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Researchers have long recommended exercise as a preventive tool to reduce the risk of breast cancer—and now new research helps explain how and why it works. The key is estrogen. Exercise appears to change the way women’s bodies metabolize, or break down, the hormone—increasing the ratio of “good” to “bad” estrogen metabolites and thereby reducing the risk of breast cancer, according to a study published in Cancer Epidemiology, Biomarkers & Prevention.

Breast cancer is the most commonly diagnosed type of cancer (other than skin cancer) in U.S. women. Each year, roughly 227,000 women are diagnosed with breast cancer and close to 40,000 die of the disease. There are a number of risk factors associated with breast cancer, including obesity, increased levels of circulating estrogen, and a sedentary lifestyle.

Regular physical activity has been shown to improve overall health and wellbeing as well as reduce the risk of cancer—but until now, there have been no clinical studies that explained the mechanism behind how exercise might reduce the risk of breast cancer. To explore this mechanism, researchers conducted a study that included 391 healthy, premenopausal women (meaning their bodies still produced estrogen) who were part of the WISER (Women in Steady Exercise Research) study. All of the women were between the ages of 18 and 30 and were considered sedentary prior to beginning the study.

The women were randomly assigned to one of two groups: one group remained inactive, while the other group performed 30 minutes of moderate-to-vigorous aerobic exercise five times a week for 16 weeks. Before and after the 16 weeks, the researchers collected urine samples from women in both groups.

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After 16 weeks, the women in the exercise group experienced significant increases in aerobic fitness and lean body mass and a significant decrease in percent body fat, whereas those in the control arm experienced no changes in body composition, aerobic fitness, and body weight despite a significant reduction in daily caloric intake. Notably, the urine samples at the end of the study indicated that the women in the exercise group had higher levels of the estrogen metabolites that reduce breast cancer risk. The researchers concluded that exercise changes the way the body breaks down estrogen, increasing the ratio of ‘good’ to ‘bad’ estrogen metabolites—and as a result, may reduce the risk of breast cancer.

The evidence in favor of exercise continues to stack up. If you needed another excuse to make exercise a priority, here it is. Get moving to reduce your risk of breast cancer.


Smith AJ, Phipps WR, Thomas W, et al. The effects of aerobic exercise on estrogen metabolism in healthy premenopausal women. Cancer Epidemiology, Biomarkers & Prevention. 2013; 22:756-764.