Predicted versus measured thoracic gas volume in estimating body fat percentage among male college athletes and sedentary controls: A cross-sectional observational study

Objectives: Despite established discrepancies between predicted thoracic gas volume (TGVp) and measured thoracic gas volume (TGVm) in Body Composition Pod (BOD POD) assessments, the influence of training modality on measurement accuracy remains unclear. This study examined whether TGVp provides an accurate body fat percentage (BF%) estimation compared to TGVm across different training modalities. Materials and Methods: In this cross-sectional study, sixty-six male participants (age 20–25 years) were categorized into three groups: sedentary individuals, anaerobic athletes (sprinters, weightlifters, throwers), and intermittent sport athletes (football, basketball, hockey) ( n = 22 each). Body composition was assessed via Air Displacement Plethysmography (ADP; BOD POD), with BF% calculated using both TGVp (demographic equations) and TGVm (direct measurement). Agreement patterns were evaluated using Bland–Altman analysis and proportional bias assessments. Results: TGVm significantly exceeded TGVp in sedentary individuals (4.67 ± 0.16 vs. 4.13 ± 0.21 L, d = −3.64), anaerobic athletes (5.02 ± 0.44 vs. 4.60 ± 0.52 L, d = −3.24), and intermittent sport athletes (6.27 ± 1.10 vs. 5.98 ± 0.92 L, d = −0.66; all p ≤ 0.006). Consequently, TGVp significantly underestimated BF% by 2.12 ± 0.44% in sedentary, 2.52 ± 1.31% in anaerobic, and 2.02 ± 0.65% in intermittent sport athletes (all p < 0.001). Bland–Altman analysis revealed proportional bias in anaerobic athletes for TGV (β = 0.158, p = 0.005) and strong proportional bias in intermittent sport athletes for BF% (β = 0.510, R 2 = 0.839, p < 0.001), indicating that measurement errors vary with the magnitude of the measurement rather than remaining constant. Conclusion: TGVp systematically underestimates BF% by 2.0–2.5% across all populations. Furthermore, the presence of proportional bias in athletic groups precludes the use of simple correction factors. Direct TGV measurement is essential for accurate body composition assessment, particularly in athletes, where training-induced adaptations create variable and unpredictable prediction errors.