Whole-body skeletal muscle mass: development and validation of total-body potassium prediction models

Background: A substantial proportion of total body potassium (TBK) in humans is found in skeletal muscle (SM), thus affording a means of predicting total-body SM from whole-body counter-measured K-40. There are now > 30 whole-body counters worldwide that have large cross-sectional and longitudinal TBK databases. Objective: We explored 2 SM prediction approaches, one based on the assumption that the ratio of TBK to SM is stable in healthy adults and the other on a multiple regression TBK-SM prediction equation. Design: Healthy subjects aged greater than or equal to 20 y were recruited for body-composition evaluation. TBK and SM were measured by whole-body K-40 counting and multislice magnetic resonance imaging, respectively. A conceptual model with empirically derived data was developed to link TBK and adipose tissue-free SM as the ratio of TBK to SM. Results: A total of 300 subjects (139 men and 161 women) of various ethnicities with a mean (+/-SD) body mass index (in kg/m(2)) of 25.1 +/- 5.4 met the study entry criteria. The mean conceptual model-derived TBK-SM ratio was 122 mmol/kg, which was comparable to the measurement-derived TBK-SM ratios in men and women (119.9 +/- 6.7 and 118.7 +/- 8.4 mmol/kg, respectively), although the ratio tended to be lower in subjects aged greater than or equal to 70 y. A strong linear correlation was observed between TBK and SM (r = 0.98, P < 0.001), with sex, race, and age as small but significant prediction model covariates. Conclusions: Two different types of prediction models were developed that provide validated approaches for estimating SM mass from K-40 measurements by whole-body counting. These methods afford an opportunity to predict SM mass from TBK data collected in healthy adults.