Lean MASLD Carries the Same Mortality Risk as Obese MASLD — Fibrosis Is What Matters
A global study of 18,326 patients shows lean individuals with fatty liver disease face similar outcomes to obese patients — fibrosis stage drives prognosis.
Summary
Fatty liver disease (MASLD) is usually linked to obesity, but a growing number of normal-weight individuals develop it too. This large multinational study examined over 18,000 biopsy-confirmed MASLD patients across 41 countries to compare lean versus overweight or obese cases. Lean patients had milder liver inflammation and less advanced fibrosis, and fewer had type 2 diabetes. Despite these apparent advantages, their long-term outcomes — including all-cause mortality and serious liver events — were statistically similar to those of heavier patients. The critical finding: fibrosis stage, not body weight, determined prognosis. Patients with advanced fibrosis faced more than double the mortality risk regardless of weight. Additionally, liver stiffness measurement outperformed the standard FIB-4 blood test for detecting advanced fibrosis in lean individuals, an important clinical tool distinction.
Detailed Summary
Metabolic dysfunction-associated steatotic liver disease (MASLD), formerly called NAFLD, is most commonly associated with excess body weight. However, a clinically distinct subset — lean MASLD — occurs in individuals with normal BMI and waist circumference, and its true prognosis has been poorly characterized until now.
Researchers from the Global NASH/MASH Council conducted a retrospective observational study of 18,326 adults with biopsy-confirmed MASLD from 41 countries. Patients were categorized as lean or overweight/obese by BMI and waist circumference. The study examined histologic severity, all-cause mortality, and major liver events including hepatocellular carcinoma, hepatic decompensation, and liver transplant.
Lean MASLD accounted for 6.7% of cases by BMI — rising to 11.3% in Asian populations. Compared with overweight or obese patients, lean individuals had lower rates of type 2 diabetes (38.2% vs. 48.3%), less advanced fibrosis (29.3% vs. 37.2%), and lower liver disease activity scores. These differences suggested a milder disease phenotype on the surface.
However, when adjusted for confounders, lean status was not independently associated with better all-cause mortality (adjusted HR 0.90) or fewer clinical events (adjusted HR 0.94). What did predict outcomes dramatically was fibrosis stage: patients with advanced fibrosis (F3–F4) faced more than twice the mortality risk and over three times the rate of serious liver events compared to those with mild fibrosis, regardless of weight category.
For noninvasive assessment, liver stiffness measurement (LSM) by transient elastography outperformed the FIB-4 score for detecting advanced fibrosis in lean individuals (AUC 0.87 vs. 0.76), a clinically important distinction. The study was limited to biopsy-confirmed cases, introducing selection bias. The summary is based on the abstract only.
Key Findings
- Lean MASLD patients had lower fibrosis rates but similar adjusted all-cause mortality versus overweight/obese patients.
- Advanced fibrosis (F3-F4) more than doubled mortality risk and tripled serious liver events regardless of body weight.
- Lean MASLD was most prevalent in Asia, affecting over 11% of MASLD patients in that region.
- Liver stiffness measurement (LSM) outperformed FIB-4 for detecting advanced fibrosis specifically in lean individuals (AUC 0.87 vs. 0.76).
- Type 2 diabetes was 10 percentage points less common in lean versus overweight/obese MASLD patients.
Methodology
Multinational retrospective observational study of 18,326 adults with biopsy-confirmed MASLD from 41 countries enrolled in the Global MASLD project. Lean status was defined using both BMI and waist circumference. Primary outcomes were all-cause mortality and composite clinical events; secondary outcomes included histologic fibrosis staging and noninvasive test performance.
Study Limitations
The retrospective design and reliance on liver biopsy cohorts introduce selection bias, as biopsy is typically reserved for higher-risk patients, likely enriching the study sample with more advanced disease. The summary is based on the abstract only, so full methodological details, covariate adjustments, and subgroup analyses cannot be fully evaluated. Agreement between BMI-based and waist circumference-based lean classifications was only moderate (κ = 0.41), highlighting definitional uncertainty.
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