May 8, 2013 | 2 minute read

For postmenopausal breast cancer, there’s a strong body of evidence that shows exercising reduces the risk. But cancer can take years to develop. A new study that may help explain the link now suggests that when young women jog and are aerobically active it causes changes in estrogen metabolism, which then plays a role in reducing later breast cancer risk.

The study, published in Cancer Epidemiology, Biomarkers & Prevention, is one of only a few clinical trials to focus on exercise and estrogen metabolism among younger women.

Study researchers wanted to focus on estrogen metabolism because the majority of breast cancers are related to the hormone estrogen. Research suggests that a higher lifetime exposure to estrogen increases a woman’s risk for breast cancer. Yet there are many forms of estrogen and they appear to play a different role in risk.

Lab studies have suggested that two of the forms, estradiol and estrone, play a role in cancer development. These forms of estrogen break down or metabolize into compounds and it’s the ratio of these metabolites that studies have suggested may influence breast cancer risk.

For this study, researchers randomly divided almost 400 sedentary young women into two groups: about half of the women were asked to exercise regularly and the others continued with their inactive lifestyle. All the women were premenopausal and the groups included women who were roughly the same age and weight.

For 16 weeks, the exercising women took to the step, the treadmill or the elliptical machine for half an hour five days a week so that they were getting a moderate to vigorous workout.

Both right before and after the 16-week exercise period, researchers collected three-days of urine samples. They then analyzed the samples for estradiol, estrone and another parent compound, and nine of their metabolites.

By the end of the study, the forms of estrogen and their metabolites were significantly different between the exercisers and the inactive women. The exercisers had far less estrone, for example, and an increase in the amount of the metabolites linked to decreased risk.

The women who exercised also led to improved fitness and less body fat.

Breastfeeding and staying a healthy weight are other ways that AICR’s expert report and its continuous updates found reduce the risk of breast cancer. You can read about the evidence in Learn More about Cancer.