Lean MASLD: Normal Weight Does Not Mean Low Liver Risk: JAMA | September 2026

What Did the Study Find?

This large Global MASLD study evaluated 18,326 patients with biopsy-confirmed MASLD across 41 countries, including 1,224 patients (6.7%) classified as lean by BMI.

• Lean MASLD was particularly common in Asia (11.3%).
• Lean patients had fewer metabolic comorbidities, including type 2 diabetes (38.2% vs 48.3%).
• Histologic disease was generally milder, yet 29.3% of lean patients already had advanced fibrosis (F3–F4).
• Despite milder histology, lean status was not independently associated with lower mortality or fewer clinical events.
• Instead, advanced fibrosis was the major prognostic determinant, doubling mortality risk (aHR 2.10) and more than tripling clinical-event risk (aHR 3.41).
• FIB-4 performed slightly worse for advanced fibrosis in lean MASLD (AUC 0.76).
• Transient elastography liver stiffness performed particularly well in lean patients (AUC 0.87).

Why Does It Matter?

Normal BMI can create false reassurance. Lean patients may have less metabolic disease on average, yet once MASLD is established, their long-term prognosis is not inherently more favourable.

GastroAGI Takeaway

In MASLD, fibrosis—not body size—determines prognosis. A lean patient should not be considered low risk simply because BMI is normal. Fibrosis assessment should be performed irrespective of body habitus, and these data suggest that liver stiffness measurement may be particularly useful in lean MASLD, while FIB-4 deserves more cautious interpretation.

Practice-informing evidence: large and globally diverse with biopsy confirmation, but retrospective and biopsy-selected rather than population-based.