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

Departments of Nutrition and Exercise Science, University of California, Davis, California 95616-8669 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 ml) 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 (V R pred vs. V R meas , respectively). Differences between V R meas and V R pred and in %BF calculated by using V R meas vs. V R pred were significant (187 ± 46 ml and 1.4 ± 0.3% BF, respectively; P