Higher Physical Activity Associated With Reduced Risk Of Breast Cancer

Breast cancer appears to be biologically heterogeneous, meaning that not all cases have the same causes or disease processes, according to background information in the article. One way cancers differ is by hormone receptor status. Tumors are classified as estrogen- and progesterone-receptor (ER/PR) positive or negative based on whether these hormones bind to the surface of the tumor. "Recent studies have found that various well-established risk factors for breast cancer vary by the ER/PR profile of the tumor, including age, menopausal status, parity, age at menarche, age at first pregnancy, hormonal use, family history, body mass index (BMI), waist-hip ratio, alcohol consumption, dietary fat intake and folate level," the authors write.

Aditya Bardia, M.D., M.P.H., of the Mayo Clinic College of Medicine, Rochester, Minn., and colleagues reported findings from the Iowa Women's Health Study, which includes 41,836 postmenopausal women who were age 55 to 69 in 1986. The women filled out a 16-page questionnaire at the beginning of the study that included information about how often and with what intensity they participated in physical activity during "free time." High physical activity (9,111 women) was defined as vigorous activity (such as jogging, swimming or racket sports) two or more times per week or moderate activity (such as bowling, golf, gardening or walking) more than four times per week; medium physical activity (10,030 women) was defined as participation in vigorous activity once per week or moderate activity one to four times per week; and low physical activity included the rest of the women (17,222). Follow-up questionnaires were completed in 1987, 1989, 1992, 1997 and 2002. Information about breast cancer cases, including ER/PR status, was identified through the Iowa Cancer Registry.

Through 2003, 2,548 cases of breast cancer were observed in the 36,363 women who were included in this analysis. Women with high physical activity levels based on the original survey had a 14 percent lower risk of developing breast cancer than those with low physical activity levels. After the researchers adjusted for body mass index (BMI), those with high physical activity levels had a 9 percent lower risk of breast cancer, suggesting that some but not all of the association was due to the effect of exercise on body weight. The inverse association between high physical activity levels and lower risk of breast cancer was strongest for women with ER+/PR- tumors; after adjusting for BMI, women with high physical activity levels had a 34 percent lower risk of developing this type of cancer.

Physical activity reduces body fat, the major source of estrogen in postmenopausal women, the authors write. "Lowering estrogen levels could lead to a decreased ER+/PR+ tumors, the opposite of which is seen in obesity, in which increased circulating estrogens are associated with increased ER+/PR+ tumors. Consistent with this mechanism, the association of physical activity with ER+/PR+ tumors attenuated after adjustment for BMI." Because adjusting for BMI did not change the risk of ER+/PR- tumors, it is possible that a different mechanism--such as certain growth factors that become more prevalent with exercise--may be involved in the association between physical activity and these tumors.

"Further studies are needed to confirm these novel findings, and to evaluate similar relationships among premenopausal women," the authors conclude. "If found to be causally related to breast cancer, physical activity would have a substantial public health effect on the prevention of this disease, along with its other positive health benefits."