Exercise may lower a woman's risk of breast cancer by affecting the way her body breaks down estrogen, according to a new study in the journal Cancer Epidemiology, Biomarkers & Prevention.
Researchers from the University of Minnesota said that observational studies have long shown associations between lowered breast cancer risk and increased exercise, but we had little information as to why this may be the case.
"Ours is the first study to show that aerobic exercise influences the way our bodies break down estrogens to produce more of the 'good' metabolites that lower breast cancer risk," study researcher Mindy S. Kurzer, Ph.D., a professor in the university's food science and nutrition department, said in a statement.
The study included 391 healthy premenopausal women who led sedentary lifestyles; 179 of them were put in a control group where they just kept living a sedentary lifestyle, while 212 were placed in an exercise group where they did 30 minutes of aerobic exercises (moderate to vigorous level, such as running on a treadmill or using the elliptical machine) five days a week for four months. Of all the women in the study, 78 percent of those from the exercise group and 86 percent of those from the control group completed the study.
Researchers took urine samples of the women for three days before the study began, in order to measure three estrogens and their metabolites (the product that remains after the estrogens are broken down). Researchers knew beforehand that a particular ratio of one kind of estrogen metabolite with another is linked with having a lower risk of breast cancer.
Indeed, researchers found that the women who were assigned to the exercise group experienced an increase in this protective estrogen metabolite ratio, while the ratio didn't change at all for those who remained sedentary.
ABC News previously reported on research published last year showing links between exercise and breast cancer risk; the news organization noted that the association may be because exercise decreases weight, and there is a lot of estrogen that arises from fat tissue. For more on that research, click over to ABC News.