Comparing DXA and MRI body composition measurements in cross-sectional and longitudinal cohorts

In this study we assessed the accuracy of dual X-ray absorptiometry (DXA) compared to magnetic resonance imaging (MRI) for evaluating body composition. Using data from 32,961 participants in the UK Biobank, including 1,928 re-scanned participants after about two and a half years, we examined cross-sectional and longitudinal agreements in DXA and MRI measurements within android and gynoid regions. Our results showed that DXA reliably captured fat measurements but overestimated lean mass compared to MRI, particularly in android regions for men (4.10 vs. 1.74 kg) and women (2.92 vs. 1.10 kg). Longitudinal MRI data revealed a 4-5% muscle and lean mass decrease, undetected by DXA, which showed lean mass increases in women at the follow-up visit. Although DXA is practical for population-level fat assessments, MRI remains the preferred method for detailed and precise longitudinal body composition analysis.