Sex-specific responses of bone metabolism and renal stone risk during bed rest

The purpose of this study was to directly assess sex differences in bone loss, bone biochemistry, and renal stone risk in bed rest. Bed rest simulates some spaceflight effects on human physiology and can be used to address the potential existence of sex-specific effects on bone metabolism and renal stone risk in space. We combined data from the control subjects in five head-down-tilt bed rest studies (combined n = 50 men, 24 women) of differing durations (14-90 days). All subjects were healthy volunteers. Mean age was 35 +/- 9 years for women and 33 +/- 8 years for men. The main outcome measures were bone density and biochemistry, and renal stone risk chemistry. Before bed rest began, men had higher bone mineral density and content (P < 0.001), and excreted more biomarkers of bone resorption and calcium per day than did women (P < 0.05). These differences remained during bed rest. A number of urine chemistry analytes increased (e.g., calcium) or decreased (e.g., sodium, citrate, and urine volume) significantly for men and women during bed rest. These changes may predispose men to higher stone risk. Men and women do not have substantially different responses to the skeletal unloading of bed rest.