Relative contributions of age and menopause to the vertebral bone density of healthy Japanese women

The relative contributions of age and menopause to vertebral bone mineral density were evaluated based on the estimated weights for age- and menopause-related bone loss components using a mathematical model in 177 healthy female volunteers ages 35-81 years, living in a community in Fukui, Japan. Bone mineral density was determined by dual X-ray absorptiometry. The model used was that which afforded the best fit among the eight possible models to the data observed. Each model was composed of a linear function for the age-related component and a different type of function for the menopausal component, without interaction between them. The weights for these components in each model were estimated by the least-squares method. The coefficient of determination and Akaike information criterion disclosed that among the eight models tested, the model affording the best fit was composed of a logarithmic decrease in bone density with an increase in years since menopause, up to 10 years postmenopausal, with no further decline thereafter. In this model, the weights for both components were statistically significant and the type III sum of squares of the menopausal component was greater than that of the age-related component. We suggest that both age and menopause made significant contributions to the decline in vertebral bone mineral density, with the contribution of menopause being greater than that of age.