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The loss of bone during spaceflight is considered a physiological obstacle for the exploration of other planets. This report of calcium metabolism before, during, and after long-duration spaceflight extends results from Skylab missions in the 1970s. Biochemical and endocrine indexes of calcium and bone metabolism were measured together with calcium absorption, excretion, and bone turnover using stable isotopes. Studies were conducted before, during, and after flight in three male subjects. Subjects varied in physical activity, yet all lost weight during flight. During flight, calcium intake and absorption decreased up to 50%, urinary calcium excretion increased up to 50%, and bone resorption (determined by kinetics or bone markers) increased by over 50%. Osteocalcin and bone-specific alkaline phosphatase, markers of bone formation, increased after flight. Subjects lost approximately 250 mg bone calcium per day during flight and regained bone calcium at a slower rate of approximately 100 mg/day for up to 3 mo after landing. Further studies are required to determine the time course of changes in calcium homeostasis during flight to develop and assess countermeasures against flight-induced bone loss.

A decrease of spongy tissue strength of iliac bone has been identified in healthy men under conditions of simulated weightlessness (bed rest). Countermeasures have significantly prevented this decrease. Additional investigations have indicated probability of degenerative-destructive bone change development under conditions of usual human activity as a consequence of bone strength decrease.

Calcium metabolism, its hormonal regulation and renal calciuretic function were investigated in 20 healthy volunteers during 120- and 182-day head-down tilt studies. The content of total and ionized calcium and parathyroid hormone was found to increase during the study. The concentration of calcitonin increased at an early stage of exposure and decreased thereafter. This resulted in negative calcium balance which was on the average 5, 6g per month. Measurements of iliac bone biopsies showed calcium decrease and phosphorus increase.