Space flight is associated with rapid decreases of undercarboxylated osteocalcin and increases of markers of bone resorption without changes in theircircadian variation: Observations in two cosmonauts

Background: Microgravity induces bone loss by mechanism(s) that remain larg ely unknown. Methods: We measured biochemical markers related to bone remodeling in two cosmonauts before, during, and after 21- and 180-day space flights, respect ively. Results: During both flights, type I procollagen propeptide and bone alkali ne phosphatase decreased as early as 8 days after launch. Undercarboxylated osteocalcin percentage increased early and remained high during both fligh ts. Vitamin It supplementation restored carboxylation of osteocalcin during the long-term flight. Urinary and serum C-telopeptide of type I collagen ( CTX) increased as early as day 8 of the flights; the increase was greater i n serum than in urine. Pyridinoline, free deoxypyridinoline, and N-telopept ide increased less than CTX during the short-term space flight. The circadi an rhythm of bone resorption assessed by urine CTX and free deoxypyridinoli ne was not altered by microgravity. Conclusion: Vitamin K metabolism or action and bone remodeling may be alter ed in cosmonauts. (C) 2000 American Association for Clinical Chemistry.