Do carbohydrates make you fat?

A debate that never ends

Few topics spark as much debate in nutrition as carbohydrates. To some, they are supposedly responsible for weight gain, promote fat storage and prevent any fat loss. Bread, pasta, rice, potatoes and even fruit are sometimes seen as foods to limit, or even avoid entirely.

Others, on the contrary, claim that carbohydrates are the body's main fuel and are essential for athletic performance. These two views seem hard to reconcile. Yet the scientific literature today offers a much more nuanced answer.

Carbohydrates do not have the power to make you fat on their own. Like protein and fat, they supply energy. It is above all overall energy balance, diet quality and lifestyle habits that determine changes in body weight.

Understanding this distinction is essential. Because the real question is not whether carbohydrates make you fat, but under what circumstances they can promote weight gain, and how to intelligently fit them into your diet.

Why are carbohydrates so criticised?

The poor reputation of carbohydrates built up gradually. With the popularisation of low-carb diets, many began directly linking their consumption to fat gain.

This idea is largely based on the role of insulin. After a carbohydrate-rich meal, blood sugar rises. In response, the pancreas secretes insulin, a hormone that facilitates the entry of glucose into cells and takes part in nutrient storage.

For a long time, this observation led some to conclude that insulin was “the hormone that makes you fat”. The explanation seemed simple: more carbohydrates would mean more insulin, and more insulin would mean more fat storage.

This view is, however, incomplete. Insulin does play a role in energy metabolism, but it does not override the fundamental laws of energy balance. Lasting weight gain can only occur when the body receives more energy than it expends over a prolonged period. This is precisely what research has confirmed.

What the scientific literature actually shows

Over recent decades, numerous studies have compared diets with very different carbohydrate proportions. Some diets were low in carbohydrates. Others contained far more.

When researchers equalise calories and protein, differences in fat loss or muscle gain generally become very small. In other words, the amount of carbohydrate consumed does not, by itself, explain variations in body weight.

Meta-analyses show that low-carb diets can be effective for losing weight. However, this effectiveness mainly stems from a spontaneous reduction in total energy intake, not from any metabolic effect specific to carbohydrates.

Over the long term, several dietary approaches produce comparable results when they allow you to maintain a calorie deficit and good adherence. The essential question therefore becomes less “how many carbs?” and more “what diet can you sustain long-term?”.

The essential question is not “how many carbs?”, but “what diet can you sustain long-term?”.

Why are carbohydrates important for athletes?

Carbohydrates are the main energy source during intense efforts. During a resistance training session, muscles mainly use glycogen, the stored form of carbohydrate found in muscles and the liver.

When these stores drop sharply, performance can progressively decline. Loads feel heavier, fatigue sets in faster and total training volume becomes harder to maintain.

In trainees seeking to build muscle mass or strength, insufficient carbohydrate intake can therefore indirectly limit progress, not because carbohydrates build muscle directly, but because they help sustain quality training.

This is also one of the reasons carbohydrate needs often increase with training volume. They depend more on physical activity than on a universal figure applicable to everyone.

Are all carbohydrates equal?

Not all carbohydrate-rich foods have the same nutritional characteristics. Wholegrains, legumes, fruit and potatoes provide not only carbohydrates but also fibre, vitamins and minerals.

Sugary drinks, confectionery or certain ultra-processed products, on the other hand, often provide a lot of energy for low nutritional density. This difference affects satiety, overall diet quality and how easily you can control your energy intake.

This does not mean a food is “forbidden”. In practice, the overall quality of the diet matters more than an occasionally eaten food.

Are low-carb diets better for losing fat?

Low-carb diets have enjoyed great popularity in recent years. In some people, they do indeed make it easier to lose weight. However, this effectiveness does not mean carbohydrates prevent weight loss.

Sharply cutting carbohydrates often means eliminating many highly calorific foods, which naturally reduces daily energy intake.

At the start of a low-carb diet, weight loss is also amplified by a drop in glycogen stores. Since each gram of glycogen is stored with several grams of water, this decrease causes a rapid water loss, sometimes mistakenly interpreted as significant fat loss.

Over the longer term, differences between dietary approaches fade. Research shows that no diet is systematically superior when it provides the same number of calories and enough protein. The best diet, then, is the one a person is able to sustain long-term, without compromising their health, performance or quality of life.

The most common mistakes

One of the most widespread mistakes is treating all carbohydrates as identical. In reality, a piece of fruit, oats or lentils do not have the same nutritional characteristics as a sugary drink or an industrial pastry.

Another mistake is attributing every weight fluctuation observed after a carbohydrate-rich meal to fat gain. In most cases, this increase mainly reflects a temporary rise in glycogen stores and the water associated with it, a perfectly normal phenomenon.

Some people also sharply cut carbohydrates while maintaining a high training volume. This strategy can lead to reduced performance, increased fatigue and make resistance training sessions harder to sustain.

Finally, many people look for a single food to blame for weight gain. Research shows, however, that body composition depends above all on overall dietary habits, physical activity and long-term energy balance, rather than on any single nutrient.

Practical recommendations

For most trainees, current recommendations can be summarised simply. Adapt your carbohydrate intake to your activity level and your goal.

Prioritise minimally processed, fibre-rich foods such as wholegrains, legumes, fruit and vegetables. Do not cut carbohydrates without reason, especially if you regularly practise resistance training or an endurance sport.

If your goal is fat loss, focus first on your overall calorie intake and on getting enough protein.

Finally, choose a diet you can sustain long-term rather than a very restrictive one that is hard to maintain.

Conclusion

Carbohydrates are among the most misunderstood nutrients in nutrition. Contrary to a widely held belief, they do not automatically cause weight gain and are not, on their own, the cause of body fat accumulation.

Scientific data shows that energy balance remains the main determinant of weight change, while carbohydrates play an essential role in supplying energy and maintaining physical performance.

For weight trainers as for the general population, the goal is therefore not to eliminate carbohydrates. It is rather to choose quality sources, adapt their quantity to your activity level and fit them into a balanced diet.

Ultimately, carbohydrates are neither enemies nor miracle foods. They are simply a nutritional tool whose value depends on context, goals and overall lifestyle habits.