Association of a novel Fat-Muscle-BMI Index (FMBI) with NAFLD, liver fibrosis, and atherosclerotic cardiovascular disease risk: a cross-sectional study in 1,592 Chinese adults

BACKGROUND: Non-alcoholic fatty liver disease (NAFLD) is a global burden with unmet non-invasive biomarker needs; BMI and FMRs have limitations, and their combined index FMBI is unstudied. We aim to explore FMBI's association with NAFLD, associated liver fibrosis and atherosclerotic cardiovascular disease risk (ASCVD) risk. METHODS: This retrospective study enrolled 1592 eligible participants aged 40-79 years from a Chinese hospital. The Fat-Muscle-BMI Index (FMBI) was calculated as Fat-to-muscle ratio (FMR) × Body mass index (BMI). Multivariable logistic/linear regression with three hierarchical models analyzed FMBI's associations with NAFLD, hepatic fibrosis and ASCVD; restricted cubic splines (RCS, subgroup and sensitivity/E-value analyses validated the findings. RESULTS: Higher FMBI quartiles exhibited a progressive increase in NAFLD prevalence (46.0% to 72.4%, P < 0.001). FMBI demonstrated a significant linear positive monotonic relationship with NAFLD (OR = 1.20 per unit increment, P < 0.001), with risk rising markedly as FMBI increased-particularly when FMBI ≥ 10. A significant additive interaction between FMR and smoking was observed (AP = 0.37, P = 0.010). Additionally, FMBI is independently associated with ASCVD risk (β = 0.32, P = 0.001) and hepatic fibrosis (OR = 1.79 for Q4 vs. Q1, P = 0.008) in NAFLD patients. E-value analysis confirmed the robustness of these findings to unmeasured confounding. CONCLUSION: This study identifies FMBI as a robust NAFLD biomarker exhibiting linear dose-response, additive synergy with smoking, and independent prediction of hepatic fibrosis and ASCVD risk, supporting its use in targeted NAFLD management.