Exercise Cuts Breast Cancer Risk

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Women who engaged in high levels of physical activity had a 14 percent lower risk of breast cancer than those who participated in low levels of activity. After the researchers adjusted for body mass index (BMI, a ratio of weight to height), they found that women with high physical activity levels had a 9 percent lower risk of breast cancer.

"Physical activity protected against breast cancer, but when we adjusted for BMI, it weakened the association," Cerhan explained.

This would imply that something in addition to exercise contributed to the protective effect.

The findings were even more striking when hormone receptor status was taken into account. Women with high physical activity levels had a 33 percent lower risk of developing estrogen receptor-positive tumors.

High levels of physical activity included vigorous exercise such as jogging, swimming or racket sports two or more times a week or moderate activity such as bowling, golf, gardening or walking more than four times a week. Medium physical activity was vigorous activity once a week or moderate activity one to four times a week.

Being overweight is a well-known risk factor for breast cancer, as body fat produces estrogen that signals some tumors to grow.

"This is another reason to be physically active. But I think much of the information is for medical scientists trying to think about the mechanisms of breast cancer," Cerhan said.

More information

To learn more about breast cancer, visit the American Cancer Society.