Biochemical parameters of bone turnover during ten days of bed rest and subsequent mobilization

Immobilization is associated with increased bone resorption. To investigate the early onset of increased bone resorption, we evaluated 14 patients who were immobilized for 10 days because of lumbar disc protrusion. The fasting urinary hydroxyproline/ creatinine ratio increased significantly after four days ( P < 0.01), reached a peak after 10 days (16.4 ± 3.3 μmol/mmol, 27.6 ± 8.8 μmol/mmol, P < 0.01) and returned slowly to baseline values after mobilization. The fasting urinary calcium/creatinine ratio followed a similar pattern. Serum calcium and phosphate increased during immobilization ( P < 0.01). Serum 1,25-dihydroxyvitamin D decreased significantly during immobilization (98 ± 33 pmol/l vs. 79 ± 36 pmol/l, P < 0.05), and reached a nadir one week after mobilization ( P < 0.01). We conclude that there is an early significant increment in resorption parameters, with a slow return during the mobilization period. Serum 1,25-dihydroxyvitamin D is suppressed as a result of the increased serum calcium and serum phosphate levels.