Body composition assessment in extreme obesity and after massive weight loss induced by gastric bypass surgery

Abstract

Body composition methods were examined in 20 women [body mass index (BMI) 48.7 ± 8.8 kg/m²] before and after weight loss [−44.8 ± 14.6 (SD) kg] after gastric bypass (GBP) surgery. The reference method, a three-compartment (3C) model using body density by air displacement plethysmography and total body water (TBW) by H₂ ¹⁸O dilution (3C-H₂ ¹⁸O), showed a decrease in percent body fat (%BF) from 51.4 to 34.6%. Fat-free mass hydration was significantly higher than the reference value (0.738) in extreme obesity (0.756; P < 0.001) but not after weight reduction (0.747; P = 0.16). %BF by H₂ ¹⁸O dilution and air displacement plethysmography differed significantly from %BF by 3C-H₂ ¹⁸O in extreme obesity ( P< 0.05) and 3C models using ²H₂O or bioelectrical impedance analysis (BIA) to determine TBW improved mean %BF estimates over most other methods at both time points. BIA results varied with the equation used, but BIA better predicted %BF than did BMI at both time points. All methods except BIA using the Segal equation were comparable to the reference method for determining changes over time. A simple 3C model utilizing air displacement plethysmography and BIA is useful for clinical evaluation in this population.