Lean Metabolic-Dysfunction-Associated Steatotic Liver Disease (MASLD): Pathophysiology, Diagnostic Challenges,...

Lean metabolic dysfunction-associated steatotic liver disease (lean MASLD) is an increasingly recognized phenotype occurring in individuals with normal body mass index (BMI), despite clinically important hepatic and cardiometabolic risk. This narrative review summarizes current evidence on its epidemiology, pathophysiology, diagnostic challenges, clinical outcomes, and management.

A narrative literature review was conducted using PubMed, Embase, and Cochrane Library from database inception to March 2026. Relevant studies on lean MASLD/lean NAFLD, including cohort studies, meta-analyses, clinical trials, consensus statements, and practice guidelines, were prioritized.

Lean MASLD reflects interactions between visceral adiposity, insulin resistance, genetic susceptibility, sarcopenia, dietary and lifestyle factors, vitamin D deficiency, and gut microbiome alterations. Diagnosis is challenging because BMI and aminotransferase levels may underestimate metabolic vulnerability, MASH, or clinically significant fibrosis. Available data suggest increased liver-related events, liver-related mortality, and all-cause mortality compared with individuals without steatotic liver disease, although comparisons with non-lean MASLD remain heterogeneous. Resmetirom and semaglutide have expanded treatment options for noncirrhotic MASH with moderate to advanced fibrosis, but lean patients are underrepresented in pivotal trials.

Lean MASLD is an underrecognized but clinically important phenotype. Earlier recognition, fibrosis risk stratification, sarcopenia assessment, cardiometabolic optimization, and lean-specific therapeutic research are needed to improve outcomes.