Nearly 7,000 women recently ran the Los Angeles Marathon — and despite some blisters and sore leg muscles, they were probably boosting their health in more ways than they even realized.
For years, physicians have recommended running, walking or other regular aerobic exercise to boost heart health and lower risk of diabetes. According to a City of Hope researcher, though, exercise also lowers the risk of breast cancer.
“Across the world, we’ve seen consistent results showing a reduction in risk with increasing levels of activity,” said epidemiologist Leslie Bernstein, Ph.D., director of the Department of Etiology in the 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 explore the role of physical activity in reducing breast cancer risk. It seemed a natural leap.
By the 1980s, she explained, researchers understood that female hormones were linked to breast cancer’s development and growth. After all, studies showed that women are more likely to develop breast cancer if they have certain reproductive risk factors such as 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.
She and her colleagues knew that female competitive athletes often have irregular or absent menstrual cycles which means they produce lower levels of key hormones; but, importantly, hormone levels can drop even in moderately active women with seemingly normal periods. Bernstein and her colleagues began digging to discover just how much activity could affect hormone levels, and whether this translated into lowered breast cancer risk.
After studying nearly 1,400 girls in fourth through seventh grades — monitoring their eating habits and exercise — Bernstein and her colleagues found that girls who played sports, danced or did other moderately strenuous activities for at least five hours each week tended to start menstruation later than less active girls. Diet had no effect.
In another study of teenagers, they found that performing enough exercise to burn 600 kilocalories a week — about two hours’ worth of activity — was sufficient to interfere with ovulation, the release of an egg from the ovary.
Knowing they were onto something, the researchers began exploring relationships between exercise and breast cancer in adults through a case-control study. The researchers first identified hundreds of women age 40 or younger who were recently diagnosed with breast cancer; then they matched them, one by one, to similar study volunteers who had not developed cancer.
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.
When they performed a similar study in postmenopausal women, they found that over a lifetime, two and a half hours of strenuous activity — or three or more hours of moderate activity a week — can drop breast cancer risk significantly, too. Overall, activity seems to help protect women regardless of age.
Now Bernstein devotes much of her time to the California Teachers Study, an investigation of health and risk factors among female educators in the California public school system. The study continues to illuminate relationships between cancer and exercise, she said.
For one, researchers have found that exercise influences insulin sensitivity and levels of certain proteins important to the immune system. The study has also shown that activity lowers risk of estrogen-receptor-negative breast cancer in particular, which hints that exercise works through slightly different mechanisms than investigators first thought.
The field remains fertile ground for further research, 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.”