Methodology guide
Measuring body composition in research
DXA, MRI-PDFF, BIA, hydrostatic weighing — methodology, accuracy, cost and which technique fits which metabolic-peptide research question.
The four mainstream methods
For metabolic peptide research, four body-composition methodologies dominate published literature: dual-energy X-ray absorptiometry (DXA), magnetic resonance imaging with proton density fat fraction (MRI-PDFF), bioelectrical impedance analysis (BIA) and hydrostatic (underwater) weighing. Each has a defined window of accuracy, cost profile and methodological vulnerability.
DXA — the modern standard
DXA is the most-cited body-composition method in incretin and GH-axis peptide research. It separates fat mass, lean soft tissue and bone mineral content at regional resolution (arms, legs, trunk, android, gynoid), with typical test-retest variation around 1-2% for total fat mass. Limitations include hydration-status confounding and an upper weight ceiling on most scanners (typically 200 kg).
MRI-PDFF — the hepatic-fat reference
MRI proton density fat fraction is now the standard non-invasive method for quantifying hepatic steatosis. SURMOUNT-MMO and several semaglutide hepatic sub-studies have used MRI-PDFF as the primary readout for non-alcoholic steatohepatitis (NASH) endpoints. Centre availability is the main limiting factor — fewer than half of UK research-active hospitals have MRI-PDFF capability.
BIA — practical but variable
BIA estimates body composition from electrical impedance. Modern multi-frequency devices (InBody, Tanita BC-series) are inexpensive and tolerable for repeated measurement but carry higher hydration-dependent variability than DXA. They are useful for screening and frequent monitoring; less suitable as primary end-point measurements in pivotal trials.
Hydrostatic weighing — historical reference
Densitometry by underwater weighing remains a gold-standard validation method but has largely been displaced by DXA in active metabolic research because of the participant-experience disadvantage and the comparable accuracy of DXA.
Choosing for a peptide trial
For most GLP-1 / dual-agonist research, DXA is the primary body-composition endpoint, with MRI-PDFF added where hepatic effects are the focus. For growth-hormone-axis research targeting visceral adipose tissue specifically, MRI or CT volumetry of VAT is the standard (replicating the Falutz tesamorelin Phase 3 methodology).