Sure, they may have gotten some temporary blisters and achy muscles, but the nearly 7,000 women who recently ran the Los Angeles Marathon probably boosted their health in more ways than they thought.

Regular aerobic exercise like running, biking or walking helps the heart and lowers the risk of diabetes, making it a standard suggestion from family doctors. According to a City of Hope researcher, though, exercise lowers the risk of developing something less obvious: breast cancer.

“Across the world, we’ve seen consistent results showing a reduction in breast cancer risk with increasing levels of activity,” said epidemiologist Leslie Bernstein, Ph.D., director of the Department of Etiology in City of Hope’s Division of Population Sciences. “We’ve observed this in pre- and post-menopausal women and both for recreational activity and occupational activity.”

Bernstein was one of the first researchers to investigate whether physical activity could influence breast cancer risk. It only made sense.

Researchers know that women are more likely to develop breast cancer if they have characteristics associated with reproductive health: starting menstruation early, entering menopause late, using hormone replacement therapy, postponing pregnancy until later in life or having no children at all. These factors raise women’s lifetime exposure to estrogen and other female hormones, which got Bernstein thinking.

“We sat back and asked, ‘What can lower your estrogen levels?’ — and we started looking at exercise,” she said.

After all, female athletes sometimes have irregular periods or miss periods altogether, which means they produce lower levels of key hormones. And according to work by Bernstein and her colleagues, it doesn’t take Olympic-level training to lower levels. Their studies of school-age girls showed that even a few regular hours a week of dancing or sports could push back the age when girls got their first period and influence whether their ovaries released an egg during the menstrual cycle.

So the researchers took the next step: looking for any connections between exercise and breast cancer risk in adults.

They carefully compared women 40 years old and younger with breast cancer to other similar women who remained cancer-free. They found that women who exercised three or more hours a week in the 10 years after their first period had a 30 percent reduction in risk compared to less active women, and women who exercised nearly four hours a week — even as adults — had a 58 percent reduction in risk.

It seems to help after menopause, as well. Their study of postmenopausal women showed that over a woman’s lifetime, two and a half hours of strenuous activity a week — or three or more hours of moderate activity — can drop breast cancer risk significantly, too. Overall, activity seems to help protect women regardless of age.

Researchers still have a lot to learn about how exercise influences risk, but Bernstein says certain messages already are clear. “We can’t recommend that all women become marathon runners,” she said. “But we can say that even a few hours of consistent exercise each week can lower risk.”

The American Cancer Society offers a variety of resources on physical activity for adults and kids. Visit its Staying Active page for more information.