Beyond BMI: central obesity identifies overlooked fatty liver disease risk in adults with normal body mass index undergoing routine health examinations.

Background: Fatty liver disease (FLD) is increasingly common in the general population, yet risk stratification in routine health examinations still relies heavily on body mass index (BMI). Because BMI does not capture abdominal fat distribution, adults with normal BMI but excess central adiposity may be overlooked despite having elevated hepatic risk. We examined the association of combined BMI-waist phenotypes with ultrasound-diagnosed FLD in a Chinese health examination population. Methods: This retrospective cross-sectional study included 2,336 adults undergoing routine health examinations at the Health Examination Center of the Third People's Hospital of Chengdu, China. Participants were classified into four BMI-waist phenotypes: normal BMI/no central obesity, normal BMI/central obesity, overweight-obesity/no central obesity, and overweight-obesity/central obesity. The primary outcome was ultrasound-diagnosed FLD; moderate-to-severe steatosis was analyzed as a secondary outcome. Logistic regression models were fitted with sequential adjustment for major covariates. Additional analyses were conducted among adults with normal BMI, including a sensitivity analysis using waist-to-height ratio. Results: The overall prevalence of FLD was 28.3%. FLD prevalence increased across BMI-waist phenotypes, from 11.2% in the normal BMI/no central obesity group to 27.8% in the normal BMI/central obesity group, 27.9% in the overweight-obesity/no central obesity group, and 51.5% in the overweight-obesity/central obesity group. In the fully adjusted model, compared with the normal BMI/no central obesity group, the odds ratios (ORs) for FLD were 2.46 (95% CI: 1.73-3.48) for normal BMI/central obesity, 2.74 (95% CI: 2.01-3.72) for overweight-obesity/no central obesity, and 6.78 (95% CI: 5.26-8.79) for overweight-obesity/central obesity. Among adults with normal BMI, central obesity remained associated with FLD in the fully adjusted model (OR 2.48, 95% CI: 1.73-3.52). Similar patterns were observed for moderate-to-severe steatosis and in sensitivity analyses using waist-to-height ratio. Conclusion: Central obesity was associated with higher observed odds of ultrasound-diagnosed FLD beyond BMI alone. These findings apply to etiology-agnostic ultrasound-detected fatty liver rather than MASLD/MASH-specific disease and suggest that waist circumference may complement BMI in routine health examinations.