The study, published online ahead of the print edition of the journal Cancer, found women could reduce their breast cancer risk by exercising and maintaining their weight.
Lauren McCullough, a doctoral candidate at the University of North Carolina at Chapel Hill Gillings School of Global Public Health, and colleagues said the study included 1,504 women with breast cancer -- 233 non-invasive and 1,271 invasive -- and 1,555 women without breast cancer who were ages 20-98 and were part of the Long Island Breast Cancer Study Project, an investigation of possible environmental causes of breast cancer.
Women who exercised either during their reproductive or post-menopausal years had a reduced risk of developing breast cancer.
The study found women who exercised 10 to 19 hours per week experienced the greatest benefit, with an approximate 30 percent reduced risk.
"The observation of a reduced risk of breast cancer for women who engaged in exercise after menopause is particularly encouraging given the late age of onset for breast cancer," McCullough said.
The researchers looked at the joint effects of physical activity, weight gain and body size, and found that even active women who gained a significant amount of weight -- particularly after menopause -- had an increased risk of developing breast cancer, the study said.