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 within-subject 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 within-day 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.