Space flight is associated with rapid decreases of undercarboxylated osteocalcin and increases of markers of bone resorption without changes in their circadian variation: observations in two cosmonauts.

Background: Microgravity induces bone loss by mechanism(s) that remain largely unknown.
Methods: We measured biochemical markers related to bone remodeling in two cosmonauts before, during, and after 21- and 180-day space flights, respectively.
Results: During both flights, type I procollagen propeptide and bone alkaline phosphatase decreased as early as 8 days after launch. Undercarboxylated osteocalcin percentage increased early and remained high during both flights. Vitamin K 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 in serum than in urine. Pyridinoline, free deoxypyridinoline, and N-telopeptide increased less than CTX during the short-term space flight. The circadian rhythm of bone resorption assessed by urine CTX and free deoxypyridinoline was not altered by microgravity.
Conclusion: Vitamin K metabolism or action and bone remodeling may be altered in cosmonauts.