Comparison of DXA-Derived Body Volume Estimation Equations (Wilson vs Smith-Ryan) in Collegiate Rugby Union Athletes

Accurate estimation of body volume (BV) is required for multicomponent body composition models. PURPOSE: Compare two published DXA-derived BV equations (Wilson vs Smith-Ryan) using whole-body Hologic DXA tissue masses in collegiate rugby union athletes. METHODS: Twenty-one male rugby union athletes underwent whole-body DXA (Hologic Horizon W; APEX v5.6.1.3 Rev 007; Classic calibration [NHANES BCA disabled]; standard tissue analysis; daily QC within tolerance) during morning testing (06:00–09:00 h) while fasted and post-void; caffeine was avoided (≥12 h) and exercise was avoided (≥24 h). Hydration was assessed prior to scanning via urine specific gravity (Siemens CLINITEK Status+; Multistix 10 SG) and urine color (clear yellow). Whole-body fat mass (FM), lean soft tissue mass (LST), and bone mineral content (BMC) were exported (g) and converted to kg. BV was computed using a Wilson-derived Hologic equation: BV=(FM/0.88)+(LST/1.05)−(BMC/4.85)+0.01 and Smith-Ryan: BV=(FM/0.84)+(LST/1.03)+(BMC/11.63)−3.12. Agreement was evaluated with paired t-test, Pearson r, ICC(2,1), and Bland–Altman bias/limits of agreement (LoA). RESULTS: Mean body mass was 89.70 ± 22.53 kg with DXA percent fat 22.20 ± 8.02%. BV estimates were nearly identical (Wilson: 88.66 ± 23.50 L; Smith-Ryan: 88.88±24.46 L), with a small, non-significant mean difference of 0.22±0.97 L (LoA -1.69 to 2.12 L; p=0.312). Estimates were essentially collinear (r=0.99999; ICC=0.99918). CONCLUSION: Wilson and Smith-Ryan DXA-derived BV equations yield near-identical BV in rugby union athletes with trivial bias and extremely high agreement. Investigators should pre-specify equation choice and report LoA to contextualize individual-level differences when criterion of BV is unavailable.