Energy, protein, and substrate metabolism in simulated microgravity

Abstract

Whole body protein turnover and energy expenditure before and during an oral glucose tolerance test (1 g/kg body wt) were studied on separate occasions in six healthy young men before and during 3 days of simulated microgravity using the 6 degrees head-down tilt (HDT) method. After 42-47 h of HDT, basal insulin concentrations increased significantly from 9.4 +/- 1.9 to 13.1 +/- 0.1 microU/ml (P < 0.002). No significant differences in glycemia, insulinemia, or free fatty acid concentrations were observed in response to the oral glucose load. With HDT, increases were observed in basal postabsorptive resting metabolic rate (8%; P < 0.05), lipid oxidation (33 +/- 2 to 51 +/- 5 mg/min; P < 0.02), and the thermic effect of glucose (7.7 +/- 1 to 10.7 +/- 0.6%; not significant). Protein turnover (arithmetic mean of ammonia and urea flux rates) was unchanged by HDT, but a significant increase was seen when calculated from ammonia alone (P < 0.02). The present data show that HDT results in an increased energy requirement through elevations in both the basal metabolic rate and the thermic response to food ingestion. These changes may have been brought about by a cephalic shift of body fluids similar to that experienced in microgravity.

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

“…It has been reported that the increase in non protein RQ also corresponds to an increase in glucose oxidation which is associated with increased insulinaemia without overt changes in plasma glucose concentration (Lane 1992;Acheson et al 1995). Unloading has been found to promote glucose oxidation in muscles (Baldwin 1996).…”

“…During bed-rest, it has been found that energy intake is either decreased (Demaria-Pesce et al 1992) or unchanged (Gretebeck et al 1995). There has been discrepancy in the literature about whether resting energy expenditure (REE) is decreased (in most cases, by 2% to 22%; review in Greenleaf 1989) unchanged (review in Greenleaf 1989) or increased (Acheson et al 1995) as a consequence of microgravity. The FFM is the most important determinant of REE and therefore a decrease in REE would seem logical in line with the decline in FFM.…”

Detrimental Effects Of Inactivity On Insulin Action

“…exercise restriction), comes from bed-rest studies with or without head-down tilt to mimic the effects of antigravity. Significant increases in fasting insulin concentrations were observed after bed rest (1,8,137,144), suggesting a decrease in insulin action, although this has not been a consistent finding (32,90,116 increased insulin response to an oral glucose load as a result of sustained bed rest in healthy subjects (14,32,116,137). Both outcomes are indicative of reduced insulin action, which is usually attributed to a decreased peripheral glucose uptake (90,100,137).…”

“…However, not all studies suggest altered substrate oxidation following inactivity. Acheson et al (1) reported that in response to 3 days of bed rest with head-down tilt, fat oxidation during fasting was significantly increased, while carbohydrate oxidation in response to a glucose load was unchanged. Despite this inconsistent finding, the majority of the human studies support the hypothesis that inactivity shifts fasting substrate metabolism toward increased reliance on carbohydrate utilization and reduced reliance on fat oxidation and impairs insulin-stimulated glucose oxidation and suppression of lipolysis, all of which are characteristic features of metabolic inflexibility.…”

Are energy metabolism alterations involved in cardiovascular deconditioning after weightlessness? An hypothesis

“…Krebs et al have shown that the body weight of subjects consuming a constant diet for 6 weeks before and 5 weeks during horizontal bed rest does not change, although lean body mass decreased and body fat mass increased (both significantly) [30]. A short microgravity simulation (3 days) has shown that HDBR increases the post-absorptive metabolic rate by 8% and that this is linked to decreased glucose oxidation and increased lipid oxidation [1]. Gretebeck et al [20] have used the doubly labelled water method to investigate total energy expenditure (TEE) during a 10-day HDBR (with a -6°tilt).…”