Many studies have shown that exercise protects against the development of breast cancer after menopause. A new study in the Journal of the National Cancer Institute shows that exercise reduces the risk of breast cancer before menopause as well.

Investigators from the Washington University School of Medicine analyzed information from 64,777 premenopausal women who participated in the Nurses' Health Study II. These women were followed for six years, and 550 of them developed breast cancer in that time. Based on analysis of self-completed questionnaires, the investigators found that the most active women had a 39% lower risk of breast cancer than the least active women. The most active women reported exercise that was equivalent to 13 hours or walking or 3.25 hours of running every week. The most important association for premenopausal breast cancer risk was total activity, rather than the type or relative difficulty of the exercise performed. Unfortunately, the reduction in risk for premenopausal breast cancer was not found to be associated with physical activity after the age of 35.

A related study in the British Journal of Sports Medicine examined 62 studies on the relationship between exercise and breast cancer and also showed that the reduction in breast cancer risk was proportional to the amount of physical activity performed. Further study is needed to understand exactly how exercise protects against breast cancer. However, since physical activity is one of the few modifiable risk factors for breast cancer, women of all ages should be encouraged to exercise more.