Exercise Effects on Bone Mineral Density: Relationships to Changes in Fitness and Fatness Original Research Article

Background Weight loss typically reduces bone mineral density (BMD). Exercise may preserve or increase BMD even while reducing fatness. We examined the relationships among exercise-induced changes in fitness and fatness with BMD. Design Randomized controlled trial conducted between July 1999 and November 2003. Participants Men and women (n =115) aged 55 to 75 years. Intervention Six months of exercise training. Main Outcome Measures Fitness measured as peak oxygen uptake and muscle strength, body composition by anthropometry, dual-energy x-ray absorptiometry, and magnetic resonance imaging. Results A total of 51 men and 53 women completed the trial. Exercise increased aerobic and strength fitness and lean body mass, and reduced general and abdominal obesity. BMD did not change among men in either group. Among women exercisers, there were reductions in total skeleton BMD (p =0.02) and greater trochanter BMD (p =0.02). By bivariate correlation, among women, increased femoral neck BMD was associated with increased aerobic fitness (p =0.01) and with reduced body weight (p =0.02) and BMI (p =0.02). In the final regression model, 13% of the change in femoral neck BMD was explained by the change in aerobic fitness (p <0.01). Among the men, increased total hip BMD and femoral shaft BMD were associated with increased lean mass and lower-body strength. In the final regression models, the change in lean mass explained 9% of the variance in total hip BMD (p =0.04). The change in lean mass explained 20% of the change in femoral shaft BMD (p <0.01), and the change in lower-body strength explained an additional 6% (p <0.04). Conclusions When examined by group assignment, 6 months of exercise had no effect on BMD among men, and reduced BMD among women. When examined by change in fitness and fatness, women who had the greatest increases in aerobic capacity and men who had the greatest increases in strength and lean mass were more likely to increase their BMD. Exercise-induced reductions in fatness did not lead to bone loss.