The effects of different intensities of exercise and active vitamin D on mouse bone mass and bone strength.

Physical exercise is beneficial to bone health. However, little is known how different intensities of exercise affect bone mass and strength. In the present study, we used young mice to study the effects of different intensities of exercise on bone mass and bone strength in comparison to pharmacological doses of active vitamin D (calcitriol). We found that only the medium level of exercise tested showed a positive effect on bone mineral density, trabecular bone volume, and bone strength, which are attributable to a decrease in bone resorption and an increase in bone formation, with the latter being accompanied by an increase in the number of osteogenic mesenchymal stem cells in the bone marrow. Calcitriol increases bone volume and bone strength, yet the combination of calcitriol and medium-intensity exercise did not further improve bone mass or strength. Moreover, calcitriol also showed some protective effect on the bone in mice with high levels of exercise. These results indicate that exercise at medium intensity increases bone mass and strength via affecting both bone formation and resorption and that its beneficial effects on bone mass cannot be further improved by calcitriol.