Effect of aging on bone mass in adult women.

Total-body calcium was measured in 40 adult women by total-body neutron activation analysis (TBNAA). Procedures for normalizing the absolute calcium measurements for the parameters of size and age were developed in order to effect a direct comparison of women of age 30-78 yr. The normal total-body calcium (TBCa) for an individual can be predicted by a formula developed in the present study to within +/- 11% (1.62 SD) at the 90% confidence level. The TBCa loss can be characterized by two components: one with a slower rate, 0.37%/yr, and the other with a faster rate, 1.08%/yr. The latter, a more rapid postmenopausal loss, started at 50-60 yr and was superimposed on the slower rate of loss that started in the fourth decade and continued throughout life. The bone mineral content (BMC) of the radius, measured by the absorptiometric technique, correlated well with the total-body skeletal calcium in this population (r = 0.813, P less than .001). However, for intercomparisons of the BMC values of individuals, normalization of the BMC values for size and age is required, as it is for the TBCa data. Normalization provided by the ratio of BMC to radius width is not adequate for comparative studies.