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One expert wasn't surprised by the findings.
"As a breast surgeon, one of my roles is to discuss prevention strategies for women," said Dr. Alison Estabrook, chief of the division of breast surgery at Mount Sinai St. Luke's and Mount Sinai Roosevelt hospitals in New York City.
"Exercise is certainly one prevention strategy I discuss for many reasons, and this study emphasizes the importance of physical activity and of its continuation in the postmenopausal years," she said.
In the study, a team led by Agnes Fournier at the Institut Gustave Roussy in Villejuif, France, tracked more than 59,000 postmenopausal women in France who were followed for an average of 8.5 years.
During that time, more than 2,100 of the women were diagnosed with primary invasive breast cancer.
Women who in the previous four years had done regular exercise equivalent to at least four hours of walking or cycling per week were 10 percent less likely to be diagnosed with breast cancer than those who did less exercise.
The breast cancer-reducing impact of regular exercise was independent of weight, body fat, waist circumference and exercise levels from five to nine years earlier, according to the study published Aug. 11 in the journal Cancer Epidemiology, Biomarkers & Prevention.
"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," Fournier said in a journal news release.
"Our study answers these questions. We found that recreational physical activity, even of modest intensity, seemed to have a rapid impact on breast cancer risk," she said.
However, because the effect quickly diminished after exercise ceased, "postmenopausal women who exercise should be encouraged to continue," Fournier added.
The study also "shows that it is not necessary to engage in vigorous or very frequent activities; even walking 30 minutes per day is beneficial," Fournier noted.
Dr. Stephanie Bernik is chief of surgical oncology at Lenox Hill Hospital in New York City. She said that the French study "gives us more evidence that indeed, exercise is one way post menopausal women can reduce their risk of invasive breast cancers."
The study couldn't prove cause-and-effect, and "the exact reason for the decreased risk is not known -- perhaps these women lived a healthier lifestyle overall," Bernik said.
"Regardless, it seems that adding exercise to one's routine improves health and decreases cancer risk," she concluded.
-- Robert Preidt
Copyright © 2014 HealthDay. All rights reserved.
SOURCES: Alison Estabrook, M.D, chief, division of breast surgery, Mount Sinai St. Luke's and Mount Sinai Roosevelt, New York City; Stephanie Bernik, M.D, chief, surgical oncology, Lenox Hill Hospital, New York City; Cancer Epidemiology, Biomarkers & Prevention, news release, Aug. 11, 2014
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