MRI-Derived Muscle Composition and Visceral Adiposity in Relation to Mortality and Vascular Outcomes in MASLD

Background/Objectives: Metabolic dysfunction-associated steatotic liver disease (MASLD) is a cardiometabolic phenotype rather than a single-pathway liver disorder, yet prognostic work tends to privilege liver fat over the rest of body composition. We examined which magnetic resonance imaging (MRI)-derived compartment, including hepatic, abdominal adipose, or skeletal muscle, best explained risk within MASLD. Methods: We investigated 24,976 UK Biobank adults with abdominal MRI, including 10,490 with MASLD, 2577 with MASLD with increased alcohol intake, and 11,909 without steatotic liver disease. Five MRI parameters were examined: proton density fat fraction (PDFF), visceral adipose tissue (VAT), abdominal subcutaneous adipose tissue (ASAT), muscle fat infiltration (MFI), and thigh fat-free muscle volume (FFMV). Primary outcomes were all-cause mortality, incident cardiovascular disease, and incident cerebrovascular disease. Cox models jointly included all five MRI variables and adjusted for demographic, metabolic, and biochemical covariates. Results: Within MASLD, liver PDFF was not independently associated with all-cause mortality or cerebrovascular disease after multivariable adjustment, whereas an inverse association was observed with cardiovascular disease. By contrast, higher MFI was associated with all-cause mortality and cerebrovascular disease, whereas greater FFMV was associated with lower cardiovascular risk. VAT showed an independent association with cardiovascular disease, whereas ASAT had no significance. Clustering identified a lean-sarcopenic phenotype with higher mortality and cerebrovascular risk than a balanced reference phenotype. Conclusions: MRI-derived body-composition parameters showed outcome-specific associations with clinical outcomes in MASLD. Muscle-related measures were particularly associated with all-cause mortality and cerebrovascular disease, whereas visceral adiposity and muscle volume contributed to cardiovascular risk. These findings support the potential value of integrated body-composition assessment rather than reliance on hepatic steatosis alone, although further validation is needed to establish its incremental clinical utility.