Physical activity and postmenopausal breast cancer: effect modification by other breast cancer risk factors

Epidemiological evidence suggests an inverse association between physical activity (PA) and postmenopausal breast cancer risk. Breast cancer is a heterogeneous disease, influenced by reproductive factors, lifestyle pattern, and predispositions. We investigated whether these risk factors modify the effect of PA on breast cancer risk.

We analyzed data from 2004 hormone-receptor-positive postmenopausal breast cancer cases and 6569 controls from the population-based MARIE study conducted 2002-2005 in Germany. Interaction was statistically tested using adjusted unconditional logistic regression models.

The inverse association between leisure-time PA and risk of postmenopausal hormone-receptor-positive breast cancer was not heterogeneous by family history of breast cancer or by hormone therapy. PA showed a significant interaction with benign breast diseases (p = 0.023) and with breastfeeding (p = 0.045) but not with parity (p = 0.94), with clear risk reductions only for women who ever had breastfed or who ever had a benign breast disease (among ever breastfed: odds ratio = 0.63; 95% confidence interval = (0.52, 0.77), highest vs. lowest PA quartile). Interaction with BMI was weak (p = 0.053).

Breastfeeding and benign breast diseases modified the effect of PA on postmenopausal breast cancer risk. If other studies find similar modifications, increasing knowledge about these risk factors may contribute to a better understanding of the mode of action of PA on breast cancer risk. For women who are at higher risk for breast cancer due to family history or due to hormone therapy use, it is encouraging that they might lower their risk by being physically active.