A low-fat diet leads to a greater loss of body fat compared to a low-carb diet, according to scientists of the US National Institutes of Health.

Researchers strictly controlled the diets of 19 people so they all had exactly the same calorie intake. The results showed 67 per cent more fat was lost on a low-fat diet than a low-carb one.

Experts at the annual Endocrine Society meeting questioned the findings, suggesting it is not clear which diets people could stick to in real life and outside strict lab conditions.

The participants spent five days on a diet designed to match how many calories their bodies were burning. It consisted of 50 per cent carbohydrate, 35 per cent fat and 15 per cent protein.

Calorie intake was cut by 30 per cent, or around 800 calories, for each of the next six days. Of these participants, half were placed on a low-carb diet of 30 per cent carbohydrate, 49 per cent fat and 21 per cent protein. The other half were put on a low-fat diet – 72 per cent carbohydrate, 7 per cent fat and 21 per cent protein.

After the six days, the test subjects had a couple of weeks to recover before starting all over again and trying the other diet – leading to the overall result that body fat loss was 67 per cent greater after six days of low fat versus low carbohydrate.

The low-fat, high-carb diet increases the amount of fat being burned by the body, but preventing the fat entering the body in the first place seems to have a greater impact, according to the study.

“It’s not easy, these folks had to eat the same meals every day,” Dr Kevin Hall from the NIH National Institute of Diabetes and Digestive and Kidney Diseases told the BBC. “This was very strictly controlled and they had to eat the food provided and nothing else, the diets got pretty boring pretty quickly.”

While the study investigates the metabolism of fat and carbohydrate under highly controlled conditions, it doesn’t account for longer-term weight control where behavioural factors play an important role. The acceptability of a dietary programme is an important aspect associated with people’s adherence to a diet. Some experts said the total fat intake of the low-fat diet of 7 per cent is too low to be sustainable and would lead to nutritional deficiencies.