share and narrow the range of choice — all things that can lower energy intake without anyone counting.

This framing is not a recommendation for or against any dietary pattern. It only describes what the observed effect rests on.

Sugar, added and naturally occurring

Chemically, the sugar in an apple is no different from the sugar in a drink. The difference lies in the food around it: an apple brings water, fibre and a structure that has to be chewed; a sweetened drink delivers the same amount of sugar in seconds and without chewing.

This is why recommendations from professional bodies generally refer to added sugar rather than to sugar naturally present in fruit or milk. That is not a statement about a different effect of the molecule, but about the amount that can practically be consumed with the respective food.

What can be said — and what cannot

What holds up: carbohydrates are an energy source at around four kilocalories per gram; the body can store them, and the stores are limited; for intense efforts they are the preferred fuel; and their amount in a diet can be varied within wide limits without a difference in body weight necessarily following.

Less robust are almost all statements presenting a particular amount, timing or type as right for everyone. Individual needs depend on body size, muscle mass, activity, training volume and preferences — and preferences are not a soft factor but decide whether a way of eating is sustained over months.

Anyone looking for orientation gets further with a simple order than with nutrient debates: total energy first, then sufficient protein, then distributing the rest between carbohydrates and fat according to what fits everyday life.

Last reviewed: 14.08.2026

Created by AI