DXA-based estimation of body volume in 4-compartment models: Validating and modifying the Smith-Ryan equation

Accurate body composition assessment is crucial for evaluating health and guiding interventions. Four-compartment (4 C) models, which separately quantify fat mass, lean body mass, bone mineral content and total body water, offer improved accuracy but usually require multiple devices. This study aimed to validate dual-energy X-ray absorptiometry (DXA)-derived body volume (BV) equations-specifically those from Wilson and Smith-Ryan-against air displacement plethysmography (ADP), and to modify the Smith-Ryan equation for enhanced BV estimates in a 4 C model. Ninety healthy adults (50 females, 40 males; aged 18-66 years; BMI 18-34 kg/m²) were recruited at a single-centre facility. Participants underwent DXA scanning (GE Lunar DXA) for tissue composition, ADP (Bod Pod) for BV, and bioelectrical impedance spectroscopy for total body water, with all measurements performed by trained technicians. BV was estimated using the original Wilson and Smith-Ryan equations and a regression-modified Smith-Ryan equation, and these estimates were incorporated into the 4 C model to calculate percent body fat (%BF) and fat mass (FM). All DXA-based BV estimates correlated highly with ADP (r ≥ 0.99). Although the original equations showed small, statistically significant differences in %BF and FM compared to ADP (p < 0.05), the modified Smith-Ryan equation produced BV estimates equivalent to ADP (p = 0.998). These findings suggest that calibrated DXA-derived BV equations can reliably substitute for ADP in 4 C models, enhancing the accessibility and cost-effectiveness of body composition analysis. Future research should validate these findings in more diverse populations.