Association of Systemic Immune-Inflammation Index with Non-Alcoholic Fatty Liver Disease: A Population-Based Cross-Sectional Study

Background: The aim of this study was to explore the relationship between systemic immune-inflammation (SII) index with non-alcoholic fatty liver disease (NAFLD) in the general population of the United States (U.S.).Methods: We conducted a cross-sectional study of subjects in the National Health and Nutrition Examination Survey 2017-2018. For the analysis of the association between SII index and risk of NAFLD, the restricted cubic spline (RCS) plot, we performed multivariable logistic regression models and subgroup analysis. In addition, generalized additive models with smooth functions were conducted for the relationship between the SII index and the ZJU index, the BARD score, and the NAFLD fibrosis score.Results: There were a total of 1197 individuals in our study. Taking into account known confounding variables, compared with the lowest quartiles, the odds ratios with 95% confidence intervals for NAFLD across the quartiles were 0.923 (0.585, 1.455), 0.563 (0.351, 0.901), and 1.061 (0.669, 1.682), respectively. As shown by the RCS plot, the SII index was linked with NAFLD risk in a U-shaped pattern. Based on the results of subgroup analysis, SII index and NAFLD risk were U-curve correlated among participants in all age groups, male or female, with or without hypertension, with diabetes mellitus, and with a BMI of <30 or >30 kg/m2. The SII index was linearly positive with the ZJU index but negative with the NAFLD fibrosis score. However, the SII index and BARD score showed a trend of first decreasing, then increasing, and then decreasing.Conclusion: The U-shaped relationships exist between SII index and risk of NAFLD, which highlighted that we should focus on the dynamic change of SII index.