Abstract

Weight-bearing exercise and calcium intake are known to contribute to bone density. However, the relative significance of physical activity and calcium intake in the development of bone characteristics in functionally different weight-bearing and nonweight-bearing bone sites at different ages is poorly known. A total of 422 women in three age groups (25-30, 40-45, and 60-65 years) were screened from 1017 women and divided into four groups by their level of physical activity (high [PA+] and low [PA-]) and calcium intake (high [Ca+] and low [Ca-]). Total body bone mineral content (TBBMC), areal bone mineral density (BMD) of the femoral neck and distal radius, and selected dimensions and estimated strength variables (bone width, cortical wall thickness, cross-sectional moment of inertia, and section modulus of the femoral and radial shafts) were measured with dual-energy X-ray absorptiometry. Both high physical activity and high calcium intake were associated with a higher TBBMC when compared with low activity and calcium intake (1.8% and 4.6%, respectively). The BMD of the weight-bearing femoral neck was 5% higher in the PA+ groups than in the PA- groups, whereas calcium intake showed no such significant association. Neither physical activity nor calcium intake was associated with the BMD of the nonweight-bearing radius. However, both high physical activity and high calcium intake were related to larger and mechanically more competent bones in the femoral and radial shafts, the association for physical activity being stronger with increasing age. No significant interaction between physical activity and calcium intake was found with respect to any of the bone variables. These data from a cross-sectional study suggest that a moderate level of physical activity or a sufficient level of calcium intake, if maintained from childhood, can result in considerable long-term improvement in the mechanical competence of the skeleton. The clinical relevance of these findings is further emphasized by the fact that the observed patterns of physical activity and calcium intake pertain to customary lifestyle and are thus feasible targets for the primary prevention of osteoporosis.