Body density and potassium 40 measurements of body composition as related to age.

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Abstract
The body fat content of 100 males ranging in age from 15 to 87 years was estimated from body density and K-40 methods. Body density was determined by underwater weighing and measurement of residual volume; K-40 activity was measured by a whole-body 4-pi liquid scintillation counter. Results obtained by the 2 methods agreed well, the correlation being 0.87. The K-40 method, however, gave values that were higher than the corresponding values obtained from body density in 82 of the cases The mean differences between the estimates of body fat obtained from the 2 methods were highly significant. These differences were greater in the older subjects; the relation of age to the magnitude of these differences was statistically significant, It is suggested that this discrepancy depends on an increase with aging in the ratio of proteins low in K, as in connective tissue, to proteins high in potassium, as in muscle. Such replacement would lead to an underestimate of lean body mass by the scintillation counting procedure but would not affect the estimate obtained by densitometry.