Exercising reduces breast cancer risk.

Women who exercise 3 or more hours a week in the decade following menarche -- the onset of their menstrual cycle -- can lower their risk of breast cancer 30 percent by age 40, compared to their more sedentary peers. Continuing that moderately active lifestyle until at least age 40 can cut their risk by almost 60 percent, a new study reports.

What makes these findings particularly important, says study leader Leslie Bernstein of the University of Southern California School of Medicine in Los Angeles, is that unlike many risk factors for this cancer -- such as early menarche, late menopause, having a mother or sister with the disease, and higher socioeconomic status -- exercise is something over which women have considerable control.

The California researchers studied 545 Los Angeles County breast cancer patients -- all age 40 or younger -- and an equal number of cancerfree neighbors of the same age with the same number of children. Questionnaires documented in detail each woman's exercise and reproductive history.

Bernstein and her coworkers focused on premenopausal women because previous studies by several groups, including hers, had shown that moderate exercise can reversibly shut down ovulation in adolescents and young women. This tends not to be true of women beyond their middle to late 20s, Bernstein notes. Because it appears that the more often a woman ovulates -- each time boosting the circulation of certain reproductive hormones -- the greater her breast cancer risk, the California group reasoned that early exercise might prove most protective and show up best in young women.

In fact, consistently exercising about 4 hours per week appeared to be protective, even if begun well after adolescence, the group now reports in the Sept. 21 JOURNAL OF THE NATIONAL CANCER INSTITUTE.

"That to me was the most surprising finding," says Bernstein, because it suggests that exercise affects hormone production beyond young adulthood. How? It may lower the secretion of ovulatory hormones in each menstrual cycle, she speculates, "so estrogen levels may not peak as high."

Moreover, the study found that exercise exerts this effect independent of other factors, such as weight, age at menarche, or number of children.

A companion study is examining cancer risk in 8,000 women 35 to 64 years old. Personally, Bernstein says, "I think we will also see an effect [of exercise] among postmenopausal women."

In an editorial accompanying the new study, Louise A. Brinton of the National Cancer Institute in Bethesda, Md., notes that earlier studies had hinted that exercise might lower breast cancer incidence. But Bernstein's team offers "fairly convincing evidence," she says, that "physical activity can lead to substantial reductions in breast cancer risk."