Effects of artificial gravity during bed rest on bone metabolism in humans

Abstract

We report results from a study designed to explore the utility of artificial gravity (AG) as a countermeasure to bone loss induced by microgravity simulation. After baseline testing, 15 male subjects underwent 21 days of 6 degrees head-down bed rest to simulate the deconditioning associated with spaceflight. Eight of the subjects underwent 1 h of centrifugation (AG; 1 G(z) at the heart, 2.5 G(z) at the feet) each day for 21 days, whereas seven of the subjects served as untreated controls (Con). Blood and urine were collected before, during, and after bed rest for bone marker determinations. Bone mineral density (BMD) and bone mineral content (BMC) were determined by dual-energy X-ray absorptiometry and peripheral quantitative computerized tomography before and after bed rest. Urinary excretion of bone resorption markers increased during bed rest, but the AG and Con groups did not differ significantly. The same was true for serum C-telopeptide. During bed rest, bone alkaline phosphatase (ALP) and total ALP tended to be lower in the AG group (P = 0.08, P = 0.09). Neither BMC nor BMD changed significantly from the pre-bed rest period in AG or Con groups, and the two groups were not significantly different. However, when AG and Con data were combined, there was a significant (P < 0.05) effect of time for whole body total BMC and total hip and trochanter BMD. These data failed to demonstrate efficacy of this AG prescription to prevent the changes in bone metabolism observed during 3 wk of bed rest.

Fig. 1.

Cumulative calcium balance of control (Con) and artificial gravity (AG) subjects before, during, and after 21 days of bed rest. This was calculated for each individual as the difference between dietary intake of calcium and the combined urinary and fecal excretion of calcium. Cumulative balance was “restarted” with each phase of the study (before, during, and after bed rest; break points are indicated by vertical dotted lines), so that balance calculations are cumulative for days BR−9 through BR1 (before), BR2 through BR21 (during), and BR+0 through BR+7 (after bed rest). Values are means ± SD; n = 6 Con subjects and 8 AG subjects. Dietary intake methodology is described in a companion article (54). There were no differences between the 2 groups, but time had a significant effect. Calcium balance for BR11 through BR21 was significantly more negative than before bed rest (P < 0.001).