Women who are aggressively seeking to reduce their risk of breast cancer are probably frustrated by the lack of clearly defined risk factors that they can alter. The majority of established factors that predispose women to breast cancer are those where control is very limited or impossible: family history, genetics, early onset of menstruation, late onset of menopause, and early age of first childbirth.
Exercise Does Reduce Risk
It's extremely welcome news, therefore, that exercise appears to be a lifestyle characteristic that women can alter to reduce their lifetime risk of breast cancer. The role of physical activity, either recreational or occupational, in reducing breast cancer risk has been studied for over a decade with generally encouraging results.
However, designing research studies on this topic is extremely difficult and has led to somewhat inconsistent and controversial data regarding: the overall magnitude of the risk reduction; whether the risk reduction is restricted to premenopausal or postmenopausal women; and the effects of timing, frequency, duration, or intensity of exercise. In addition, studies are often challenged because of small sample sizes, the difficulty in collecting accurate data on the extent and timing of exercise, and incomplete accounting for such related factors as dietary patterns and caloric intake. Nonetheless, recent well-designed studies and analyses do strongly support the protective effect of physical activity.
In a comprehensive review by Dr. Marilie Gammon and her colleagues at the Columbia University School of Public Health, published in the Journal of the National Cancer Institute (January 21, 1998), eleven out of sixteen investigations on recreational exercise reported a 12%-60% decrease in risk among both premenopausal and postmenopausal women. Seven of nine studies suggest that higher levels of occupational physical activity may be associated with a reduction in risk.
Included in Dr. Gammon's review was one of the largest studies ever done on the topic, conducted by researchers in Norway. In this study, the physical activity of 25,000 women was tracked throughout the 1970's and 1980's. The age range of the study group was 20 to 54 at time of entry and each woman was followed for an average of 14 years. Their findings, published in The New England Journal of Medicine (May, 1997), demonstrated that women who exercised at least four hours per week during leisure time reduced their risk of breast cancer by approximately 37 percent. In women who exercised regularly, the reduction in risk was greater in premenopausal as compared to postmenopausal women, and greater in women younger than 45 as compared to those over 45.
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This study also found that women whose job descriptions involved high levels of activity, such as walking or lifting, reduced their risk by approximately 25 percent, with a more pronounced effect among premenopausal women.
Another interesting finding of this study was a significantly decreasing risk of breast cancer with increasing activity (that is, the more a woman exercised, the less breast cancer risk she had). Other studies, however, have not confirmed this relationship, suggesting that once a woman performs a certain level of exercise, the maximum possible amount of risk reduction has been achieved and breast cancer risk is not driven down further by additional physical activity.
Why Does Physical Activity Reduce Risk?
Just how does physical activity influence the development of breast cancer? A combination of factors may be at play. Some investigators have proposed that the risk of breast cancer is related to lifetime exposure to estrogen and perhaps progesterone. Physical activity may influence risk through its effect on these hormones. Recreational exercise is associated with various changes in menstrual characteristics, with the most well studied being the delay of the onset of menstruation.
Since obesity is a major determinant of circulating estrogen, exercise may reduce breast cancer risk by preventing weight gain or promoting weight loss. Although it has been found that heavier women may have a decreased risk of developing breast cancer during the premenopausal years, weight gain in postmenopausal women is associated with increased risk. According to Dr. Gammon at Columbia, "Physical activity may be more useful as a preventive measure against breast cancer by reducing the likelihood of weight gain, particularly among postmenopausal women.... Thus, exercise through the postmenopausal years may be important in lowering breast cancer risk."
Of course, the overall effects of exercise on the body must not be forgotten in terms of reducing the risk of breast cancer. Low-to-moderate levels of exercise have been shown to elevate the body's immune system, a key factor in the defense against such conditions as osteoporosis, diabetes mellitus, and stroke. In cancer, exercise may influence the immune system's ability to slow the growth rate and killing capacity of tumor cells.
Exercise has also been found to be helpful to breast cancer survivors and women who are undergoing some types of breast cancer treatment. With light aerobic activity of their choice (exercise bike, stair-climber, treadmill etc.), breast cancer survivors may find themselves less stressed, less anxious, and less depressed. Of course, women who have been diagnosed with breast cancer or any other serious disease should always consult their physician before beginning an exercise program. Determining the biological mechanism by which exercise reduces breast cancer risk would be extremely helpful in enabling us to make more precise recommendations about the connection. Currently, researchers are not certain regarding the exact time in a woman's life when exercise would be most effective. The optimal intensity, frequency, and duration of exercise necessary to reduce risk are also not known.
Conclusions
However, at this point, the body of research on the connection between breast cancer and exercise points to the following conclusion: By following the general guidelines for exercise as they pertain to the reduction of risk for cardiovascular diseases, you are likely to reduce your risk of breast cancer to some degree as well. You can help yourself by sticking to a weekly exercise routine, perhaps a brisk 20-minute walk three times a week, and taking the stairs instead of the elevator. Try to be more active with your daily routines.
"The only thing I can tell you," says Dr. Gammon, "is that it doesn't have to be a marathon." So considering the frustration and anxiety women often experience in trying to reduce their risk of this disease, it appears that exercise is on their side, and indeed may be a powerful weapon. Although there are a number of questions that remain unanswered, the outlook for the exercise connection is very encouraging.
