Fibrosis-4 (FIB-4) Index Is Associated With Skeletal Muscle Mass in Japanese Patients With Type 2 Diabetes: A Single-Center Retrospective Cross-Sectional Study.

Introduction Liver fibrosis is increasingly recognized as a metabolic complication that may contribute to reduced skeletal muscle mass in type 2 diabetes. While the fibrosis-4 (FIB-4) index is a widely used non-invasive liver fibrosis marker, its clinical utility for assessing muscle mass remains unclear. Chronic inflammation and insulin resistance, which drive liver fibrosis, are also implicated in muscle degradation via the liver-muscle axis. We investigated the association between the FIB-4 index and low skeletal muscle mass index (SMI) in Japanese patients with type 2 diabetes, focusing on the influence of age. Methods This retrospective, cross-sectional study included 476 outpatients. SMI was assessed via bioelectrical impedance analysis; low SMI was defined by the Asian Working Group for Sarcopenia (AWGS) 2019 criteria. The relationship between the FIB-4 index and low SMI was evaluated using multiple logistic regression analyses and the Cochran-Mantel-Haenszel test, stratified by age (<65 vs. ≥65 years). Results Low SMI was identified in 111 (23%) of participants. In multivariable models adjusted for sex, BMI, and cerebrovascular disease, FIB-4 index was significantly associated with low SMI (OR 1.65, 95% CI 1.07-2.56, P=0.02). However, when age was added as a covariate, this association lost statistical significance (OR 0.83, 95% CI 0.48-1.42, P=0.50). Subgroup analysis revealed that in patients aged <65 years, those with FIB-4 index ≥1.3 had a significantly higher prevalence of low SMI (χ2=4.44, P=0.04), whereas no such association was observed in those aged ≥65 years. Conclusions The apparent association between the FIB-4 index and low SMI is predominantly driven by the confounding effect of age. While exploratory analyses suggest a potential signal in non-elderly patients (<65 years), the independent clinical utility of the FIB-4 index for assessing muscle mass remains uncertain and requires further prospective validation.