Postmenopausal Obesity and Dyslipidemia as Risk Factors for Breast Cancer in Korean Women: Analysis of a National Health Screening Cohort

<b>Background/Objectives</b>: Over the past two decades, the incidence of breast cancer has been increasing in Korea, which is potentially attributable to longer life expectancies, Westernized lifestyles, and declining fertility rates. However, the contributions of modifiable metabolic and behavioral risk factors in Asian populations remain underexplored. This study aimed to assess the associations between health-related factors and incidence of breast cancer in a large Korean cohort. <b>Methods</b>: We analyzed data from the Korean National Health Insurance Service and included women who underwent health screening in 2009. Cases of breast cancer diagnosed between 2010 and 2021 were identified using medical claims and registration codes. The breast cancer cases were matched to controls in a 1:4 ratio based on age, income, and region of residence. Conditional logistic regression was used to calculate the adjusted odds ratios (aORs) and 95% confidence intervals (CIs) for key exposures. <b>Results</b>: In total, 52,869 breast cancer cases and 211,476 matched controls were included. The peak age group at diagnosis was 50-54 years. Dyslipidemia was associated with a 12% increase in the risk of breast cancer across all age groups (95% CI, 1.10-1.14). In women ≥ 50 years of age, a dose-response relationship was observed between body mass index (BMI) and breast cancer risk: aORs were 1.04 (95% CI, 1.01-1.08) for overweight, 1.14 (95% CI, 1.11-1.17) for obesity class I (BMI ≥ 25 to < 30 kg/m²), and 1.33 (95% CI, 1.26-1.41) for obesity class II (BMI ≥ 30 kg/m²). Conversely, being underweight was associated with a decreased risk (aOR, 0.81; 95% CI, 0.74-0.89). No consistent associations were observed with alcohol consumption, cigarette smoking, or the presence of diabetes mellitus. <b>Conclusions</b>: Postmenopausal obesity and dyslipidemia contribute to the risk of breast cancer among Korean women. Promoting healthy behaviors throughout life may support long-term risk reduction.