Severe negative energy balance during 21 d at high altitude decreases fat-free mass regardless of dietary protein intake: a randomized controlled trial

In this 2-phase randomized controlled study, we examined whether consuming a higher-protein (HP) diet would attenuate fat-free mass (FFM) loss during energy deficit (ED) at high altitude (HA) in 17 healthy males (mean +/- sd: 23 +/- 6 yr; 82 +/- 14 kg). During phase 1 at sea level (SL, 55 m), participants consumed a eucaloric diet providing standard protein (SP; 1.0 g protein/kg,) for 21 d. During phase 2, participants resided at HA (4300 m) for 22 d and were randomly assigned to either an SP or HP (2.0 g protein/kg) diet designed to elicit a 40% ED. Body composition, substrate oxidation, and postabsorptive whole-body protein kinetics were measured. Participants were weight stable during SL and lost 7.9 +/- 1.9 kg ( P < 0.01) during HA, regardless of dietary protein intake. Decrements in whole-body FFM (3.6 +/- 2.4 kg) and fat mass (3.6 +/- 1.3 kg) were not different between SP and HP. HP oxidized 0.95 +/- 0.32 g protein/kg per day more than SP and whole-body net protein balance was more negative for HP than for SP ( P < 0.01). Based on changes in body energy stores, the overall ED was 70% (-1849 +/- 511 kcal/d, no group differences). Consuming an HP diet did not protect FFM during severe ED at HA.