In a multinational retrospective study of 18,326 biopsy-confirmed MASLD patients across 41 countries, lean individuals (6.7% by BMI) had meaningfully lower rates of type 2 diabetes (38.2% vs 48.3%), advanced fibrosis (29.3% vs 37.2%), and histologic activity scores compared with overweight/obese counterparts. Yet despite these apparent advantages, lean MASLD carried statistically equivalent long-term mortality (adjusted HR 0.90, 95% CI 0.69–1.16) and clinical event rates — including hepatocellular carcinoma, decompensation, and liver transplant. Advanced fibrosis (F3–F4) remained the dominant prognostic driver regardless of body weight, doubling all-cause mortality risk (aHR 2.10).
The finding reframes a common clinical assumption: normal BMI does not confer meaningful protection once liver disease is established. This matters because lean MASLD patients are frequently under-screened precisely because they lack the obesity signal clinicians rely on. The study adds important nuance to diagnostic practice — the FIB-4 score performed modestly worse in lean patients (AUC 0.76 vs 0.79), while transient elastography was actually more accurate (AUC 0.87 vs 0.83), suggesting clinicians should preferentially use elastography in this subgroup. The higher prevalence in Asian populations (>11%) underscores why BMI thresholds calibrated to Western bodies may obscure risk. Limitations include retrospective design, biopsy-selected cohorts (introducing ascertainment bias toward more severe disease), and moderate agreement between BMI and waist circumference lean classification (κ=0.41). Overall, this is a clinically significant, confirmatory dataset that should elevate lean MASLD from footnote to front-line screening consideration.