Obesity affects more than 650 million people worldwide. Although low-carbohydrate and low-fat diets are effective at helping people lose weight, the health benefits and sustainability of the two dietary approaches are matters of controversy. A recent trial weighed the benefits of low-fat versus low-carbohydrate diets.

The causes of obesity and overweight are not fully known. Some scientists have suggested that consuming high-glycemic carbohydrates increases insulin levels, ultimately driving a vicious cycle of body fat accumulation, hunger, and food intake, commonly referred to as the “carbohydrate-insulin” model of obesity. Others have suggested that consuming high-fat foods drives overconsumption of calories due the foods' high caloric levels, poor ability to provide satisfaction and fullness, and high “pleasure factor.”

The four-week crossover trial involved 20 healthy men and women (average age, 30 years). Half of the participants ate an animal-based, ketogenic, low-carbohydrate diet that provided about 10 percent of its calories from carbohydrates and about 75 percent from fat and high calorie foods. The other half ate a plant-based, low-fat diet that provided about 10 percent of its calories from fat and about 75 percent from carbohydrates and low-calorie foods. After two weeks on their respective diets, participants switched diets and adhered to the new diet for another two weeks. All meals were prepared and served in an in-patient metabolic ward to ensure compliance. The study investigators monitored the participants' weight, vital signs, blood ketones, energy expenditure, activity, and other measures throughout the study.

Although both diets promoted weight loss, participants on the low-carbohydrate diet lost more weight (1.5 pounds) and faster, but the difference was not statistically or clinically significant. The participants who ate the low-fat diet had higher glucose and insulin levels compared to those who ate the low-carbohydrate diet. They didn’t report any differences in hunger, fullness, or satisfaction with their meals. When eating the low-fat diet, participants ate about 690 fewer calories per day than when eating the low-carbohydrate diet over the two-week period.

These findings suggest that whereas eating a low-carbohydrate diet is beneficial in reducing glucose and insulin levels, the low-fat diet reduces appetite, a finding that contradicts the carbohydrate-insulin model of obesity.

• Link to full study.