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

Detailed body composition assessment, through measurement of adipose tissue and muscle distribution, is essential for understanding population health trends, guiding targeted interventions, evaluating lifestyle effects, and monitoring age-related changes such as sarcopenia. Dual X-ray absorptiometry (DXA) provides estimates of fat and lean mass from low-dose X-ray images, while magnetic resonance imaging (MRI) offers three-dimensional measurements of adipose tissue and muscle distribution. Both methods have their strengths and weaknesses, but comparison between them at scale is missing. We assessed the accuracy of DXA compared to 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. MRI scans were acquired using a Siemens 1.5 T MRI scanner, and DXA scans on a GE iDXA. Our results show that DXA is able to capture fat measurements, including visceral adipose tissue and fat mass, but overestimates lean mass compared to MRI, particularly in android regions for men (4.10 kg measured by DXA vs. 1.74 kg by MRI) and women (2.92 vs. 1.10 kg). Longitudinal MRI data reveal a 4-5% muscle and lean mass decrease, undetected by DXA, which shows lean mass increases in women at the follow-up visit. Although DXA is practical for population-level fat assessments, it may not be as precise for lean mass assessments, especially in longitudinal studies. If economically and practically feasible, MRI remains the preferred method for detailed and precise longitudinal body composition analysis. Basty et al. compare DXA and MRI measurements of body fat and muscle in over 32,000 UK Biobank participants, including some repeat scans after about two and a half years. We find that both methods measure fat similarly, but DXA overestimates muscle and may miss muscle loss over time compared to MRI. This study looked at different ways to measure fat and muscle, which are important for understanding health, ageing, and the effects of lifestyle. Two common whole body imaging approaches are DXA, which takes 2D images and is faster, simpler, cheaper, and automated, and 3D MRI, which provides more detailed images but is slower, more expensive, and requires expert analysis. We compared these two methods in over 32,000 people in the UK population, with some scanned again after about two and a half years. We found that both methods measured body fat similarly, but DXA reported higher muscle values than MRI and may miss changes in muscle over time. This suggests that DXA may be useful for large-scale fat measurements, while MRI may be better for detailed muscle assessments and tracking changes over time.