Low-carb Diets Help Overweight People Burn More Calories, Weight Loss Trial Reveals

A low-carb diet appears to help overweight and obese people maintain weight loss as it increases the number of calories a person burns, scientists have discovered.

In a trial of 164 adults aged between 18 and 65 with a BMI over 25, researchers controlled what people ate over the course of 20 weeks. During this period, the team, led by Boston Children's Hospital and Framingham State University, tracked each person's weight, insulin secretion, metabolic hormones and the number of calories they burned.

The findings, published in the BMJ, suggest a low-carb diet and its metabolic effect could be used as a treatment for obesity.

David Ludwig, one of the study authors and co-director of the New Balance Foundation Obesity Prevention Center in Boston Children's Division of Endocrinology, said this is the biggest and longest feeding study looking at the Carbohydrate-Insulin Model. "According to this model, the processed carbohydrates that flooded our diets during the low-fat era have raised insulin levels, driving fat cells to store excessive calories," he said in a statement. "With fewer calories available to the rest of the body, hunger increases and metabolism slows—a recipe for weight gain."

During the randomized study, participants were split into three groups—high-, moderate- or low-carb diets. For each group, calorie intake was adjusted to maintain weight loss—rather than for them to continue losing weight. Over the 20 weeks, the team monitored energy expenditure. They found the low-carb group burned far more calories compared to the high-carb group—250 kilocalories on average.

This was most pronounced among people with the highest insulin secretion at baseline. This can be a sign of insulin resistance, which is characteristic of type 2 diabetes.

The authors say that the difference in calorie expenditure between the groups continued throughout the 20 weeks with no sign of the effect dropping off. This would equate to 20lb weight loss over three years, despite no change to overall calorie intake.

Study co-author and co-director of the New Balance Foundation Cara Ebbeling said: "Our observations challenge the belief that all calories are the same to the body. Our study did not measure hunger and satiety, but other studies suggest that low-carb diets also decrease hunger, which could help with weight loss in the long term."

The authors note there are several limitations to the study. For example, they make certain assumptions relating to the total energy expenditure that could lead to errors. They also did not carry out the study in a clinical setting—participants were provided with meals and given strategies to comply with the diets at home. This could have resulted in non-compliance, and other unrecognized dietary factors could have produced the results seen.

The final issue relates to how you might translate the findings into public health recommendations. Meta-analysis—where scientists evaluate lots of studies together—of previous research shows only small advantages to low-carb diets.

Concluding, the team says more research will be needed to assess low-carb diets and their effectiveness at maintaining weight loss.

The health risks of low-carb diets are also being explored at the moment. Research presented at the European Society of Cardiology Congress in August indicated that people who consume a low-carb diet were at greater risk of death from coronary heart disease, stroke and cancer. The findings, based on research that has not yet been published, indicate that the long-term safety of low-carb diets has not been fully assessed.

Maciej Banach, of the Medical University of Lodz, Poland, said in a statement: "Low-carbohydrate diets might be useful in the short-term to lose weight, lower blood pressure, and improve blood glucose control, but our study suggests that in the long-term they are linked with an increased risk of death."