Healthnotes Newswire (January 11, 2007)—A higher level of physical activity is known to lower risk of breast cancer, but exercise apparently affects different types of breast cancer to different degrees. Researchers are gaining new insights into what effects physical activity has on breast cancer occurrence, and how those effects may differ from person to person.

“Physical activity is a potentially modifiable breast cancer risk factor,” said James R. Cerhan, MD, PhD, of the Department of Health Sciences Research at the Mayo Clinic College of Medicine and one of the authors of the new study. “However, a few studies have reported no benefit or an increased risk.” Cerhan suggests that since not all breast cancer is the same, risk factors may differ based on the tumor characteristics, including how the tumor cells respond to hormones such as estrogen and progesterone.

If cells have receptors for both estrogen and progesterone or receptors for one of the two hormones, the cancer is considered hormone-receptor-positive.

About 75% of breast cancers are estrogen-receptor-positive (“ER-positive” or “ER+”) and about 65% of ER-positive breast cancers are also progesterone-receptor-positive (“PR-positive” or “PR+”). Approximately 25% of breast cancers are ER-negative (“ER–”) and PR-negative (“PR–”) or of unknown status. About 10% of breast cancers are ER+ and PR–, and 5% are ER– and PR+.

Breast cancer is the second leading cause of cancer-related death among women in the United States, and its incidence is increasing around the world. This increase appears to be related, at least in part, to lifestyle factors such as diet and exercise.

As part of the Iowa Women’s Health Study (IWHS), which began in 1986 and enrolled 41,836 women ages 55 to 69, participants rated their physical activity during “free time” on a questionnaire. The researchers found a 14% decreased risk of postmenopausal breast cancer among the most physically active women. This protective association was most marked for ER+ tumors, particularly ER+/PR− tumors, which are more aggressive.

“Physical activity decreases the body’s production of estrogen by reducing fat tissue, the major source of estrogens in postmenopausal women,” Cerhan explained. Lowering estrogen levels could lead to decreased ER-positive tumors. Cerhan says that further studies are needed to confirm these findings.

(Arch Intern Med 2006;166:2478–83)