Energy expenditure in rhesus monkeys (Macaca mulatta) during spaceflight using doubly labeled water (²H₂¹⁸O)

The mean daily energy expenditure rates of three rhesus monkeys (Macaca mulatta) were determined during spaceflight on the joint US-Russian Cosmos 2044 and 2229 missions by the doubly labeled water (²H₂¹⁸O) method. In- flight values were compared with ground data obtained from seven measurements taken from six chair-adapted control monkeys. The mean energy expenditure for the ground control determinations was 94.5 ± 6.4 kcal · kg^-1 · day^-1 (n = 6). The mean in-flight energy expenditure, 55.1 ± 8.0 kcal · kg^-1 · day^-1 (n = 3), was significantly less than the mean ground control value (P < 0.05). These data suggest that energy expenditure in restrained rhesus monkeys is significantly reduced during spaceflight.