Changes in total body, lean body, and fat mass will be assessed at sea level and after a 22-d energy deficit at HA. Regulation of muscle mass will be assessed using stable isotope methodology, muscle biopsies, and various molecular techniques, to directly measure muscle protein synthesis, whole body protein balance, and the cellular mechanisms that regulate these processes. Furthermore, exercise testing, substrate oxidation rates, and expired carbon dioxide labeled with the stable isotope of carbon dioxide (one extra neutron) will be used to assess total, exogenous, and endogenous carbohydrate oxidation and performance capacity.