Diets that promote either restriction in dietary carbohydrate or higher protein intakes are very popular, but official agencies are reluctant to promote them because they conflict with current recommendations that focus on lowering total fat. Proponents of low carbohydrate or high protein diets claim they enhance body composition compared to traditional diets by either enhancing fat loss or augmenting muscle gain or both. A comprehensive review was published in the latest issue of the American Journal of Clinical Nutrition that examined a total of 86 scientific studies that had investigated the effects of dietary carbohydrate and protein on weight loss and body composition. The results from these studies were statistically analyzed using a meta-regression procedure that allows them to control for such confounding variables as age, gender, caloric intake, etc. The results clearly showed that diets lower in carbohydrate were associated with greater weight loss and greater fat loss, and the effect was independent of caloric intake. So even when calories are matched, a diet lower in carbohydrate enhances weight and fat loss more than a high carbohydrate diet. Higher protein intake was associated with greater retention of lean body mass, and this effect was also independent of caloric intake. Overall, the results of this extensive review provide convincing evidence that lowering carbohydrates and increasing protein can have a significant benefit on body composition.

Krieger JW, Sitren HS, Daniels MJ, Langkamp-Henken B. Effects of variation in protein and carbohydrate intake on body mass and composition during energy restriction: a meta-regression. Am J Clin Nutr. 2006 Feb;83(2):260-74.