Vitamin D Is Associated with Severity and Mortality of Non-alcoholic Fatty Liver Disease: A US Population-based Study

Background and Aims: There has been increasing evidence that vitamin D deficiency may increase the risk of metabolic syndrome. Since metabolic syndrome is a major risk factor for non-alcoholic fatty liver disease (NAFLD), we aimed to investigate the association between vitamin D and the severity and mortality of NAFLD. Methods: Data was obtained from the United States Third National Health and Nutrition Examination Survey conducted in 1988-1994, with follow-up mortality data through 2011. NAFLD was defined by ultrasonographic detection of hepatic steatosis in the absence of other liver diseases and categorized as normal, mild, moderate or severe. The severity of hepatic fibrosis was determined by NAFLD fibrosis score (NFS). ANOVA (F-test) was used to evaluate the association between vitamin D level and degree of NAFLD, and Cox proportional hazards regression analysis was used for survival analyses. Results: Vitamin D levels for normal, mild, moderate and severe steatosis were 25.1 ± 0.29 ng/mL, 24.7 ± 0.42 ng/mL, 23.7 ± 0.37 ng/mL and 23.6 ± 0.60 ng/mL, respectively (trend p < 0.001). Likewise, vitamin D levels for low, intermediate and high NFS categories were 24.7 ± 0.38 ng/mL, 23.4 ± 0.42 ng/mL and 21.5 ± 0.57 ng/mL, respectively (trend p < 0.001). After median-follow up over 19 years, vitamin D deficiency was significantly associated with diabetes- and Alzheimer's disease-related mortality (hazard ratio (HR): 3.64, 95%CI: 1.51-8.82 and HR: 4.80, 95%CI: 1.53-15.1, respectively), with a borderline significance in overall mortality (HR: 1.16, 95%CI: 0.99-1.36, p = 0.06). Conclusions: Vitamin D level was inversely related to the degree of liver steatosis and fibrosis. Moreover, vitamin D deficiency was associated with diabetes- and Alzheimer's disease-related mortality in NAFLD patients.