University of Aberdeen, University Medical School, Foresthill, Aberdeen, AB9 2ZD
There is much interest in the measurement of body fat content in view of the well established association between an increased degree of fatness and a variety of disease processes. Direct measurements of fat content are not possible except after death, and in humans, only a limited number (n=10) of such measurements have been made. A variety of indirect methods have been proposed; of these, hydrostatic weighing, which measures body density, is generally recognized as the most “accurate” method, but this may be misleading as none of these indirect methods has ever been validated directly (Martin and Drinkwater, 1991).
This study compared three methods:
1) Hydrostatic weighing, which is based on assumptions regarding the constancy of the density of the fat and lean components of the body.
2) Electrical impedance, which is based on differences in the impedance of the tissues to a high frequency current; this assumes a constant water content of the non-fat tissue mass, and uses equations derived from comparisons between the method and hydrostatic weighing.
3) Skinfold thickness, which again uses equations derived from comparisons between skinfold thickness and body fat content estimated from body density.
Hydrostatic weighing was performed as described by Pollock & Wilmore (1990) with residual volume being measured by oxygen rebreathing. Electrical impedance was measured using a Bodystat 500 analyser (Bodystat Ltd, Douglas, Isle of Man). Body fat was estimated from skinfold thickness using the method and equations of Durnin and Rahaman (1967). Twenty eight subjects (10 male, 18 female) were used; mean (sem) age was 32±2y, height 167±2cm and body weight 64.8±2.0 kg. mean body fat content, expressed as a percentage of body weight, estimated by the three methods was 22.7% by hydrostatic weighing, 21.2% by impedance and 23.3% by skinfold thickness.
Since both the skinfold thickness equations and those used in the impedance measurements are based on comparisons with hydrostatic weighing, albeit on different subject populations, it is hardly surprising that correlations among the methods are highly significant (Figures 1,2).
These results suggest that electrical impedance measurements may be made using this apparatus with no loss of reliability and without the extensive practice necessary to achieve reproducible results using the skinfold thickness method.