Relationship between the Incidence of Metabolic Syndrome and Breast Cancer

Purpose Breast cancer affects females from puberty onward, with incidence rates increasing with age. Although metabolic syndrome (MetS) has reportedly increased the incidence of almost all cancers, no clear consensus on the role of MetS in breast cancer development exists. We aimed to clarify the effects of MetS on breast cancer incidence. Methods To investigate this relationship, we analyzed Japanese healthcare data of females from 2005 to 2020 and examined the incidence of breast cancer. MetS was evaluated based on the Japanese criteria or the NCEP ATP III guidelines at enrollment. Of 1,144,791 participants without missing data in our general public cohort, 32,775 with breast cancer at the beginning of the observation period were excluded; 54,330 participants with breast cancer were identified during the observation period. Results Both pre-stage MetS and MetS, defined using the Japanese criteria, decreased the incidence of breast cancer (hazard ratios [HRs], 0.90 and 0.83; p  < 0.005). Furthermore, MetS using NCEP ATP III decreased the HR (0.87, p  < 0.005), and the number of the factors from 1 to 5 gradually decreased the HRs. Analysis according to age group revealed that this observation was the most prominent in the < 50-year-old group. Conclusion MetS is associated with decreased breast cancer incidence in females, especially aged < 50 years.