Modifications of bone and connective tissue after orthostatic bedrest

Eight male volunteers were submitted to a 6-week anti-orthostatic bedrest t rial followed by a 1-month reambulation period. We prospectively monitored whole-body composition by dual-energy X-ray absorptiometry, bone and connec tive tissue metabolism by biochemical markers and calcium regulating hormon es by 1-84 parathyroid hormone and 1,25-dihydroxyvitamin D-3. Bone mineral density (BMD) did not vary significantly; however, a trend toward an increa se in head BMD and a decrease in trunk, lumbar vertebrae and lower limb BMD was observed. A decrease in the lower limb lean content occurred by day 27 and was maximum by day 42 after the beginning of bedrest; it normalized by day 30 after bedrest, The serum levels of both osteocalcin and C-terminal crosslinked telopeptide of type I collagen increased as a consequence of be drest. A slight increase in the serum levels of the N-terminal propeptide o f type III collagen, a marker of connective tissue metabolism, was observed during the bedrest period. Except for the C-terminal extension propeptide of type I collagen, all markers decreased to baseline preimmobilization lev els during the 1-month recovery phase, Serum PTH and 1,25-dihydroxyvitamin D-3 levels were low during the bedrest period and rose during the reambulat ion phase. These results seem to reflect early changes in bone and connecti ve tissue metabolism as a result of bedrest unloading. but their order of m agnitude remains moderate, thus emphasizing the necessity to perform longer -duration trials.