Physical activity lowers a woman's risk of breast cancer, researchers find.

The benefit is modest -- a 20 percent lower risk of breast cancer at ages 35 to 64 -- but it's real. It's just as true for black women as it is for white women.

And it doesn't take all that much exercise, report University of Southern California researcher Leslie Bernstein, PhD, and colleagues in the Nov. 16 issue of the Journal of the National Cancer Institute.

"An annual average of at least 1.3 hours of exercise activity per week ... from age 10 years onward was associated with a nearly 20 percent reduction in breast cancer risk," they write.

The Latest Treatments for Breast Cancer

Reducing Risk

Bernstein and colleagues interviewed 4,538 women with breast cancer. They also interviewed 4,649 women who did not have cancer.

They found that the effect of exercise did not differ between races. The risk reduction was equally strong for all stages and kinds of breast cancer.

Only one breast cancer risk factor seemed resistant to the effects of exercise: having a first-degree relative (a mother, sister, or daughter) with breast cancer. For these women, only those who got the most exercise -- an average of three or more hours of exercise every week -- saw a benefit.

Unfortunately, the researchers were not able to tell exactly when in a woman's life exercise would be most helpful or what kinds of exercise work best.

"Our data suggest that regular exercise activity throughout a woman's life can decrease breast cancer risk and that the association of physical activity and breast cancer risk is not materially modified by race," Bernstein and colleagues conclude.

The Breast Cancer Gene: What Should You Do?

By Daniel J. DeNoon, reviewed by Louise Chang, MD

SOURCES: Bernstein, L. Journal of the National Cancer Institute, Nov. 16, 2005; vol 97: pp 1671-1679.