WAGNER, D. R., V. H. HEYWARD, and A. L. GIBSON. Validation of air displacement plethysmography for assessing body composition. Med. Sci. Sports Exerc., Vol. 32, No. 7, pp. 1339-1344, 2000.
Purpose: The purpose of this study was to verify the validity of an air displacement plethysmography device (Bod Pod®) for estimating body density (Db).
Methods: The Db from the Bod Pod (DbBP) was compared with the Db from hydrostatic weighing (DbHW) 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 DbHW as the dependent variable and DbBP as the predictor, produced an R2 = 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.
Multicomponent models, combining the body density (Db), total body water, and total body mineral data obtained from several different laboratory procedures, are frequently used to derive criterion measures of body composition. However, this approach is costly, time consuming, and often not practical. Thus, many researchers and clinicians continue to rely on a two-component model that simply separates the body into fat and fat-free components to assess body composition. Commonly, the two-component model involves measuring Db and then using a conversion formula to estimate relative body fat (%BF). Regardless of which body composition model is used, multicomponent or two-component, an accurate measurement of Db is critical to obtain valid and reliable estimates of %BF.
Db is the ratio of body mass to body volume (Vb). Traditionally, hydrostatic weighing (HW) has been used to determine Vb. This technique, which was pioneered by Behnke et al (2)., is based on Archimedes principle-a body immersed in a fluid is buoyed by a force equal to the weight of the displaced fluid.
The HW procedure requires the subject to perform the maneuver of maximally exhaling while completely submerged under water. Additionally, the subject needs to remain relatively motionless underwater in order for a trained technician to get an accurate reading of the subject's underwater weight. This procedure, often considered unpleasant by some subjects, must be repeated multiple times for a valid and reliable reading. Furthermore, to ensure an accurate measure of Vb from HW, residual lung volume (VR) must also be measured. Thus, HW is time-consuming, requires a skilled technician, and is difficult, and sometimes impossible, for some subjects to perform.
An alternative to HW is the Bod Pod® (Life Measurement Instruments, Concord, CA), a large, egg-shaped, fiberglass chamber. Based on air displacement plethysmography, the Bod Pod uses a pressure-volume relationship to derive Vb for a subject seated inside the chamber. The Vb is equal to the volume of air in an empty chamber minus the volume of air remaining in the chamber after the subject enters the chamber. The physical design and the operating principles of the Bod Pod have been described in detail elsewhere (5).
The Bod Pod method is much easier and more accommodating for subjects than HW, thereby potentially reducing subject error. Furthermore, it is a faster, more convenient, and easier test to administer, thereby reducing potential error associated with technician skill. Thus, the Bod Pod appears to be a promising tool for the determination of Vb and Db, and a method that could potentially replace HW.
Although this device has been extensively advertised and commercially available for several years, there remains a paucity of research on the Bod Pod. The published research is limited to only one study that demonstrated its validity and reliability for measuring the volume of inanimate objects (5), and one study that verified that it was a reliable and valid tool for estimating %BF in adult humans (15). Thus, the purpose of this study was to cross-validate Db measured by the Bod Pod to that obtained from HW. Additionally, the %BF values estimated from the Bod Pod and HW were compared with those obtained from dual-energy x-ray absorptiometry (DXA), a method that estimates %BF independent of Db.