Critical appraisal of the estimation of body composition via two-, three-, and four-compartment models.

This review explores the robustness of the assumptions underpinning the two- (fat mass [FM], fat-free mass [FFM]), three- (FM; total body water [TBW], fat free dry solid), and four- (FM; TBW; bone mineral [BM], residual) compartment models of body composition. The measurement of body density (BD) via underwater weighing (UWW) and TBW via isotopic dilution are the two most frequently used two-compartment techniques. The former assumes that the FM and FFM have densities of 0.9007 g/cm(3) and 1.1000 g/cm(3), respectively, while the latter uses a FFM hydration constant. Although both techniques can estimate body composition precisely (technical error of measurement [TEM]: UWW = 0.4 %BF; TBW = 0.6 %BF), the validity of these estimates is adversely affected by biological variability in the assumed percentages for the FFM components (TBW = 73.72%; protein = 19.41%; BM = 5.63%; non-BM = 1.24%). The three-compartment model, which incorporates measures of BD and TBW, greatly increases validity by removing errors relating to variability in TBW, which comprises the largest percentage of the FFM and is furthermore acutely variable. The four-compartment model marginally improves on the three-compartment model by additionally controlling for BM, which displays less variability than the TBW component of the FFM. The three- and four-compartment models therefore provide more valid estimates of body composition than the two-compartment model, and this increased accuracy is not offset by propagation of errors (TEM = 0.7 %BF for both models) from the combinations of multiple measurements (BD, TBW, BM). Am. J. Hum. Biol. 11:175-185, 1999. Copyright 1999 Wiley-Liss, Inc.