Purpose: This study tested the hypothesis that hypoxia exacerbates reductions in body mass observed during unloading.
Methods: To discern the separate and combined effects of simulated microgravity and hypoxia, 11 healthy males underwent three 21-day campaigns in a counterbalanced fashion: (1) normoxic bed rest (NBR; FiO₂ = 0.209; PiO₂ = 133.1 ± 0.3); (2) hypoxic ambulatory confinement (HAMB; FiO₂ = 0.141 ± 0.004; PiO₂ = 90.0 ± 0.4; ~4,000 m); and (3) hypoxic bed rest (HBR; FiO₂ = 0.141 ± 0.004; PiO₂ = 90.0 ± 0.4). The same dietary menu was applied in all campaigns. Targeted energy intakes were estimated individually using the Harris-Benedict equation taking into account whether the subjects were bedridden or ambulatory. Body mass and water balance were assessed throughout the campaigns. Whole body and regional body composition was determined before and after the campaigns using dual-energy X-ray absorptiometry. Before and during the campaigns, indirect calorimetry and visual analogue scores were employed to assess the resting energy expenditure (REE) and perceived appetite sensations, respectively.
Results: Energy intakes were lower than targeted in all campaigns (NBR: -5%; HAMB: -14%; HBR: -6%; P < 0.01). Body mass significantly decreased following all campaigns (NBR: -3%; HAMB: -4%; HBR: -5%; P < 0.01). While fat mass was not significantly altered, the whole body fat free mass was reduced (NBR: -4%; HAMB: -5%; HBR: -5%; P < 0.01), secondary to lower limb fat-free mass reduction. Water balance was comparable between the campaigns. No changes were observed in REE and perceived appetite.
Conclusions: Exposure to simulated altitude of ~4,000 m does not seem to worsen the whole body mass and fat-free mass reductions or alter resting energy expenditure and appetite during a 21-day simulated microgravity.