Potassium content of the fat free body in children.

Previous estimates of body composition in children by measurement of total body potassium (TBK) have employed adult derived values for the ratio of K/fat free body mass (FFBM) to calculate the FFBM. To evaluate the appropriateness of this practice, body composition was determined in 30 children (14 boys, 16 girls) between the ages of 9 and 14 years by hydrostatic weighing, bioelectric impedance, and whole body counting of 40K. Estimates of body fat percentage by TBK measurements were significantly greater (p less than 0.05) than by either the hydrostatic or bioelectric impedance techniques. When current estimates for the FFBM density in children were employed to derive the K/FFBM, the value for boys was 58.9 mEq and for girls, 54.2 mEq. These data indicate that the FFBM will be underestimated in pediatric populations when adult derived values for K/FFBM are used.