NEWS

Exercise and Breast Cancer Risk: Lacking Consensus

Lou Fintor

Skipping the elevator and taking the stairs has long been touted for improving health. While the cardiovascular benefits of physical activity are well known, the evidence that exercise reduces breast cancer risk remains controversial.

"There are probably more studies in favor of an exercise benefit for breast cancer, but also some that show little or no benefit," says Louise Brinton, Ph.D, chief of the National Cancer Institute's Environmental Epidemiology Branch.



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Dr. Louise Brinton

 
Although various cancer organizations suggest that women exercise to reduce breast cancer risk — and in some cases engage in regular physical activity even after a breast cancer diagnosis — none have developed specific exercise guidelines. The age at which exercise may confer maximum benefit, the extent of that potential benefit, the exact mechanism by which it may act, and even what constitutes "exercise" continues to lack widespread scientific consensus.

Reducing Estrogen

Supporters of the exercise hypothesis argue that a regular exercise regimen or simply increasing physical activity may provide breast cancer benefit both indirectly and directly. Body fat is a source of estrogen, so leaner women have less estrogen, thereby reducing risk. Exposure of the breasts to estradiol over time appears to increase the chances of developing breast cancer. Women who exercise regularly are not only thought to produce less estradiol but the increased activity may also stimulate their immune systems.

In studies conducted by Harvard researchers Rose Frisch, Ph.D., and Grace Wyshak, Ph.D., 82% of former college athletes who started exercising in their high school years or earlier, had a significantly reduced risk of developing breast cancer.

Exercise Histories

Frisch and colleagues collected and compared exercise history and health outcomes data for 5,400 alumnae athletes and their sedentary classmates at eight institutions. Among women athletes between the ages 20 and 80, the incidence of breast, cervical, ovarian, and uterine cancers were reduced as was the incidence of adult-onset diabetes. Fertility in the two groups was similar both for number of pregnancies and number of live births.

"Lower benign breast disease, less late-onset diabetes, and no increase in bone fracture risk for the athletes in their menopausal years were also important findings. But the lower incidence of breast cancer was obviously our most dramatic one," Wyshak said.

Frisch and Wyshak recently completed a 15-year follow-up study of these women and according to Wyshak are "very confident" that their original results will hold up.

"If one stands back and thinks about the biological mechanisms involved in breast cancer and take what we know about physical activity, physical activity should reduce disease risk," said Leslie Bernstein, Ph.D., Professor of Preventive Medicine at the University of Southern California's Norris Comprehensive Cancer Center.



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Dr. Leslie Bernstein

 
In a 1994 retrospective study, Bernstein and colleagues reported that women of reproductive age (age 40 and younger) who engage in 4 or more hours of physical activity per week reduce their risk of developing breast cancer by more than 50% when compared with less active women. This "protective association" was found in evaluating more than 500 breast cancer patients who were individually matched with disease-free controls.

Women who engaged in at least 2 hours per week of physical activities such as walking, jogging, swimming, exercise classes, dance, gymnastics, or gym workouts had a lower risk of breast cancer relative to sedentary women. At least 3 hours of regular exercise each week in the first 10 years after menarche resulted in a 30% decline in risk.

Bernstein explains that while the circumstantial evidence linking exercise and reduced breast cancer risk continues to grow, the exact mechanisms remain unclear, which make the prospects for developing national guidelines or official recommendations somewhat remote.

"We are dealing with a very complex exposure," Bernstein said. "Obtaining detailed lifetime histories that allow us to look at critical age periods is very difficult to do. Researchers disagree on the exact mechanisms by which exercise modifies breast cancer risk since we don't know the true mechanism."

No Protective Effect

While the evidence linking physical activity and breast cancer mounts, it is far from conclusive. A Harvard University prospective study of more than 100,000 nurses recently sparked intense debate when researchers found no breast cancer benefit from exercise in pre-menopausal women.

"We found no protective effect whatsoever," said Beverly Rockhill, Ph.D., of Harvard Medical School and a study collaborator.

According to Rockhill, it is possible that no protective effect was detected because exercise levels have to be "fairly intense" in order to suppress or reduce hormone production in a way that provides significant benefit. Among young women, it may be that a relatively large proportion of breast cancer is attributable to highly penetrant genetic mutations and therefore may be less amenable to primary prevention through physical activity.

"If you take the bulk of all the evidence, women are deriving very modest benefit from exercise in relation to breast cancer and it appears that the physical activity must be sustained over very long periods. There is currently no real agreement on how long this period should be nor on the level of intensity of the exercise," Rockhill said.

In a recently completed analysis, Rockhill observed a stronger protective effect for physical activity among post-menopausal women. Weight gain and higher body fat scores are linked to increased risk of post-menopausal breast cancer. Therefore, a woman who is physically active enough to avoid weight gain in later life and to maintain a relatively low level of fat stores post-menopausally would be expected to lower her risk. Because the risk of breast cancer rises sharply with age, reducing the risk of post-menopausal breast cancer is significant, Rockhill explained.

Relatively Rare

"Pre-menopausal breast cancer is a relatively rare disease; a finding that physical activity may not offer much protection against this disease in no way invalidates the importance of physical activity from an overall health perspective," Rockhill cautioned.

Even though women who begin an exercise regimen — or increase the level of physical activity at age 50 — after not being active for years are unlikely to substantially alter breast cancer risk, there are other benefits of such activity. For women already diagnosed with breast cancer, there is likewise increasing evidence that exercise provides a variety of both emotional and physical benefits, Rockhill said.

When incorporated into post-diagnosis management and treatment planning, regular physical activity may minimize accompanying fear, depression, anxiety, and other psychological symptoms. In addition, it is possible that regular exercise may help stabilize and improve treatment-associated changes in weight, muscle strength and flexibility, appetite, and stamina, says Alberta (Canada) Cancer Board epidemiologist Christine Friedenreich, Ph.D.

"Research related to exercise and survival after a breast cancer diagnosis hasn't been as well-researched as risk," said Friedenreich. "Studies so far have generally consisted of small samples of women studied in controlled conditions."

Lifetime Activity

Friedenreich explained that there are at least 17 ongoing physical activity and breast cancer risk-related studies that measure lifetime activity over a lifetime and have much larger numbers of women. In addition, there is growing interest in studies assessing the success of physical activity-related treatment interventions such as those incorporating home-based walking programs.

Rockhill, Frisch, Wyshak, Bernstein and Friedenreich all agree on the overall benefits of physical activity and are alarmed by the lack of exercise and physical activity among so many Americans.

According to a landmark 1996 U.S. Surgeon General's report, more than 60% of all Americans are not engaging in regular exercise, 25% are not exercising at all, and almost 15% of young people are not physically active.

Poor diet and lack of exercise are thought to be contributing factors in the deaths of more than 300,000 Americans each year.Go



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The ability of exercise to lower a woman's breast cancer risk is still uncertain, but experts agree it is better to get a good workout — whether it's aerobics or some other exercise — than to sit on the sidelines admiring one's nails.

 



             
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