Measurement of total body water using 2 H dilution: impact of different calculations for determining body fat

Abstract

This study estimated total body water (TBW) in four groups (twelve per group; sedentary and highly trained men and women) at the time of 2 H dosing (T 0 ) and after a 3·5 h equilibration period (T eq ). Standard TBW calculations were employed at T 0 (no correction for disproportionate urinary tracer loss) and T eq (correction for urinary tracer loss only), plus those calculations that corrected for a disproportionate urinary tracer loss and insensible tracer loss respectively. The measurement of body density enabled the four TBW estimates to be compared for the determination of three-compartment % body fat (BF). The very small difference between the standard and corrected T 0 TBW data was not significant ( P =0·914) and no Group×TBW interaction was identified ( P =0·125). These results reflect the closeness of the 2 H concentration in the urine produced during the equilibration period and the T eq saliva samples. The associated mean % BF values were essentially identical. Although correcting for insensible 2 H losses in addition to urinary losses at T eq produced a statistically significant ( P <0·001) lower mean TBW (about 200 g) than the standard calculation, this translated to a small difference in % BF (0·3). The larger difference (about 500 g, P <0·001) between the two (T 0 , T eq ) corrected TBW calculations was also associated with a small body composition difference (0·1 % BF), which was less than the propagated error (0·3 % BF) for the three-compartment body composition model. Corrections to the standard calculations of TBW at T 0 and T eq for a protocol employing a brief equilibration period (3·5 h) were therefore of marginal use for improving the accuracy of % BF estimates. The TBW difference over time (T 0 v. T eq ) also had little impact on % BF values.