Measured thoracic gas volume using the bod pod air displacement plethysmography system versus three predictions

Bod Pod air displacement plethysmography is a common method to estimate body fat percentage (%BF). To determine body volume from this method, thoracic gas volume (TGV) must be quantified. Measured TGV was compared to three predictions: the default prediction formula in the Bod Pod software from Crapo (CRAPO), a relatively new prediction that relies on height and body mass as the predictors from Ducharme (DUCHARME), and a child-specific formula from Fields (FIELDS).

University club sport athletes (121 men, 80 women) completed the breathing maneuver in the Bod Pod necessary to obtain measured TGV. Values from the three TGV predictions were compared to measured TGV with a one-way repeated measures ANOVA. Linear regression was used to obtain standard error of estimate (SEE), and Bland-Altman plots were used to evaluate bias.

Significant (P < .001) bias exists for all three prediction equations such that individuals with small TGV are overpredicted and those with large TGV are underpredicted. SEEs were similar (0.63-0.66 L) for all three equations. On average, CRAPO and FIELDS underestimated (P < .001) measured TGV by -0.25 L and -0.43 L, respectively, but the difference between DUCHARME and measured TGV (0.07 L) was not significant (P = .813). There were fewer large errors (≥ 2%BF) from DUCHARME (25%) than from CRAPO (31%) or FIELDS (37%).

TGV should be measured. If it must be predicted, DUCHARME offers a slightly better prediction than CRAPO or FIELDS for young adult athletes, particularly those at the extremes of the TGV continuum.