Association of body mass index with risk of luminal A but not luminal B estrogen receptor-positive and HER2-negative breast cancer for postmenopausal Japanese women

Background The impact of body mass index (BMI) on the risk of postmenopausal estrogen receptor (ER)-positive breast cancers has been well documented. However, the mechanism for the impact of BMI on the etiology of luminal A and luminal B subtypes has not yet been identified. Methods We analyzed associations between BMI and breast cancers stratified by immunohistochemically defined intrinsic subtypes, and 1,297 Japanese women (615 breast cancer patients and 682 healthy women from a breast cancer screening program) were enrolled in a case–control study. ER-positive/human epidermal growth factor receptor 2 (HER2)-negative breast cancers were classified into luminal A and B subtypes according to Ki67 expression levels. Results Higher BMI was significantly positively associated with postmenopausal breast cancer risk for one-unit increase in BMI (adjusted odds ratio (aOR) 1.09, 95 % confidence interval (CI) 1.04–1.15; P = 0.0008). Analyses of postmenopausal women revealed that BMI was consistently and exclusively associated with luminal A incidence (aOR 1.18, 95 % CI 1.10–1.26; P