Subjects will be admitted to the Rockefeller University Hospital for the 10 week in-patient phase of the study. The first 2 weeks consist of a weight stabilization period, during which subjects will be required to consume an average American diet. This is followed by the 6 week weight loss period where subjects will be randomized to either a low calorie high carbohydrate or a low calorie high fat/protein diet. On both diets there will be a 50% calorie deficit compared to the weight stabilization period. This will be followed by a 2 week weight maintenance period during which calories will be added to the diet to maintain the new weight. Appropriate adjustments will be made to keep the subject's weight stable.