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

Abstract

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 x min(-1) (day 0) to 32 (SEM 10) mg x min(-1) (day 8, P = 0.04) and 36 (SEM 8) mg x min(-1) (day 40, P = 0.06). Mean post-absorptive glucose oxidation rose from 126 (SEM 15) mg x min(-1) (day 0) to 164 (SEM 14) mg x min(-1) (day 8, P = 0.04) and 160 (SEM 20) mg x 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 x day(-1) to 1589 (SEM 42) kcal x day(-1) (P = 0.056). Changes in REE were accounted for by changes in FFM. Mean energy intake decreased from 2532 (SEM 43) kcal x day(-1) to 2237 (SEM 50) kcal x 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.

The effect of inactivity on dietary intake and energy homeostasis

“…Demaria-Pesce et al (1992) showed a reduction in spontaneous intake in three subjects during 28d bed-rest in an HDT position. We have observed similar findings during 42 d bed-rest in the study mentioned previously (Ritz et al 1998). While this latter study was not specifically designed to examine in detail appetite sensations and feeding behaviour, a number of interesting patterns emerged.…”

Reduced whole-body fat oxidation in women and in the elderly

“…10 Usual physical activity modulates resting fat oxidation, as it is higher in very active young and elderly persons than in sedentary ones. 7,9,10 Furthermore, resting fat oxidation is reduced by bed-rest 12 and increased by endurance training in young 13 ± 15 and elderly subjects. 16 ± 18 Since their usual physical activity is reduced, 19 elderly persons are expected to show a lower fat oxidation than young subjects.…”

“…Indeed sleeping fat oxidation represents about 24% of daily fat oxidation. 18 The remaining 76% can be in¯uenced by physical activity (since for low-intensity exercise fat is the main fuel 37 ) and by meals (since carbohydrates decrease fat oxidation, 12,38 ). Sial et al 10 have shown that, when compared to young subjects, elderly men (or women) rely more on fat than on glucose for energy production during exercise.…”

Detrimental Effects Of Inactivity On Insulin Action

“…After 7 days of head-down bed rest, fasting lipid oxidation was reduced by ~80% in both men and women (14). Similarly, in a study by Ritz et al (127), fat oxidation during fasting was significantly lower and carbohydrate oxidation higher following long-term simulated microgravity (42 days). In addition, both fasting and insulin-stimulated levels of lipogenesis were significantly increased in women following 7 days of bed rest (14).…”