Calcium metabolism before, during, and after a 3-mo spaceflight: kinetic and biochemical changes

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