Deconditioning increases bone resorption and decreases bone formation in the rat.

The response of calcium metabolism and bone turnover to deconditioning after exercise training was studied in three groups of Sprague-Dawley female rats: age-matched controls, 12-week treadmill training, and 8-week treadmill training followed by 4-week discontinuation of training. The exercised rats had a higher absorption efficiency of calcium that decreased to control levels as early as 2 weeks following discontinuation of training. Urinary calcium excretion increased by 2 weeks of deconditioning and then reverted to control levels; bone mineral content measurements declined following deconditioning. Tracer uptake of 45Ca in the femur demonstrated an increase in bone formation that was present during exercise, but was not evident following 4 weeks of deconditioning. The urinary excretion of prelabeled 3H-tetracycline was used to investigate bone resorption in two groups of exercised rats; one group continued to exercise and in the other group the exercise was discontinued. Bone resorption increased within 4 to 11 days after deconditioning and returned to basal levels by the 10th day, at which time the net bone retention of 3H-tetracycline was 86% of that of the group that continued exercising. We conclude that exercise must be continued to sustain any gain it produces in bone mineral. Bone mass gained through exercise will be lost during deconditioning as a result of a decline in bone formation and an increase in bone resorption.