Introduction
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,
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.
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.
