To reduce body fat, eating less fat may be more effective than eating less carbohydrate

"Calorie for calorie, reducing dietary fat results in more body fat loss than reducing dietary carbohydrate when men and women with obesity have their food intake strictly controlled," said lead study author Kevin D. Hall, PhD, senior investigator at the National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK) of the National Institutes of Health (NIH) in Bethesda, Maryland.

Nutrition recommendations for people with obesity often conflict as to whether restricting fat or carbohydrate is better for body fat loss.

"Ours is the first study to investigate whether the same degree of calorie reduction, either through restricting only fat or restricting only carbohydrate, leads to differing amounts of body fat loss in men and women with obesity," Dr. Hall said

The authors studied 10 men and 9 women with obesity. The average age of the participants was 24 years and their average body mass index was 36 kg per meter squared.

All participants were admitted to the metabolic ward of the NIH Clinical Center and resided there 24 hours per day. All food eaten was strictly controlled and the daily activities of the participants were monitored. For 5 days, everyone was fed a eucaloric baseline diet (consisting of 50% carbohydrate, 35% fat, and 15% protein) that gave them the exact number of calories they needed to maintain their body weight.

For the next 6 days, the participants were randomly assigned to one of two groups where they received a 30% reduced-energy diet by having either their fat or carbohydrate intake restricted.

After a 2- to 4-week washout period, all participants were readmitted and they repeated the same 5-day eucaloric diet. Those who had eaten 6 days of reduced-fat diet in the first phase now ate a reduced-carbohydrate diet, and those who had eaten the reduced carbohydrate diet now ate the reduced fat diet.

The researchers measured the amount of fat eaten and the amount of fat burned, and the difference between them determined how much fat was lost from the body during each diet. Compared to the reduced carbohydrate diet, the reduced fat diet led to a roughly 67% greater body fat loss.

The Intramural Research Program of the NIDDK, NIH funded the study.