Dual-energy X-ray absorptiometry vs underwater weighing comparison of strengths and weaknesses.

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.

A new equation set for converting body density to percent body fat.

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.