Postmenopausal overweight and breast cancer risk; results from the KARMA cohort.

Purpose: To study the risk of incident breast cancer and subtype-specific breast cancer in relation to excess body weight in a contemporary Swedish prospective cohort study, The Karolinska Mammography Project for Risk Prediction of Breast Cancer, KARMA.

Methods: A total of 35,412 postmenopausal women attending mammography and included in the KARMA study provided baseline data on body mass index (BMI) and potential confounders. During eight years of follow-up, 822 incident invasive breast cancer cases were identified.

Results: Women with overweight (BMI ≥ 25-< 30 kg/m) constituting 34% of the study cohort had an increased risk of incident breast cancer with an adjusted Hazard Ratio (HR) 1.19 (95% CI 1.01-1.4). A similar, however, non-significant, association was found for women with obesity (BMI ≥ 30 kg/m) conferring 13% of the cohort, with a HR of 1.19 (95% CI 0.94-1.5). Overweight was associated with risk of node-negative disease (HR 1.29, 95% CI 1.06-1.58), whereas obesity was associated with node-positive disease (HR 1.64, 95% CI 1.09-2.48). Both overweight and obesity were associated with risk of estrogen receptor positive (ER+) disease (HR 1.20, 95% CI 1.00-1.44 and HR 1.33, 95% CI 1.03-1.71, respectively), and low-grade tumors (HR 1.25, 95% CI 1.02-1.54, and HR 1.40, 95% CI 1.05-1.86, respectively). Finally, obesity was associated with ER+HER2 negative disease (HR 1.37, 95% CI 1.05-1.78) and similarly luminal A tumors (HR 1.43, 95% CI 1.02-2.01).

Conclusion: Overweight and obesity are associated with an increased risk of developing breast cancer, specifically ER+, low-grade, and for obesity, node-positive, high-risk breast cancer indicating a further need for risk communication and preventive programs.