The assumption that lean individuals with fatty liver disease face better outcomes than their overweight counterparts has been fundamentally challenged by new evidence spanning over 186,000 patients across Western and Asian populations. This finding could reshape clinical approaches to metabolic dysfunction-associated steatotic liver disease (MASLD) screening and treatment priorities.
Analysis of three major population cohorts—UK Biobank, Kailuan, and China Kadoorie Biobank—revealed that lean MASLD patients experienced 2.14 times higher risk of liver-related events and 2.31 times greater liver-related mortality compared to non-lean MASLD patients over 14 years of follow-up. The study tracked 5,030 lean patients (BMI under 25 kg/m² in Western populations, under 23 kg/m² in Asian cohorts) against 181,191 non-lean MASLD cases, documenting 2,501 liver events, 22,482 deaths, and 375 liver-specific deaths.
This paradoxical finding challenges the clinical intuition that lower BMI confers protection in metabolic liver disease. Lean MASLD likely represents a distinct phenotype driven by genetic susceptibility, visceral adiposity, or metabolic dysfunction independent of overall body weight. The elevated mortality risk suggests these patients may harbor more aggressive disease mechanisms or receive delayed diagnosis due to their seemingly healthy appearance. Current screening protocols predominantly target overweight individuals, potentially missing this high-risk lean population. The consistency across diverse ethnic populations strengthens the clinical relevance. However, the observational design cannot establish causality, and the specific mechanisms driving worse outcomes in lean patients remain unclear. This represents a paradigm-shifting finding that demands immediate attention to lean MASLD identification and management strategies.