Body mass index (BMI) is an independent risk factor for
luminal-type
breast cancer in Western populations. However, it is unclear whether the impact of BMI differs according to
breast cancer subtype in Japanese populations. We conducted a case-control study with 715 cases and 1430 age- and menopausal status-matched controls to evaluate the associations of BMI and its change (from age 20 years to the current age) with
breast cancer risk. We applied conditional logistic regression models to estimate odds ratios (
ORs) and 95% confidence intervals (CIs).
Tumor subtypes were divided into four subtypes, namely the
luminal,
luminal/HER2, HER2-rich, and triple-negative subtypes. Current BMI and BMI change were positively associated with postmenopausal
breast cancer risk. On stratified analysis by
tumor subtype, we observed associations between current BMI and BMI change and postmenopausal
breast cancer risk for the
luminal subtype, with OR for each 1 kg/m(2) increase in current BMI of 1.14 (95% CI: 1.07 - 1.20) and the corresponding OR of BMI change of 1.16 (1.09 - 1.23) (each P(trend) < 0.001). Additionally, we found the same tendency for the triple-negative subtype, with the OR for a 1 kg/m(2) increase in current BMI of 1.21 (1.05 - 1.39) and that for BMI change of 1.18 (1.02 - 1.36) (P(trend) was 0.008 and 0.024, respectively). In premenopausal women, a suggestive inverse association was observed between BMI change and
breast cancer risk for the
luminal subtype only, with OR of BMI change of 0.93 (0.87 - 1.00, P(trend) = 0.054). No association was seen between BMI at age 20 years and risk of any
tumor subtype. In conclusion, BMI and its change are associated with the risk of both
luminal and
triple-negative breast cancer among postmenopausal Japanese women. These findings suggest the etiological heterogeneity of
breast cancer among
tumor subtypes.
ELECTRONIC SUPPLEMENTARY MATERIAL: The online version of this article (doi:10.1186/2193-1801-1-39) contains supplementary material, which is available to authorized users.