Women of all ages can reduce their risk of breast cancer by exercising regularly. Previous studies have shown that regular exercise can protect against the disease.
Writing in the Journal of the American Medical Association, they said that even post-menopausal women can reduce their risks by exercising.
Regular exercise: Dr Anne McTiernan and colleagues from the Fred Hutchinson Cancer Research Center in Seattle based their findings on a survey of each of the women.
The researchers found that women who exercised regularly reduced their risk of developing breast cancer by 20%.
Moderate exercise, such as walking, cycling or swimming, five times a week had the greatest impact.
They found that physical activity also reduced the risks for women who may have a greater chance of developing the disease.
This includes those with a family history of breast cancer and those taking hormone replacement therapy.
Women should aim to do moderate exercise 30 minutes each day, five days a week. Women who do not exercise regularly should start off slowly, increasing their physical activity over time.
Breast Cancer Care welcomed the study. Anna Wood, its policy analyst, said: Age is the strongest risk factor for breast cancer, and this research shows that moderate exercise such as walking or swimming is something older women can do to help reduce their risk of breast cancer.
Women often want to know what they can do in order to reduce their risk of breast cancer and this research suggests a simple, positive and empowering lifestyle change.