Obesity is a growing problem whose importance is reflected in the resources that are expended each year by a large section of the population on weight reduction drugs and therapies. Although numerous weight loss diets are available, there is little agreement in the scientific literature or the lay press as to the amount of fat, protein, and carbohydrates that would comprise the most efficacious diet for weight loss and long term weight loss retention. Reliable information about the effectiveness of low calorie diets with differing macronutrient composition is clearly needed and of paramount importance to inform the choice of a weight reduction diet.