Comparison of estimates of fat-free mass in normal and obese women from measurements of body potassium, body water and body density.

The relation between the estimates of fat-free mass (FFM) obtained from measurements of density, total body potassium and water was investigated in 104 women with Quetelet's Index between 19.9 and 49.7 kg/m2, body weight between 42.3 and 133.5 kg. The mean difference between the estimates based on water and potassium, previously reported as 2.2 kg or 4.4 per cent, was found to be independent of Quetelet's Index. The mean difference (s.e.m.) between the estimates of FFM based on density, compared with the average values from water and potassium, was found to be +7.0 (1.0), +6.3 (0.7) and +7.4 (1.1) kg in the ranges of Quetelet's Index 19.9-30.0, 30.1-40.0, and 40.1-49.7, respectively. The relative independence on Quetelet's Index of the difference between the estimates based on density compared with (water + potassium) is consistent with the idea that tissues of a higher density (ie, skin and bone) make up a larger proportion of the fat-free body weight in thin subjects than in fat ones.