Altered biochemical markers of bone turnover in humans during 120 days of bed rest

Microgravity induces significant and progressive bone loss in both humans a nd animals. This is the consequence of disturbed bone remodeling. We perfor med a bed rest experiment to simulate microgravity and tried to clarify bon e metabolism by measuring biochemical markers of bone turnover. Six healthy volunteers participated in 120 days of bed rest. The parameters of calcium homeostasis, calcitropic hormones, and biochemical markers of bone turnove r were examined, After ambulatory control evaluation, all subjects underwen t 120 days of bed rest. Metabolic evaluation was performed in a baseline pe riod, and on days 7, 16, 50, 72, 92, and 108 during bed rest, and on days 1 0 and 25 during a recovery period. Bed rest induced an increase in urinary calcium (Ca) excretion and serum Ca and bone resorption markers. Urine pyri dinoline, deoxypyridinoline, and type I collagen cross-linked N-telopeptide increased more rapidly than urinary Ca excretion and serum Ca. Tartrate-re sistant acid phosphatase (TRAP) increased even in the recovery period, Carb oxy-terminal propeptide of type I collagen, a bone formation marker, signif icantly decreased on days 50, 92, and 108 of bed rest. These changes of bio chemical markers of bone metabolism, except for TRAP, rapidly returned towa rd control levels in the recovery period. Immunoreactive parathyroid hormon e showed a modest decrease during bed rest and a significant increase in th e recovery period. Insulinlike growth factor I (IGF-I) and its binding prot ein, insulinlike growth factor binding protein-3, increased during bed rest , indicating the possibility of resistance to IGF-I in bones under reduced mechanical stress and strain. Bone loss from unloading results from the com bination of acceleration of bone resorption and subsequent retardation of b one formation. (C) 2000 by Elsevier Science Inc. All rights reserved.