Alcohol, physical activity, smoking, and breast cancer subtypes in a large, nested case–control study from the Norwegian Breast Cancer Screening Program

Background: To what extent alcohol, smoking, and physical activity are associated with the various subtypes of breast cancer is not clear. We took advantage of a large population-based screening cohort to determine whether these risk factors also increase the risk of the poor prognosis subtypes. Methods: We conducted a matched case–control study nested within the Norwegian Breast Cancer Screening Program during 2006–2014. A total of 4,402 breast cancer cases with risk factor and receptor data were identified. Five controls were matched to each case on year of birth and year of screening. Conditional logistic regression was used to estimate ORs of breast cancer subtypes adjusted for potential confounders. Results: There were 2,761 luminal A–like, 709 luminal B–like HER2-negative, 367 luminal B–like HER2-positive, 204 HER2-positive, and 361 triple-negative cancers. Current alcohol consumption was associated with breast cancer risk overall [OR 1.26; 95% confidence interval (CI), 1.09–1.45] comparing 6+ glasses a week to never drinkers. However, this risk increase was found only for luminal A–like breast cancer. Smoking 20+ cigarettes a day was associated with an OR of 1.41 (95% CI, 1.06–1.89) overall, with significant trends for luminal A–like and luminal B–like HER2-negative cancer. Current physical activity (4+ hours/week compared with none) was associated with 15% decreased risk of luminal A–like cancer, but not clearly with other subtypes. Conclusions: In this large study, alcohol, smoking, and physical activity were predominantly associated with luminal A–like breast cancer. Impact: Alcohol, smoking, and physical activity were associated with luminal A–like breast cancer subtype.