Reliability Of The Bod Pod® Compared To Traditional Reference Methods For Measuring Body Composition

ABSTRACT In the mid 1990s, a new air-displacement plethysmograph (ADP) was developed for measuring human body composition. This device (BOD POD® Body Composition System) uses the relationship between the pressure and volume of air to measure the body volume of a subject seated in the test chamber. Body density (Db) is then calculated using body mass and body volume and percent body fat (% BF) estimated using an equation such as the one by Siri (1961). This study evaluated the reliability of the BOD POD compared to dual-energy X-ray absorptiometry (DXA), underwater weighing (UWW) and isotope dilution for the measurement of percent body fat (%BF). Twenty healthy Caucasians (12 males and 8 females) aged 25 to 81 years, with mean ± SD body mass (75.9 ± 18.1 kg), height (169.9 ± 8.3 cm) and BMI (26.1 ± 5.1 kg/m2) volunteered. %BF was measured twice on day 1, after an overnight fast and once on day 2. Also assessed was within-day and between-day reliability was assessed. The two test days were usually scheduled 5 - 8 days apart and were randomized. Results showed there were no significant differences among the three test sessions in %BF measured by any individual method, or calculated by multi-compartment models. For the within-day variability, the BOD POD had the smallest within-subject SD (0.3 %BF) compared to UWW which had the highest within-subject SD (1.0 %BF). Between days, DXA had the lowest withinsubject SD (0.5 %BF) but was followed closely by the BOD POD (0.7 %BF). The actual error for estimating %BF by multi-compartment models was found to be smaller than theoretically calculated and was smallest when Dh from BOD POD was used (0.1 %BF) compared to when Db from UWW was used (0.6 %BF). Neither subject characteristics nor the time between tests was significantly associated with variability in measured %BF for any method either within or between-days. Moreover, the difference in oral and skin temperatures was not associated with the difference between-days in %BF measured by the BOD POD after controlling for confounding factors. Thoracic gas volume (TGV), associated with the BOD POD, and residual lung volume (RV) associated with UWW, had similar reliability; however heavier subjects had a higher RV on the second withinday trial. These findings indicate that the BOD POD is among the most reliable methods. This study supports the use of the BOD POD in clinical and nutrition research.