Adolescent obesity is a problem characterized by significant after-effects including altered metabolism of glucose and predisposition to diabetes mellitus. In order to better understand the metabolic abnormalities associated with adolescent obesity, we will compare the rates of glucose and whole-body protein synthesis and catabolism in morbidly obese and normal adolescents by means of the simultaneous primed-constant infusion of tracer quantities of U-13C-glucose and 15N-lysine. Whereas weight reduction by calorie restriction is the therapy of choice in obese adolescents, the risk of growth retardation is an undesirable side effect. Therefore we will also investigate the relationship between the rates of protein turnover and glucose turnover in obese adolescents under different dietary conditions. Thus, adolescents will be studied before and after two three-week intervals of hypocaloric dietary therapy. The diets will be isonitrogenous, with one diet having all non-protein energy in the form of carbohydrate, and the other diet will provide the non-protein energy in the form of fat. Each experimental subject will be studied in a randomized cross-over design. In order to control for effects of weight reduction during the dietary periods, four additional subjects will repeat the protein and fat diets. Following the second dietary period, subjects will be continued on one of the two experimental diets. Those who continue to lose weight will be restudied either when ideal body weight is achieved or after no further weight loss occurs. In addition to kinetic studies of glucose and protein metabolism, we will also estimate lean body mass by means of the oral administration of H218O before and after weight reduction. Finally, oral glucose tolerance tests will be given in order to classify our patients according to more traditional metabolic criteria. Our studies should clarify whether fat or carbohydrate is the more appropriate source of energy for supporting the growth spurt in obese adolescents receiving hypocaloric dietary therapy, and will determine if weight reduction in obese adolescents results in normalization of protein and glucose metabolism.