Most people know that regular exercise can reduce the risk of numerous ailments, including heart disease, adult-onset diabetes, and colon cancer. Now, new evidence suggests that physical activity may help protect women against breast cancer too.
An extensive study of more than 1,000 California women, published recently in the Journal of the National Cancer Institute, indicates that moderate, regular physical activity may reduce a woman's risk of developing premenopausal breast cancer by up to 60 percent. The greatest benefits came to women who exercised four hours a week, but even two or three hours of exercise weekly appeared beneficial. The protective effect appeared to be strongest among women who had borne children, but researchers were unsure why.
"Physical activity can have a dramatic effect on menstrual cycle patterns and on the production of estrogen and other sex hormones," notes study author Leslie Bernstein, a professor of preventive medicine at the University of Southern California's Norris Cancer Center. "Since breast cancer is an estrogen-dependent disease, the theory is that exercise reduces risk by lowering a woman's exposure to estrogen."
Bernstein is studying 3,000 women ages 55 to 64 to see if exercise exerts a similar protective effect on postmenopausal breast cancer. If her findings are confirmed by other studies, regular physical exercise could become the first modifiable lifestyle habit women can adopt to help reduce their risk of breast cancer Other established risk factors include family history and age at first menstrual period, at first full-term pregnancy, and at menopause.
But more data is needed to conclusively prove the link between exercise and reduced breast cancer risk, since previous studies show conflicting findings. For example, a report based on data from the Framingham Heart Study found that physically active women had a slightly higher incidence of breast cancer. "We don't know why this occurred, but it may be related to the fact that data was collected in the 1950s," said Joanne Dorgan of the National Cancer Institute, who published her findings last year in the American Journal of Epidemiology. …