Research in context
Evidence before this study
Non-alcoholic fatty liver disease (NAFLD) affects about 25% of the global population. Although commonly associated with obesity, NAFLD is increasingly being identified in non-obese individuals. However, data on the global prevalence of non-obese NAFLD and its associated outcomes are scarce. Before undertaking this study, we searched four databases (PubMed, Embase, Scopus, and the Cochrane Library) using the search terms “NAFLD” AND “Non-obese” without any language restrictions, for articles published from database inception to May 1, 2019. No meta-analysis of such a study has been published.
Added value of this study
In this systematic review and meta-analysis, we estimated that the overall prevalence of non-obese NAFLD was 40·8% among the NAFLD population and 12·1% in the general population. The prevalence of lean NAFLD was 19·2% among the NAFLD population and 5·1% in the general population. The incidence of NAFLD among non-obese people was 24·6 per 1000 person-years. Among people with non-obese or lean NALFD, about 39·0% had non-alcoholic steatohepatitis (NASH) and 29·2% had significant fibrosis; incidence of all-cause mortality was 12·1 per 1000 person-years, of liver-related mortality was 4·1 per 1000 person-years, of cardiovascular-related mortality was 4·0 per 1000 person-years, incidence of new-onset hypertension was 56·1 per 1000 person-years, new-onset diabetes was 12·6 per 1000 person-years, and new-onset cardiovascular disease was 18·7 per 1000 person-years.
Implications of all the available evidence
Around 40% of people with NAFLD are not obese but they have high mortality and are just as metabolically unhealthy as obese people with NAFLD; almost 40% of non-obese people with NAFLD have NASH and almost 30% have significant fibrosis. Therefore, screening for NAFLD should consider other metabolic risks besides bodyweight, and clinical trials of treatments for NAFLD should include participants across all body-mass index ranges.