Tracking Changes in Body Composition in Athletes: Are Rapid Four-Compartment Models Valid?

Abstract

While rapid four-compartment (4C) models have demonstrated cross-sectional validity in diverse populations, including athletes, their longitudinal validity remains unverified. This study aims to evaluate the longitudinal validity of utilizing Dual-Energy X-ray Absorptiometry (DXA) derived estimates of body volume (BV) and bioelectrical impedance spectroscopy (BIS) derived total body water (TBW) in rapid 4C models in athletes. Criterion 4C used DXA for bone mineral content, air displacement plethysmography for BV, and deuterium dilution for TBW. Analyses of longitudinal changes in fat mass (FM), in both kilograms and percentage, were performed in participants who experienced changes greater than 1% (n = 60) and in a subgroup exceeding the least significant change (1.97 kg, n = 25). All alternative estimates underestimated changes relative to criterion 4C, with 4C TBWBIS providing the smaller mean difference (0.41 kg) and DXA the larger (0.94 kg). The mean differences for 4C BVSilvaTBWBIS and 4C BVHeymsfieldTBWBIS were 0.48 kg and 0.50 kg, respectively. Bland-Altman analyses showed wide limits of agreement (LOA) for all methods. Even the alternative with the smaller 95% LOA had a wide LOA (4C TBWBIS: -2.9% to 4.0%, n = 60). This investigation demonstrates that these rapid 4C models exhibit precision superior to or equivalent to DXA alone, while offering reduced assessment times and broader accessibility than the criterion 4C. Regarding accuracy for evaluating changes over time, despite acceptable results at the group level, interpretation should be careful at the individual level. (c) The Author(s), 2026. Published by Cambridge University Press on behalf of The Nutrition Society.