Twenty-four, normal to overweight (body mass index of 18.5-29.9 kg/m2), physically active adult civilians or active duty military personnel will be recruited for the 44-d protocol, 22-d at sea level and 22-d at high altitude (HA). During the 22-d sea level phase, volunteers will receive dietary counseling to maintain baseline weight and consume protein at levels consistent with recommendations for periods of low physical activity (1.0 g/kg/d). During the 22-d at HA, all meals and beverages (water ad libitum) will be prepared and provided to volunteers by research staff. Physical activity will be increased at HA and calorie intake will be reduced 40% to create a state of negative calorie balance.