Does Reduced Skeletal Loading Account for Age-Related Bone Loss?

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Abstract
A leading theory suggests that decreasing activity and muscle mass is the main cause of age-related bone loss. However, in a population-based study of 375 women and 325 men (age, 21-97 years), we failed to find a close correspondence between these variables and changes in bone strength with aging. It has been suggested that bone strength is homeostatically adapted to habitual skeletal loading conditions and that bone loss could, therefore, result simply from age-related reductions in physical activity and muscle mass, but this notion has not been explored in detail. In a stratified random sample of Rochester, MN, women and men 21-97 years of age, indices of bone strength, flexural rigidity (EI), and axial rigidity (EA) were estimated from central QCT measurements at the femoral neck and lumbar spine and pQCT measurements at the ultradistal radius, whereas habitual skeletal loading was assessed using lean body mass, total skeletal muscle mass (TSM), body weight, and physical activity. Using regression analysis, we tested the hypothesis (H(o)) that bone strength per unit load did not vary with age. In these cross-sectional data, the null hypothesis of no age-related change was rejected in 72% of the strength-to-load ratios tested. For example, the ratio of femoral neck EI to TSM increased by 0.19%/year in women (p = 0.008) and by 0.49%/year in men (p < 0.001). There was no close correspondence between changes in habitual load and changes in bone strength, nor any consistent pattern. Moreover, interindividual variation in the strength-to-load ratios was substantial. These data suggest that the notion of reduced skeletal loading as the primary basis for age-related bone loss is oversimplified.