Teen Exercise May Help
Prevent Breast Cancer

Some teenage girls play sports for fun. Some work out to look good. Now there is an even better reason.

A new study suggests that young women who routinely exercise just a few hours a week during their teen years can lower by 30% to 35% their likelihood of developing a common type of breast cancer later in life. The research is the latest in a mounting body of science to show that working out, even in relatively small doses, promotes a biological shield against breast cancer. The findings also add support to a growing consensus among scientists that physical activity helps moderate a woman's exposure to female hormones, such as estrogen. In excessive amounts at certain times of life, such hormones can stimulate the growth of cancerous breast tumors.

Though the importance of exercise as a means of prevention against breast cancer has been documented in a number of studies, the new research is more specific. It suggests that preventive characteristics of physical activity are particularly effective during the adolescent and teenage years -- long before breast cancer is a threat, or even a worry, to most females.

Teen exercise appears to interfere with a young woman's monthly ovulation. Reduced ovulation means less estrogen production. "We believe less estrogen at a time when women's duct cells in the breasts are growing fastest, in the teen years, is protective," said Leslie Bernstein, a researcher at the University of Southern California, who led one of the new studies. Duct cells, which produce milk after pregnancy, are where most breast-cancer tumors arise.

Dr. Bernstein's project is the latest in a series of recent "energy balance" studies showing that excess weight and a lack of routine physical exertion strongly predisposes women to breast cancer. Based on the new research findings, the American Cancer Society and the World Health Organization earlier this year issued separate reports that, for the first time, identified extra pounds and lack of exercise as powerful predictors of breast cancer. This year the National Cancer Institute initiated a project aimed at determining more precisely how much physical exertion and weight loss -- and at what times during a woman's life -- can provide the best defense against the cancer.

Since exercise and weight control can also help reduce the risk of heart disease, diabetes and osteoporosis for women, researchers are now advising that females engage in a consistent exercise routine at least a few hours a week, beginning in their early teens and continuing through menopause.

"There is enough evidence now in the scientific literature to argue that exercise and weight control can have a beneficial impact on breast cancer prevention," says Rachel Ballard-Barbash, who is heading the NCI effort. "We are actively getting this message out."

For post-menopausal women, exercise works in a different, but also beneficial, way, says Anne McTiernan of the Fred Hutchinson Cancer Research Center in Seattle, who has also done research on exercise and breast cancer. In that population, excess estrogen seems to be coming from fat cells, she says. Exercise will reduce fat and therefore estrogen.

An estimated 267,000 new cases of breast cancer are expected to have been diagnosed among American women in 2003 and 39,800 women will likely have died of the disease this year. The incidence of the cancer rose by almost 4% a year between 1980 and 1987, then moderated to a 0.4% annual growth rate between 1987 and 2000. The slowing of the rate of new cases and an annual 2.3% decline in breast-cancer death rates between 1990 and 2000 are linked to public health efforts to encourage screening, and also to several new cancer drugs that work well when the disease is treated early.

But researchers are becoming increasingly convinced that the findings from the new prevention studies -- and others expected out over the next few months and years -- may also help fight the disease. Dr. Bernstein and her colleagues surveyed 600 women -- some with breast cancer and some who don't have the disease -- between the ages 35 and 64 about their early exercise experiences. They found a protective lifelong effect of teenage exercise against in-situ breast cancer, a form of the disease in which the cancer has yet to spread beyond the spot it first began. (Invasive cancers are those that have spread to nearby fatty tissue in the breast, or beyond to lymph nodes and other organs.) A report of their findings was published in the Nov. 15 issue of the journal Cancer.

The women who consistently engaged in some physical activity, even as little as one hour a week, experienced the protective effect, the report says. However, women with a family history of the disease -- meaning one or more close relatives, such as a mother, sister or daughter, had it -- didn't benefit for reasons still unclear to the researchers.

The study was released just one month after Dr. Bernstein and her colleagues reported a similar finding in a study involving post-menopausal women, ages 55 to 72. That study, published in the International Journal of Cancer, compared 1,883 women who developed breast cancer after menopause in the 1980s and 1990s with 1,628 similar women without cancer. It reported, among other findings, that women who exercised more than four hours a week for 12 or more years were 24% less likely to develop breast cancer than those who didn't exercise that much.

Researchers say the type of exercise doesn't appear to matter. Though still preliminary, the research suggests that any type of exertion appears to work in reducing ovulation in teens and excess weight in adults. But researchers say the connection still must be proven in more research.

"We need to be able to tell women exactly how much and what type of exercise is most beneficial," says Dr. McTiernan who expects to report soon on a small study looking at women who engaged in 45 minutes of aerobic exercise five times a week. "We need more information to produce specific prevention recommendations. But even without that, the evidence we have now is hard to dispute. Exercise and weight control can reduce cancer. That's an awfully powerful message."

Write to Michael Waldholz at mike.waldholz@wsj.com