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One source of error in the underwater weighing (UWW) method of body fat determination is variability in bone mass as a fraction of total nonfat mass. We examined this error theoretically and experimentally using data gathered on 219 human subjects who were measured by UWW and also by the newer technique of dualenergy X-ray absorptiometry (DXA). The experimental data suggest an error was made in the assumed bone mass fraction used in the derivation of the Brozek equation, the standard means of converting the body density values obtained in UWW to body fat percent. Using this new experimental data, a new equation (set) is proposed for use in UWW measurements.

This paper begins with a fundamental description of the dual-energy X-ray absorptiometry (DXA) technique for measurement of bone mineral. It describes how, in extending the technique to do accurate assessment of body fat and lean, it is important that material standards for fat and lean exist, and that a suitable model for fat distribution in the body be developed. The computational steps employed in DXA and in the familiar underwater weighing (UWW) technique are compared and contrasted. Experimental data on over 350 human subjects shows that the percent fat results of DXA and UWW do not agree. However when both methods are used to determine body tissue density, there is good agreement. The authors suggest that the discrepancy may lie with the equations that are used in UWW to compute % fat from body density.

This paper discusses a number of strengths and weaknesses of two methods for determination of body fat, Dual-energy X-ray Absorptiometry (DXA) and underwater weighing (UWW). Several error sources are theoretically quantified. One source of error in the UWW method, variation in bone mass fraction, is examined using data gathered on 219 human subjects who were measured by both methods. The experimental data show the expected linear form but do not exactly match the theoretical curves, indicating that all error sources are not completely understood. The data suggest a possible error in the Brozek equation which is commonly used to compute % fat in UWW.