Changes in body chemical composition with age measured by total-body neutron activation.

Total-body levels of calcium and phosphorus (reflecting skeletal mass) and total-body levels of potassium (reflecting muscle mass) were measured by neutron activation analysis in 39 men and 40 women ages 30-90 yr. In order to intercompare the total body calcium (TBCa) values in a heterogeneous population, such as this, it was necessary to normalize the data for skeletal size. The normalization consisted of dividing the absolute calcium level by the predicted calcium level for each individual matched to a set of critical parameters. The parameter used in the computation of normal values were age, sex, muscle mass, i.e., total body potassium (TBK) and height. For the calcium data of the women, it was necessary to add an age correction factor after the age of 55 yr. The calcium ratio(mean ratio of the predicted to measured TBCa) in men was 1.000 +/- 7.8% and in women 0.996 +/- 7.1%. The TBCa of normal males and females can thus be predicted to +/- 13% (at the 90% confidence level). An exception to this was found in males (70-90 yr) who exhibited a mean calcium ratio greater than 1.13. The derivative of TBCa with time was determined for this population of men and women by taking into account the dependency of calcium on three time dependent variables, height, TBK, and an explicit age correction factor in the case of the women. The mean rate of loss of TBCa in women was 0.37% and 1.1% per year before and after menopause (50 yr). In the males, the average rate of loss of TBCa was 0.7% per year after 50 yr of age. The pattern of total body phosphorus (TBP) loss with age paralleled that of TBCa as the ratio of TBP/TBCa was rather constant with age. The constancy of the ratio suggests that the mineral composition of bone does not change significantly with age. The rate of loss of TBK with age was also related directly to that of TBCa. The mean ratio of TBK/TBCa was 9.9 in females and 8.0 in males and this ratio remained relatively constant from 30-70 yr. Thus, the mechanism responsible for the loss of bone with age, whether nutritional deficiency or decreased gonadal function and physical activity may also be responsible for the loss of muscle mass with age.