A study done by a research team at University of Minnesota in St. Paul conducted a study of a group of women and results found that those who exercised showed changes in the estrogen metabolism which decreased their risk of cancer.

According to Time, scientists from the university conducted a study of 391 inactive, healthy, premenopausal women whom they split into two groups.

They concluded that "the 179 women in the intervention group, who received 30 minutes of moderate-to-vigorous exercise five times a week for over 16 weeks, showed changes in their estrogen metabolism that could explain the anti-cancer benefits of working out."

Mindy Kurzer, a professor in the Department of Food Science and Nutrition at the University of Minnesota made a statement about their unique study.

"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."

Time reported that researchers measured the levels of three parent estrogens, E1, E2 and E3 as well as nine breakdown products called metabolites in the women's urine samples.

At the end of the study period, the exercise group had healthier Body Mass Indexes and cardiovascular sustainability. The women also showed a 25% increase in the metabolite ratio 2-OHE1/16-alpha-OHE1, which has been linked to a lower breast cancer risk.

"More 2-OHE1 and less 16-alpha-OHE1 have been associated with a lower risk since 16-alpha-OHE1 is thought to encourage cancer cell growth."

These changes were not seen in the control group of women who remained inactive during the study period.

The study revealed that women who did any exercise at all had a 6% lower risk of breast cancer than idle women.

"However, women who had children and exercised about 10 to 19 hours each week either during their reproductive years or after menopause experienced a much greater benefit, with a 30% lower risk of breast cancer compared to women who exercised less or were inactive."

The study can be found in the journal Cancer Epidemiology, Biomarkers & Prevention.