Postmenopausal women who engage in more vigorous physical activity seem to have a lower risk of breast cancer. The beneficial effect was most pronounced for estrogen receptor positive/progesterone receptor negative tumors, which are generally more aggressive.

"It seems like another confirmation to the fact that exercise will help reduce the risk of breast carcinoma and may play some other interesting roles in addition to effects on cardiovascular health," said Dr. Jay Brooks, chairman of hematology/oncology at Ochsner Health System in Baton Rouge, La.

While overweight and obesity are risk factors for breast cancer, the findings also suggest that additional mechanisms may be involved in promoting the growth of tumors.

The association between exercise and a reduced risk of breast cancer is not a new one. Previous studies have shown that physical activity can reduce the risk for the disease among women of all ages.

But because breast cancer is such a varied disease, there may be different risk factors, depending on different tumor characteristics, including estrogen receptor (ER) and progesterone receptor (PR) status. Receptor status refers to whether these hormones bind to the surface of the tumor.

Only three small studies have looked at the association between physical activity and postmenopausal breast cancer defined by receptor status, the study authors said.

The new study, called the Iowa Women's Health Study, is the largest study to cross-classify estrogen receptor (ER) and progesterone receptor (PR) status, said Dr. James Cerhan, senior author of the study and professor of epidemiology at the Mayo Clinic College of Medicine in Rochester, Minn. The findings are published in the Dec. 11/25 issue of Archives of Internal Medicine.

Cerhan and his colleagues looked at 41,836 postmenopausal women who were between the ages of 55 and 69 in 1986. The participants filled out a 16-page questionnaire at the beginning of the study about their recreational physical activity and then were monitored for 18 years.

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.