Background: NAFLD is increasing in Asia including Japan, despite its lower obesity rate than the West. However, NAFLD can occur in lean people, but data are limited. We aimed to investigate the epidemiology of NAFLD in Japan with a focus on lean NAFLD. Methods: We searched PubMed, Cochrane Library, EMBASE, Web of Science, and the Japan Medical Abstracts Society (inception to 5/15/2019) and included 73 eligible full-text original research studies (n = 258,531). We used random-effects model for pooled estimates, Bayesian modeling for trend and forecasting, contacted authors for individual patient data and analyzed 14,887 (7752 NAFLD; 7135 non-NAFLD—8 studies) patients. Results: The overall NAFLD prevalence was 25.5%, higher in males (p < 0.001), varied by regions (p < 0.001), and increased over time (p = 0.015), but not by per-person income or gross prefectural productivity, which increased by 0.64% per year (1983–2012) and is forecasted to reach 39.3% in 2030 and 44.8% in 2040. The incidence of NAFLD, HCC, and overall mortality were 23.5, 7.6 and 5.9 per 1000 person-years, respectively. Individual patient-level data showed a lean NAFLD prevalence of 20.7% among the NAFLD population, with lean NAFLD persons being older and with a higher all-cause mortality rate (8.3 vs. 5.6 per 1000 person-years for non-lean NAFLD, p = 0.02). Older age, male sex, diabetes, and FIB-4 were independent predictors of mortality, but not lean NAFLD. Conclusion: NAFLD prevalence has increased in Japan and may affect half of the population by 2040. Lean NAFLD individuals makeup 20% of the NAFLD population, were older, and had higher mortality.
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