Exercise May Lower Breast Cancer Risk

Participating in regular physical "recreational" activity, even for just a few hours a week, may significantly lower a woman's risk of developing early, localized breast cancer, what doctors call breast carcinoma in situ (BCIS).

Dr. Leslie Bernstein of the University of Southern California and colleagues compared the self-reported exercise habits of 567 women diagnosed with BCIS with that of 616 "control" women who did not have the disease. All of the women were between the ages of 35 and 64.

In one-on-one interviews, the women provided information about their lifetime involvement in activities such as walking, jogging, bicycling, aerobics, swimming, sports, and dance. The researchers calculated each woman's weekly average hours and average energy spent exercising since she began menstruating.

After factoring out known breast cancer risk factors, such as a history of cigarette smoking and use of hormone replacement therapy, they found that overall the risk of BCIS was roughly 35 percent lower among women who reported "any" physical activity compared to physically inactive women. This level of reduced risk remained fairly constant with increasing levels of activity.

However, when the investigators subdivided the women into those with and without a family history of breast cancer, they found no reduction in breast cancer risk among women with a mother or sister with the disease.

"This is the first study of the impact of physical activity on BCIS," Bernstein stated.

In a previous study, her team found a "strong protective effect of lifelong exercise on the risk of invasive breast cancer," she said. "For invasive disease, the risk decreased proportionally with increasing level of exercise and was restricted to women without a family history of breast cancer."

In general, doctors are not exactly sure how physical activity may guard against early breast cancer but they have several theories. Exercise may lower levels of female hormones, especially during adolescence. The known ability of exercise to boost the immune system and make a person more sensitive to insulin may also play a role.

"By identifying risk or protective factors for BCIS, we have the opportunity to intervene a bit earlier in the cancer process," Bernstein said.

SOURCE: Cancer, November 15, 2003 (published online October 6 at www.interscience.wiley.com/cancer)

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