Energy and substrate metabolism during a 42-day bed-rest in a head-down tilt position in humans

Microgravity-induced changes in body composition (decrease in muscle mass and increase in fat mass) and energy metabolism were studied in seven healthy male subjects during a 42-day bed-rest in a head-down tilt (HDT) position. Resting energy expenditure (REE), fat and glucose oxidation were estimated by indirect calorimetry on days 0, +8 and +40 of the HDT period. Assessments were performed both in post-absorptive conditions and following two identical test meals given at 3-h intervals. Body composition (dual x-ray absorptiometry) was measured on days 0, +27, +42. Mean post-absorptive lipid oxidation decreased from 53 (SEM 8) mg · min−1 (day 0) to 32 (SEM 10) mg · min−1 (day 8, P=0.04) and 36 (SEM 8) mg · min−1 (day 40, P=0.06). Mean post-absorptive glucose oxidation rose from 126 (SEM 15) mg · min−1 (day 0) to 164 (SEM 14) mg · min−1 (day 8, P=0.04) and 160 (SEM 20) mg · min−1 (day 40, P=0.07). Mean fat-free mass (FFM) decreased between days 0 and 42 [58.0 (SEM 1.8) kg and 55.3 (SEM 1.7) kg, P<0.01] while fat mass increased without reaching statistical significance. The mean REE decreased from 1688 (SEM 50) kcal · day−1 to 1589 (SEM 42) kcal · day−1 (P=0.056). Changes in REE were accounted for by changes in FFM. Mean energy intake decreased from 2532 (SEM 43) kcal · day−1 to 2237 (SEM 50) kcal · day−1 (day 40, P<0.01) with only a minor decrease in the proportion of fat. We concluded that changes in fat oxidation at the whole body level can be found during HDT experiments. These changes were related to the decrease in FFM and could have promoted positive fat balance hence an increase in fat mass.