Validation of air displacement plethysmography for assessing body composition

Purpose: The purpose of this study was to verify the validity of an air displacement plethysmography device (Bod Pod(R)) for estimating body density (Db). Methods: The Db from the Bod Pod (Db(BP)) was compared with the Db from hydrostatic weighing (Db(HW)) at residual lung volume in a heterogeneous sample of 30 black men who varied in age (32.0 +/- 7.7 yr), height (180.3 +/- 7.5 cm), body mass (84.2 +/- 15.0 kg), body fatness (16.1 +/- 7.5%), and self-reported physical activity level and socioeconomic status. The Db for each method was converted to relative body fat (%BF) using race-specific conversion formulas and subsequently compared with %BF obtained from dual-energy x-ray absorptiometry (%BFDXA). Results: Linear regression, using Db(HW) as the dependent variable and Db(BP) as the predictor, produced an R-2 = 0.84 and SEE = 0.00721 g.cc(-1). However, the mean difference between the two methods (0.00450 +/- 0.00718 g.cc(-1)) was significant (P < 0.01). The Bod Pod underestimated the Db of 73% of the sample. The %BF estimates from the Bod Pod, HW, and DXA differed significantly (P < 0.01). The average %BFBP (17.7 +/- 7.4%) was significantly greater than %BFHW (15.8 +/- 7.5%) and %BFDXA (16.1 +/- 7.5%); however, there was no significant difference between %BFHW and %BFDXA. Conclusion: The Bod Pod significantly and systematically underestimated Db, resulting in an overestimation of %BF. More cross-validation research is needed before recommending the Bod Pod as a reference method.