ObjectiveThe main aim of the study was to evaluate the association between non-alcoholic fatty liver disease (NAFLD), estimated by fatty liver index (FLI), and the development of type 2 diabetes (T2D) in a large cohort of adult workers with pre-diabetes.DesignProspective cohort study.SettingOccupational health services from Spain.Participants16 648 adult workers (aged 20–65 years) with pre-diabetes (fasting plasma glucose (FPG) of 100–125 mg/dL).Outcome and measuresFLI was calculated based on measurements of triglycerides, body mass index, waist circumference and γ-glutamyltransferase. The population was classified into three categories: FLI<30 (no hepatic steatosis), FLI 30–60 (intermediate status) and FLI>60 (hepatic steatosis). Sociodemographic, anthropometric, dietary habits, physical activity and clinical data were collected from all subjects. The incidence rate of T2D was determined after 5 years of follow-up.ResultsAfter 5 years of follow-up, 3706 of the 16 648 participants (22.2%) were diagnosed with T2D, corresponding to an annual rate of progression of 4.5%. FLI was strongly associated with T2D conversion. The incidence rates of T2D in the FLI<30, FLI 30–60 and FLI>60 groups were significantly different after 5 years of follow-up were 19/6,421 (0.3%), 338/4,318 (7.8%) and 3,349/5,909 (56.7%), respectively. This association remained significant for FLI>60 after adjustment for, age, diet, physical activity, FPG, blood pressure, social class and smoking habits (adjusted HR=6.879; 95% CI 5.873 to 8.057 for men, and HR=5.806; 95% CI 4.863 to 6.932 for women).ConclusionNAFLD assessed by FLI independently predicted the risk of conversion to T2D among people with pre-diabetes. FLI may be an easily determined and valuable early predictor for T2D in people with pre-diabetes. FLI-based assessment of NAFLD in subjects with pre-diabetes in routine clinical practice could allow the adoption of effective measures to prevent and reduce their progression to T2D.