Tracking changes in body composition in athletes: are rapid four-compartment models valid?

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 utilising 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 ²H 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 TBW_BIS providing the smaller mean difference (MD) (0·41 kg) and DXA the larger (0·94 kg). The MD for 4C BV_SilvaTBW_BIS and 4C BV_HeymsfieldTBW_BIS 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 TBW_BIS: -2·9 %, 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.