Bone formation and resorption biological markers in cosmonauts during and after a 180-day space flight (Euromir 95)

Long-term spaceflights induce bone loss as a result of profound modifications of bone remodeling, the modalities of which remain unknown in humans. We measured intact parathyroid hormone (PTH) and serum calcium; for bone formation, serum concentrations of bone alkaline phosphatase (BAP), intact osteocalcin (iBGP), and type 1 procollagen propeptide (PICP); for resorption, urinary concentrations (normalized by creatinine) of procollagen C-telopeptide (CTX), free and bound deoxypyridinoline (F and B D-Pyr), and Pyr in a 36-year-old cosmonaut (RTO), before (days -180, -60, and -15), during (from days 10 to 178, n = 12), and after (days +7, +15, +25, and +90) a 180-day spaceflight, in another cosmonaut (ASW) before and after the flight. Flight PTH tended to decrease by 48% and postflight PTH increased by 98%. During the flight, BAP, iBGP, and PICP decreased by 27%, 38%, and 28% respectively in CM1, and increased by 54%, 35%, and 78% after the flight. F D-Pyr and CTX increased by 54% and 78% during the flight and decreased by 29% and 40% after the flight, respectively. We showed for the first time in humans that microgravity induced an uncoupling of bone remodeling between formation and resorption that could account for bone loss.