Does it matter for your weight whether your calories come from fat or carbohydrates?

In a tightly controlled study in which people with obesity ate exactly the same number of calories for six days, participants on a low-fat diet lost an average of 89 grams of body fat per day. Participants on a low-carbohydrate diet lost 53 grams per day. That is a statistically significant difference, but the same researchers predict that this advantage fades over the longer term once calorie intake is truly kept equal.

Over two years, in a study with more than 800 participants, the ratio of fat to carbohydrates made no measurable difference. Those who ate 20% fat lost on average the same amount of weight after two years as those who ate 40% fat, namely about 3.3 kg. The same applied to a diet high versus low in carbohydrates. What did correlate with greater weight loss was how often a person attended the counselling sessions. Each additional session yielded an average of 0.2 kg of extra loss, regardless of diet type. Behaviour and persistence count for more than the exact nutritional ratio.

There is a theory that highly processed carbohydrates with a high blood sugar spike disrupt hormone balance in such a way that they promote fat storage. Meta-analyses do show some advantage for diets containing less of these types of carbohydrates. But the quality of the evidence is limited: many studies struggle with poor long-term adherence, making the results difficult to interpret.

Quality does matter, although it is less about weight and more about health risks. Diets rich in wholegrains, vegetables, legumes and nuts lower the risk of type 2 diabetes and improve blood sugar and blood lipids. Replacing saturated fat with unsaturated fat lowers the risk of cardiovascular disease by roughly 17 to 21%, although this has hardly any effect on total mortality.