A new analysis has found that physical activity -- either mild or intense and before or after menopause -- may reduce breast cancer risk, but substantial weight gain may negate these benefits. Published early online in CANCER, a peer-reviewed journal of the American Cancer Society, the findings indicate that women can reduce their breast cancer risk by exercising and maintaining their weight.

While studies have shown that physical activity reduces breast cancer risk, many questions remain. For example, how often, how long, and how intense does physical activity have to be to provide benefits? Also, do women with all body types experience a reduced risk when they exercise, and does exercise reduce the risk of all types of breast cancer?

To investigate, Lauren McCullough, of the University of North Carolina Gillings School of Global Public Health in Chapel Hill, and her colleagues looked for a link between recreational physical activity, done at different time points in life, and the risk of developing breast cancer.

The study included 1,504 women with breast cancer (233 noninvasive and 1,271 invasive) and 1,555 women without breast cancer who were 20 to 98 years old 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 postmenopausal years had a reduced risk of developing breast cancer. Women who exercised 10 to 19 hours per week experienced the greatest benefit with an approximate 30% reduced risk. Risk reductions were observed at all levels of intensity, and exercise seemed to preferentially reduce the risk of hormone receptor positive breast cancer (ER or PR positive), which is the most commonly diagnosed tumor type among American women.

"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," said McCullough.

When the researchers looked at the joint effects of physical activity, weight gain, and body size, they found that even active women who gained a significant amount of weight -- particularly after menopause -- had an increased risk of developing breast cancer, indicating that weight gain can eliminate the beneficial effects of exercise on breast cancer risk.