Body composition by air-displacement plethysmography by using predicted and measured thoracic gas volumes

The BOD POD, a new air-displacement plethysmograph for measuring human body composition, utilizes the inverse relationship between pressure and volume (Boyle's law) to measure body volume directly. The quantity of air in the lungs during tidal breathing, the average thoracic gas volume (Vtg), is also measured by the BOD POD by using a standard plethysmographic technique. Alternatively, the BOD POD provides the use of a predicted Vtg (Vtg(pred)). The validity of using Vtg(pred) in place of measured Vtg (Vtg(meas)) to determine the percentage of body fat (%BF) was evaluated in 50 subjects (36 women, 14 men; ages 18-56 yr). There was no significant difference between Vtg(meas) and Vtg(pred) (mean difference +/- SE, 53.5 +/- 63.3 mi) nor in %BF by using Vtg(meas) vs. Vtg(pred) (0.2 +/- 0.2 %BF). On an individual basis, %BF measured by using Vtg(meas) vs. Vtg(pred) differed within +/-2.0% BF for 82% of the subjects; maximum differences were -2.9 to +3.0% BF. For comparison, data from 24 subjects who had undergone hydrostatic weighing were evaluated for the validity of using predicted vs. measured residual lung volume (VRpred VS VRmeas, respectively). Differences between VRmeas and VRpred and in %BF calculated by using VRmeas vs. VRpred were significant (187 +/- 46 mi and 1.4 +/- 0.3% BF, respectively; P < 0.001). On an individual basis, %BF determined by using VRmeas vs. VRpred differed within +/-2.0% BF for 46% of the subjects; maximum differences were -2.9 to +3.8% BF. With respect to %BF measured by air displacement, our findings support the use of Vtg(pred) for group mean comparisons and for purposes such as screening in young to middle-aged individuals. This contrasts with the use of VRpred in hydrostatic weighing, which leads to significant errors in the estimation of %BF. Furthermore, although the use of Vtg(pred) has some application, determining Vtg(meas) is relatively simple in most cases. Therefore, we recommend that the use of Vtg(meas) remain as standard experimental and clinical practice.