Magnitude and variation of ratio of total body potassium to fat-free mass:a cellular level modeling study

Potassium is an essential element of living organisms that is found almost exclusively in the intracellular fluid compartment. The assumed constant ra tio of total body potassium (TBK) to fat-free mass (FFM) is a cornerstone o f the TBK method of estimating total body fat. Although the TBK-to-FFM (TBK /FFM) ratio has been assumed constant, a large range of individual and grou p values is recognized. The purpose of the present study was to undertake a comprehensive analysis of biological factors that cause variation in the T BK/FFM ratio. A theoretical TBK/FFM model was developed on the cellular bod y composition level. This physiological model includes six factors that com bine to produce the observed TBK/FFM ratio. The ratio magnitude and range, as well as the differences in the TBK/FFM ratio between men and women and v ariation with growth, were examined with the proposed model. The ratio of e xtracellular water to intracellular water (E/I) is the major factor leading to between-individual variation in the TBK/FFM ratio. The present study pr ovides a conceptual framework for examining the separate TBK/FFM determinan ts and suggests important limitations of the TBK/FFM method used in estimat ing total body fat in humans and other mammals.