Long-duration space flight and bed rest effects on testosterone and other steroids.

CONTEXT Limited data suggest that testosterone is decreased during space flight, which could contribute to bone and muscle loss. OBJECTIVE The main objective was to assess testosterone and hormone status in long- and short-duration space flight and bed rest environments and to determine relationships with other physiological systems, including bone and muscle. DESIGN Blood and urine samples were collected before, during, and after long-duration space flight. Samples were also collected before and after 12- to 14-d missions and from participants in 30- to 90-d bed rest studies. SETTING Space flight studies were conducted on the International Space Station and before and after Space Shuttle missions. Bed rest studies were conducted in a clinical research center setting. Data from Skylab missions are also presented. PARTICIPANTS All of the participants were male, and they included 15 long-duration and nine short-duration mission crew members and 30 bed rest subjects. MAIN OUTCOME MEASURES Serum total, free, and bioavailable testosterone were measured along with serum and urinary cortisol, serum dehydroepiandrosterone, dehydroepiandrosterone sulfate, and SHBG. RESULTS Total, free, and bioavailable testosterone was not changed during long-duration space flight but were decreased (P < 0.01) on landing day after these flights and after short-duration space flight. There were no changes in other hormones measured. Testosterone concentrations dropped before and soon after bed rest, but bed rest itself had no effect on testosterone. CONCLUSIONS There was no evidence for decrements in testosterone during long-duration space flight or bed rest.