Substrate Utilization, Exercise Performance, and Skeletal Muscle Response to Energy Deficit and Altitude Acclimatization

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. 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. A comprehensive cognitive/behavioral test battery will be performed at sea level and HA. Sleep will be assessed by actigraphy. Eating behavior and appetite will be assessed at sea level, acute HA, and after the 22-d energy deficit at HA using validated measures. Gut health will be assessed using sugar absorption tests and the bacterial composition of the gut will be characterized using advanced molecular techniques.