Female athletes know that their menstrual periods often stop occurring when they're training and exercising vigorously, a phenomenon called amenorrhea. For years, doctors have wondered if the reduction of menstrual hormones that comes with amenorrhea could help protect against the formation of breast cancer tumors, especially those affected by estrogen, the primary female hormone.

Dr. Leslie Bernstein, an epidemiologist from the University of Southern California, has been studying the relationship between breast cancer and exercise for several decades. In 1995, she and her research partners recruited more than 100,000 California teachers, to observe how many of them developed breast cancer over time. They asked the women about their exercise habits.

"We asked about strenuous activities," says Dr. Bernstein. "Jogging, running, doing calisthenics, or aerobic activity, being in some of these really strenuous aerobics classes, swimming laps, working out in a swimming pool rather than just playing in the water, then we asked about more moderate activities, like brisk walking."

Bernstein's research indicated that the more exercise the women did, the lower their chances of breast cancer, no matter their other risk factors for the disease, such as weight, family history and age.

"[Among] women who participated during this long-term period that covered their reproductive years, those women who exercised on average five hours a week or more had 20 percent reduction in risk of invasive breast cancer, but a 30 percent reduction of risk in in-situ breast cancer," she says.

In-situ breast cancer is an early-stage tumor that predisposes women for future cancers. Bernstein says they were surprised to find that the risk reduction held true for both hormone sensitive and non-hormone sensitive tumors.

She says women don't have to do recreational exercise to see these results. Those working at heavy labor or doing strenuous housework for more than five hours a week can also benefit.

"We believe the effect is universal," says Leslie Bernstein. "And so we recommend women exercise, and exercise as much as they can each week. We realize that five hours a week is too much to recommend for every woman, but in prior studies, we've seen effects at lower levels of activity, so any activity is better than none."

 Bernstein points out that continuing to exercise also reduces the risk of heart disease, obesity, diabetes and depression, among other conditions. Her study appears in the Archives of Internal Medicine.