COMPARISON OF BODY-FAT ESTIMATES DERIVED FROM UNDERWATER WEIGHT AND TOTAL-BODY WATER

The purpose of this study was to compare the techniques of underwater weighing and analysis of total body water for estimating body fat with that derived from a 3-compartment model combining total body density and total body water. Body fat was estimated from: (1) body density using the Siri equation; (2) total body water assuming a hydration factor of 0.73 for fat free mass; and (3) the 3-compartment model combining body density and body water. The criterion method in this study was body fat mass determined from Siri's 3-compartment model. The subjects in this study included 10 obese men (41 +/- 10 years; 115.5 +/- 16.6 kg), 7 elderly men (68 +/- 6 years, 77.1 +/- 7.4 kg), and 18 young men (24 +/- 5 years; 71.0 +/- 9.2 kg). Body density was obtained by underwater weight with simultaneous determination of lung volume by helium dilution. Body water was obtained from zero-time extrapolation of isotope washout over 10-14 days following an oral dose of (H2O)-H-2 and (H2O)-O-18. Estimates of body fat derived from total body water were significantly higher than that derived from underwater weight in the elderly (1.4 +/- 1.5 kg; P < 0.05) and younger (3.0 +/- 2.8 kg; P < 0.001) men. In the obese subjects, there was no significant difference in body fat estimates between the two techniques (mean difference = 1.3 +/- 5.1 kg). The overestimation of body fat from total body water relative to underwater weight was negatively associated with body fat mass estimated from the 3-compartment model combining measurement of density and water (r = -0.35; P = 0.04). It is concluded that total body water overestimates body fat relative to underwater weight, and that this effect is more pronounced in leaner subjects.