Background: Lifestyle factors are associated with overall breast cancer risk, but less is known about their associations, alone or jointly, with risk of specific breast cancer subtypes.
Methods: We conducted a case-control subjects study nested within a cohort of women who participated in the Norwegian Breast Cancer Screening Program during 2006-2014 to examine associations between risky lifestyle factors and breast cancer risk. In all, 4402 breast cancer cases subjects with information on risk factors and hormone receptor status were identified. Conditional logistic regression was used to estimate odds ratios (ORs), with 95% confidence intervals (CIs), in relation to five risky lifestyle factors: body mass index (BMI) of 25 kg/m² or greater, three or more glasses of alcoholic beverages per week, ever smoking, fewer than four hours of physical activity per week, and ever use of menopausal hormone therapy. Analyses were adjusted for education, age at menarche, number of pregnancies, and menopausal status. All statistical tests were two-sided.
Results: Compared with women with no risky lifestyle behaviors, those with five had 85% (OR = 1.85, 95% CI = 1.42 to 2.42, P trend < .0001) increased risk of breast cancer overall. This association was limited to luminal A-like (OR = 2.20, 95% CI = 1.55 to 3.12, P trend < .0001) and luminal B-like human epidermal growth factor receptor 2 (HER2)-positive (OR = 1.66, 95% CI = 0.61 to 4.54, P trend < .004) subtypes. Number of risky lifestyle factors was not associated with increased risk of luminal B-like HER2-negative, HER2-positive, or triple-negative subtypes (P trend > .18 for all).
Conclusions: Number of risky lifestyle factors was positively associated with increased risk for luminal A-like and luminal B-like HER2-positive breast cancer.