As the obesity epidemic pushes more and more women into the plus-size shopping section, it also bumps up their risk for breast cancer, especially after menopause. Fat cells produce the hormone estrogen, and high levels of that hormone have been shown to increase the risk of breast cancer. But whether or not women can improve their odds and reduce the levels of estrogen in their bodies through exercise is still unknown. Researchers from around the country studied a group of Seattle women to try to find out.

What the researchers wanted to know: Can a regular exercise program decrease body fat and reduce estrogen levels in the body?

What they did: The researchers recruited about 170 women who lived near Seattle and split them into two groups. They told both groups not to change their eating habits but gave one group a strict exercise regimen. Those women worked out at least five days a week for 45 minutes a day for 12 months, sometimes in supervised gym sessions and sometimes at home. The second group was required to attend only one 45-minute stretching session a week and told not to change any of their other exercise habits. At the beginning, after three months, and at the end of the study, both groups had a blood sample drawn and were given a physical exam, a cardio-fitness test, surveys about how much they ate, and X-ray body scans to measure body fat concentrations.

What they found: The women in the exercise group lost an average of about 3 pounds; the women in the stretching group gained, on average, a little less than one quarter of a pound. The women in the exercise control group also lost a significant amount of fat around their waists, an area where studies have shown fat is especially dangerous, compared with a small gain in the other group. When the researchers looked at the blood samples from the women, they found that women who lost more than 2 percent of their body fat also had declines in the amount of estrogen their body produced—potentially lowering their risk for breast cancer.

What it means to you: The authors concluded that, for overweight, postmenopausal women, exercise can reduce body fat, especially abdominal body fat, which probably helps prevent cancer because a body with fewer fat cells produces less estrogen. The women were not asked to change their diet, so the weight loss in the exercise group most likely came from sweating, not starving. The bottom line: Exercise can help postmenopausal women lose weight and potentially reduce their risk of breast cancer.

Caveats: This study was done on overweight, postmenopausal women so it's unclear if it applies to thinner women, In addition, previous studies have shown slightly less risk of breast cancer for overweight women who have not yet hit menopause, so the effects of exercise would most likely be different for them. In addition, for women in the exercise group, they relied on diaries kept by the women themselves on how much they exercised. Though the women might have a tendency to lie out of the guilt of skipping a day, that would strengthen the argument that exercise might be a prevention tool for cancer.

Find out more: Another study, done earlier than these two, offers more evidence that exercise can help reduce the risk of breast cancer. Find out about that study on the nonprofit website breastcancer.org.

If you're having trouble figuring out where to get started, the National Safety Council has 10 tips to help you fit exercise into your daily life.

Read all three articles on this study: McTiernan, A. et al. "Effect of Exercise on Serum Estrogens in Postmenopausal Women: A 12-month Randomized Clinical Trial" Cancer Research. April 15, 2004, Vol. 64, No. 8, pp. 2923–2928.

Abstract online: http://cancerres.aacrjournals.org

McTiernan, A. et al. "Effect of Exercise on Serum Androgens in Postmenopausal Women: A 12-month Randomized Clinical Trial." Cancer Epidemiology, Biomarkers & Prevention. July 2004, Vol. 13, No. 7, pp. 1099–1105.

Abstract online: http://cebp.aacrjournals.org

Irwin, M. L. "Effect of Exercise on Total and Intra-abdominal Body Fat in Postmenopausal Women: A Randomized Controlled Trial." Journal of the American Medical Association. Jan. 15, 2003, Vol. 289, No. 3, pp. 323–330.

Abstract online: http://jama.ama-assn.org