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

Area of Science:

• Human Physiology
• Biomedical Engineering
• Body Composition Analysis

Background:

• Accurate body composition assessment is vital for health evaluation and interventions.
• Four-compartment (4C) models offer high accuracy but often require multiple devices.
• Dual-energy X-ray absorptiometry (DXA) is a common tool, but its body volume (BV) estimation for 4C models needs validation.

Purpose of the Study:

• To validate DXA-derived BV equations (Wilson and Smith-Ryan) against air displacement plethysmography (ADP).
• To modify the Smith-Ryan equation for improved BV estimation in 4C models.
• To assess the impact of DXA-derived BV on 4C model accuracy for body fat percentage (%BF) and fat mass (FM).

Main Methods:

• Ninety healthy adults (50 female, 40 male) underwent DXA, ADP (Bod Pod), and bioelectrical impedance spectroscopy.
• DXA-derived BV was calculated using original Wilson and Smith-Ryan equations, and a regression-modified Smith-Ryan equation.
• BV estimates were integrated into a 4C model to compute %BF and FM.

Main Results:

• All DXA-based BV estimates showed high correlation with ADP (r ≥ 0.99).
• Original equations yielded small, significant differences in %BF and FM compared to ADP.
• The modified Smith-Ryan equation produced BV estimates statistically equivalent to ADP (p = 0.998).

Conclusions:

• Calibrated DXA-derived BV equations can reliably replace ADP in 4C models.
• This enhances the accessibility and cost-effectiveness of body composition analysis.
• Further validation in diverse populations is recommended.