Assessment of cellular mass and lean body mass by noninvasive nuclear techniques.

With the nuclear techniques employed in this study, it has been possible to measure total body potassium (TBK), lean body mass (LBM), and body cell mass (BCM). The latter two are derived independently of TBK. LBM is derived from the sum of total body water (TBW), total body protein, and bone mineral ash measurements. The BCM value is, in turn, based on the difference between the LBM (as measured above) and the sum of the extracellular water and extracellular solids. It was demonstrated in 123 normal individuals that, although the TBK/LBM ratio decreases with age, the TBK/BCM ratio does not change significantly with age. It can be deduced that the ratio of TBK to intracellular water should be relatively constant with age. This relative constancy with age was demonstrated independently with the absolute measurements of TBK and intracellular water in normal individuals. Because the BCM is physiologically and chemically more homogeneous than LBM and because it reflects the actively metabolizing cellular compartment more accurately than LBM, it is the preferred parameter to be used for reference or normalization in body composition studies. For most applications, BCM is readily derived from TBK measurement by either whole body counting or isotope dilution techniques.