Abstract: Objective To investigate the metabolic characteristics of patients with lean non-alcoholic fatty liver disease (NAFLD) and to analyze the severity of hepatic steatosis and liver fibrosis, thereby providing evidence for the clinical identification and risk assessment of lean NAFLD. Methods Between January 2021 and June 2025, study participants were classified into three groups according to body habitus and the presence of NAFLD: a lean NAFLD group (n=35), an obese NAFLD group (n=42), and a lean healthy control group (n=28). Differences among the three groups in metabolic parameters, the degree of hepatic steatosis and related indicators, as well as liver fibrosis-related indices were compared. Results Compared with the lean healthy control group, the lean NAFLD group showed significantly higher fasting insulin levels [FINS: 11.8 (8.9, 15.2) IU/mL vs 7.4 (5.6, 9.2) IU/mL] and homeostatic model assessment of insulin resistance (HOMA-IR) values [2.9 (2.1, 3.7) vs. 1.6 (1.2, 2.1)]. Triglyceride levels [TG: (1.7±0.6) mmol/L vs. (1.1±0.4) mmol/L] were increased, whereas high-density lipoprotein cholesterol levels [HDL-C: (1.1±0.3) mmol/L vs. (1.4±0.3) mmol/L] were decreased (all P<0.05). Indicators related to hepatic steatosis showed that liver fat content [(11.4±4.6)%] and controlled attenuation parameter (CAP) values [(262±38) dB/m] in the lean NAFLD group were significantly higher than those in the lean healthy control group [(3.2±1.5)%, (208±31) dB/m], but lower than those in the obese NAFLD group [(18.9±6.7)%, (302±46) dB/m] (P<0.05). In the lean NAFLD group, 18 patients (51.4%) had mild steatosis and 17 patients (48.6%) had moderate-to-severe steatosis. Assessment of liver fibrosis showed that liver stiffness measurement (LSM) [(7.9±2.6) kPa], the FIB-4 index [1.4 (1.0, 1.9)], and the NAFLD fibrosis score (NFS) [(-0.3±0.1)] were all significantly higher in the lean NAFLD group than those in the lean healthy control group [(4.8±1.3) kPa, 0.9 (0.6, 1.1), (-1.3±0.6)], while the platelet count was lower than that in the control group [(212±46)×10⁹/L vs. (236±41)×10⁹/L] (all P<0.05). In the lean NAFLD group, the proportion of patients with significant fibrosis (≥F2) was 25.7% (9/35), and the proportion with advanced fibrosis (≥F3) was 11.4% (4/35). Conclusion Although patients with lean NAFLD do not exhibit overt obesity, they already demonstrate a certain degree of metabolic abnormality and may be accompanied by a substantial risk of hepatic steatosis and liver fibrosis. In clinical practice, greater attention should be paid to screening and evaluation of NAFLD in lean individuals, so as to avoid underestimating disease severity based solely on body habitus.

Key words: Lean non-alcoholic fatty liver disease, Metabolic characteristics, Hepatic steatosis, Liver fibrosis