Association between metabolic dysfunction-associated steatotic liver disease and breast cancer risk in Korean women: a nationwide population-based cohort study

Metabolic dysfunction-associated steatotic liver disease (MASLD) is a common chronic liver disorder linked to systemic metabolic imbalance. Although MASLD has been associated with extrahepatic cancers, including breast cancer, evidence is limited, especially among Asian populations. Given the younger peak incidence of breast cancer in Korea, this study examined the association between MASLD and breast cancer risk in middle-aged Korean women. We conducted a nationwide, population-based cohort study of 483,279 randomly selected Korean women aged 40–60 years who underwent health checkups from 2012 to 2015. MASLD was defined using the International Classification of Disease-10 (ICD-10) codes or a fatty liver index (FLI ≥ 60) with metabolic criteria. Breast cancer cases were identified using ICD-10 codes (C50, D05) and followed through 2021. Adjusted hazard ratios (HRs) were estimated with Cox proportional hazards models, including subgroup analyses by body mass index (BMI) and menopausal status. MASLD was not significantly linked to overall breast cancer risk (adjusted HR = 1.089, 95% CI 0.984–1.205, p = 0.099). However, women with BMI 25–30 kg/m² showed a significantly higher risk (adjusted HR = 1.077, p = 0.011). Although postmenopausal women had a lower overall risk, MASLD was significantly associated with increased risk in postmenopausal women with BMI 25–30 kg/m² (adjusted HR = 1.203, 95% CI 1.029–1.407, p = 0.021). In this large Korean cohort, MASLD was not independently linked to overall breast cancer risk, but moderate obesity and menopause may influence this relationship. These findings highlight the need to consider metabolic and hormonal factors in risk assessment.