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A new study examined the effects of exercise on breast cancer risk for postmenopausal women. The researchers found that when postmenopausal women started a regular exercise regimen, their risk of developing breast cancer fell significantly.

For this study, the researchers measured physical activity based on metabolic equivalent task-hours (MET-h) per week. They analyzed data taken from biennial questionnaires that were filled out by 59,308 postmenopausal women. The participants were a part of E3N, which was the French portion of the European Prospective Investigation Into Cancer and Nutrition (EPIC) Study. The follow-up study lasted a mean of 8.5 years in which 2,155 people were diagnosed with primary invasive breast cancer.

"Twelve MET-h per week corresponds to walking four hours per week or cycling or engaging in other sports two hours per week and it is consistent with the World Cancer Research Fund recommendations of walking at least 30 minutes daily," said Agnès Fournier, PhD, a researcher in the Center for Research in Epidemiology and Population Health at the Institute Gustave Roussy in Villejuif, France.

The researchers compared the women's risk of breast cancer based on their MET-h per week. They found that women who took part in 12 or more MET-h per week of physical activity reduced their invasive breast cancer risk by 10 percent when compared to women who did not exercise as often. Activity did not have to be rigorous, intensive exercises.

"Physical activity is thought to decrease a woman's risk for breast cancer after menopause. However, it was not clear how rapidly this association is observed after regular physical activity is begun or for how long it lasts after regular exercise stops. Our study answers these questions," Fournier said reported in the press release. "We found that recreational physical activity, even of modest intensity, seemed to have a rapid impact on breast cancer risk. However, the decreased breast cancer risk we found associated with physical activity was attenuated when activity stopped. As a result, postmenopausal women who exercise should be encouraged to continue and those who do not exercise should consider starting because their risk of breast cancer may decrease rapidly."

The study, "Postmenopausal Breast Cancer Risk Decreases Rapidly After Starting Regular Physical Activity," was published in the journal, the American Association for Cancer Research.