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Background/Aims Sarcopenia has emerged as an important risk factor for nonalcoholic fatty liver disease (NAFLD). Although aging is the main cause of sarcopenia, the longitudinal association between age-related body composition changes and NAFLD development has not been fully investigated. Thus, we evaluated whether age-related increased fat mass or decreased muscle mass is an independent risk factor for incident NAFLD. Methods : We conducted a retrospective cohort study involving 4,398 initially NAFLD-free subjects who underwent routine health examinations during 2004 to 2005 and returned for a follow-up during 2014 to 2015. Their body composition was measured by bioelectrical impedance analysis, and fatty liver was diagnosed by abdominal ultrasonography. Results : At the 10-year follow-up, 591 out of 4,398 participants (13.4%) had developed NAFLD. In men and women, both increased fat mass and decreased appendicular skeletal muscle mass (ASM) with aging were significantly associated with incident NAFLD after adjustment. A subgroup analysis according to the baseline obesity status showed that increased fat mass was significantly associated with incident NAFLD in obese and nonobese subjects. However, decreased ASM was significantly associated with incident NAFLD in nonobese but not in obese subjects. According to ΔASM tertiles (decrease of ASM), the odds ratios for incident NAFLD in nonobese subjects were 1.38 (95% confidence interval [CI], 1.04 to 1.84) for the second tertile and 1.81 (95% CI, 1.34 to 2.45) for the third tertile after adjustment (p=0.001). Conclusion : s A progressive increase in fat mass and a loss of ASM with aging were significantly associated with incident NAFLD. This association was more prominent in nonobese subjects.