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Exercise Is Key to Breast Cancer Prevention

By Peggy Vaughn

On the Front Page...

Exercise can play as important a role in cancer prevention as the latest screening tool or chemoprevention drug, said Dr. Leslie Bernstein, professor of preventive medicine at the Keck School of Medicine, University of Southern California in Los Angeles.


Epidemiological studies strongly suggest that just a few hours each week of moderate to vigorous exercise can reduce a woman's exposure to ovarian hormones that cause breast cancer, she said.

"My message is...we have wonderful opportunities in chemoprevention, vaccines, surgical interventions and removal of carcinogens from the environment," she told the audience filling Lister Hill Auditorium on Aug. 1 for the third annual Advances in Cancer Prevention Lecture, sponsored by the National Cancer Institute. "But what's really important, also — and it's mostly free — is that lifestyle can offer important opportunities in the prevention of breast cancer."

Dr. Leslie Bernstein
Unfortunately, that message is not reflected in the choices made by an American public that is becoming overweight and obese at an alarming rate. Obesity prevalence increased by 50 percent between 1991 and 1998, she said.

"Even worse, by 1998 more than 21 percent of African-American children, and more than 21 percent of Hispanic children were overweight. More than 12 percent of non-Hispanic, white children were overweight," she said. "We have a tremendous problem we need to deal with."

Studies show that overweight and obesity accounts for 10 percent of cancer mortality among men, 15-20 percent among women and affects the risk of colorectal, endometrial, breast, kidney, prostate and esophageal cancers.

"The issue with obesity is that the gene pool hasn't changed," she said. "The (obesity) epidemic is solely due to lifestyle changes. We don't need to look for genetic causes or intermediate biomarkers."

When Bernstein first started looking at breast cancer risks in the early 1980's, it was already clear that incidence rates increased most rapidly during the reproductive years. Early menarche (first menstruation) and late menopause increased the risk, as did a woman's lactation history and number of births.

Studies also revealed a direct correlation between these life events and the total levels of hormone exposure. A study of 200 Finnish schoolgirls that followed them into their 20's and 30's found the earlier the menarche, the higher the levels of circulating estrogen during each menstrual cycle.

"(Menarche) is not just a chronological marker, it doesn't just mark the onset of the exposure to cyclic hormones," Bernstein said. "It also indicates the intensity of hormone exposure during adolescence. It's an effect that persists."

Having been an athlete all her life, Bernstein knew from personal experience that intensive training alters ovarian hormone production. For example, female marathon runners often experience anovulation, where there is still a menstrual flow but the levels of estrogen and progesterone hormones are much lower.

"An athlete has nearly a six-fold risk of being anovulatory," she said. "You might miss a cycle or cycles become irregular. Even if a woman ovulates, if she's an athlete she's likely exposed to lower levels of hormones."

Bernstein decided to test the connection between moderate physical exercise and ovarian function after menarche. She wanted to determine whether or not exercise affected the age at menarche and if it reduced the likelihood of developing breast cancer before and after menopause.

In a cohort study of 210 schoolgirls aged 14-17, she tracked menstrual bleeding and rates of physical exercise for 9 months. She found that girls who exercised moderately for 2 to 3 hours a week were twice as likely to be anovulatory. She conducted a second, larger study of 1,378 elementary school girls and found that 5 hours per week of physical activity delayed menarche.

She next did a case-control study of breast cancer among women aged 40 or younger and tracked the average hours of exercise per week in the 10 years after menarche.

"There was a reduction in risk of breast cancer among women who were most physically active in the first 10 years after menarche," she said. "Then we looked at (exercise rates) over their lifetime. Women who exercised on average about 4 hours per week over their reproductive years had more than a 50 percent reduction in risk in our study."

A similar study of postmenopausal women, aged 55 to 64 years, found a similar reduction in risk when women exercised moderately for about 4 hours a week, she said.

However, the effects of exercise on breast cancer risk were stronger among thinner women or those who had not gained substantial amounts of weight during adulthood.

"The ovaries may have turned off...but a heavy woman becomes her own estrogen factory," she said, since the body converts the testosterone and androstenedione found in body fat into estrogen.

The effects of exercise on breast cancer risk were restricted to women who did not have a family history of breast cancer, she added. In contrast, the effects of obesity were stronger among women with a family history of the disease.

Bernstein laments that only 25 percent of adults, and 27 percent of students, engage in recommended levels of physical activity. She believes getting children to exercise regularly is key to ending the obesity epidemic.

"By promoting physical activity, we not only impact breast cancer, but also colon cancer and possibly endometrial cancer," she said. "We can look towards lifestyle, in particular physical activity and the reduction of obesity, as important targets for cancer prevention."

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