ALT/AST ratio as an early marker of metabolic multimorbidity in young adults: A community-based study.

Background: Metabolic multimorbidity, characterized by the clustering of metabolic dysfunction-associated steatotic liver disease (MASLD, formerly NAFLD) and metabolic syndrome (MetS), is increasingly recognized in young adults and may reflect early systemic metabolic dysfunction. However, simple biomarkers for identifying metabolic abnormalities in asymptomatic populations remain limited. Objective: To evaluate the ALT/AST ratio as a marker of metabolic multimorbidity and examine its association with modifiable behavioral factors. Methods: This community-based cross-sectional study included 3,796 asymptomatic adults aged 20-40 years. Moderate-to-severe fatty liver was assessed by ultrasonography. The discriminatory performance of the ALT/AST ratio was evaluated using ROC analysis and compared with ALT, white blood cell count, and neutrophil-to-lymphocyte ratio. Multivariable logistic regression identified independent associations. Results: An ALT/AST ratio >1.3 showed moderate discriminatory performance for fatty liver (AUC 0.791) and MetS (AUC 0.780), with modestly improved discrimination compared with ALT alone and inflammatory markers, particularly in reclassification analyses. Individuals with ALT/AST >1.3 were more likely to have fatty liver (OR 6.13, 95% CI 5.15-7.30) and MetS (OR 5.81, 95% CI 4.61-7.34). Regular physical activity and dental scaling were associated with lower odds of metabolic abnormalities, whereas smoking and betel nut chewing were associated with higher odds of metabolic abnormalities. Conclusion: The ALT/AST ratio was associated with fatty liver and metabolic syndrome in young adults and showed moderate discriminatory performance. Further longitudinal and external validation is needed before clinical implementation.