Influence of Metabolic Syndrome on Risk of Breast Cancer: A Study Analyzing Nationwide Data from Korean National Health Insurance Service

Background: To investigate the influence of metabolic syndrome and its components on the risk of breast cancer. Methods: Retrospective nationwide cohort study analyzing data of 13,377,349 women older than 19 years from Korean National Health Insurance Service was performed. Cox proportional hazards model was used to calculate HR and 95% confidence interval (CI) of breast cancer risk. Results: The presence of metabolic syndrome decreased the risk of all breast cancer types in all subjects (HR, 0.954; 95% CI, 0.939–0.970). In women with age ≤50 years, metabolic syndrome decreased the risk of all breast cancer types, with similar findings for all subject groups (HR, 0.915; 95% CI, 0.892–0.939). In women with age >50 years, metabolic syndrome increased the risk of all breast cancer types (HR, 1.146; 95% CI, 1.123–1.170), especially in age groups of more than 55 years. In women with age >50 years, HRs increased as the number of metabolic syndrome components increased, while HRs decreased as the number of metabolic syndrome components increased in women with age ≤50 years. Conclusions: The presence of metabolic syndrome increased the risk of breast cancers in postmenopausal women, but decreased the risk in premenopausal women. Every metabolic syndrome component played similar roles on the risk of breast cancer as metabolic syndrome, and their effects became stronger when the number of components increased. Impact: Metabolic syndrome is associated with the risk of breast cancer having different effect according to age groups.