Relationship between single point insulin sensitivity estimator index and nonalcoholic fatty liver disease in Chinese middle-aged and older adults: a cross-sectional study

Background Nonalcoholic fatty liver disease (NAFLD) is a major public health issue, especially in China’s middle-aged and elderly population. Insulin resistance (IR) is a key pathogenic factor, yet its gold-standard assessment methods are complex and invasive, limiting clinical use. The Single Point Insulin Sensitivity Estimator (SPISE) index—based on routine measures such as body mass index (BMI), triglycerides (TG), and high-density lipoprotein cholesterol (HDL-C)—offers a simple, insulin-free surrogate marker for IR. However, evidence for its association with NAFLD in China’s middle-aged and elderly population remains limited. Methods This cross-sectional study included 1,592 Chinese adults aged 40–79 years. NAFLD was diagnosed by abdominal ultrasonography. The SPISE index was analyzed as both a continuous variable and in quartiles. To assess its association with NAFLD, multivariable logistic regression models were employed, adjusting for a comprehensive set of covariates covering demographics, lifestyle, comorbidities, and metabolic parameters. A restricted cubic spline (RCS) model evaluated the dose–response relationship. Subgroup analyses and receiver operating characteristic (ROC) curve analysis were conducted to test the robustness and diagnostic performance of the association. Results The prevalence of NAFLD was 61.12%. A significant inverse association was found between the SPISE index and NAFLD prevalence. In the fully adjusted model, each 1-unit increase in the SPISE index was associated with a 45% lower odds of NAFLD. Participants in the highest SPISE quartile (Q4) had an 86% lower odds of NAFLD compared to those in the lowest quartile (Q1). RCS analysis confirmed a linear inverse dose–response relationship. This inverse association remained consistent across all pre-specified subgroups, including age, gender, BMI, and metabolic status. Furthermore, the SPISE index demonstrated superior diagnostic performance for NAFLD, with an area under the curve (AUC) of 0.78, significantly outperforming its individual components (BMI, TG, or HDL-C alone). Conclusion In this Chinese middle-aged and older adult population, a higher SPISE index was independently associated with a significantly lower prevalence of NAFLD, showing a clear dose–response relationship. As a low-cost tool calculated from routine clinical measures, the SPISE index shows promise for the early identification and risk stratification of NAFLD in community and clinical settings.