Exercise and Breast Cancer

A THOROUGH new study of more than 1,000 California women has found that moderate but regular physical activity can reduce a woman's risk of developing premenopausal breast cancer by as much as 60 percent. The benefits seen were greatest among women who had borne children and those who were physically active in their teen-age years and early 20's.

The most striking reduction in risk was found among women who exercised for four hours a week, doing activities like jogging, swimming laps or playing tennis. But even as few as two to three hours of weekly exercise was beneficial, reported Dr. Leslie Bernstein, who directed the study. The results are described in today's issue of The Journal of the National Cancer Institute by Dr. Bernstein and her colleagues at the University of Southern California's North Cancer Center.

If confirmed by further studies, the finding, which at the moment applies only to women 40 or younger, singles out exercise as the first risk factor for breast cancer that women can readily control.

"This could be an extremely important finding," said Dr. Lynn Rosenberg, an epidemiologist at Boston University School of Medicine. "There's not any method at the moment that women could practically use to reduce their risk of getting breast cancer."

In an editorial accompanying the study, Dr. Louise A. Brinton of the National Cancer Institute said the new finding was especially important because it seemed to identify exercise as an independent factor influencing the risk of breast cancer.

Dr. Rosenberg commented that "even if the results are not confirmed, there are a thousand other reasons that women should exercise." Among other things, regular exercise reduces a woman's chances of developing heart disease, osteoporosis, diabetes and hypertension.

Other established risk factors for breast cancer include family history, age at menarche, age at first pregnancy, number of pregnancies and socioeconomic status. Risk is lowest among those who start menstruating late, who have their first child by the age of 20 and who have a greater number of pregnancies. There may also be some protective effect from breast-feeding. All these factors are associated with a diminished release of the ovarian hormones estradiol and progesterone, which appear to influence the development or growth of breast cancer.

Although various dietary factors have been proposed as influencing the risk of breast cancer, none have been firmly established.

In the new study, lifetime exercise habits and other relevant factors were determined through personal interviews with 545 women 40 years oldand younger with newly diagnosed breast cancer and an equal number of women free of cancer but otherwise comparable.

Dr. Bernstein said in an interview that there were still many unanswered questions. Among them, she said, are when in life women must exercise to be protected and whether breast cancer in postmenopausal women might also be reduced by physical activity. Data gathered in a study of more than 3,000 postmenopausal women will soon be analyzed to see whether those who developed breast cancer were more sedentary than those who did not, she said.

Although the current study showed that physical activity in adolescence and early adulthood had the greatest benefit, Dr. Bernstein said, "We still saw a very strong protective effect among the women who became active later on." Also unknown is why women who have had children are most likely to benefit from regular exercise. This finding, she said, might be related to ovarian problems that prevented the childless women from conceiving and, at the same time, muted the benefits of exercise.

Dr. Bernstein's initial interest in the relationship between exercise and breast cancer arose from an earlier finding that Radcliffe alumnae who were college athletes were less likely to develop breast cancer than their nonathletic classmates. Although it has long been known that extreme physical activity can disrupt a woman's menstrual cycle, stopping ovulation and menstrual bleeding, the effect of moderate exercise on women's hormones was not known.

Dr. Bernstein found that teen-age girls who were physically active were less likely to ovulate every month, even though they continued to have regular menstrual cycles. She explained that without ovulation, less estradiol would be released during the menstrual cycle and no progesterone would be produced during the second half of the cycle. But women having difficulty getting pregnant should not stop exercising unless ovulation failure is found to be the cause of their infertility, Dr. Bernstein said.

"Now we want to find out whether there is diminished hormone production in active women, even if they continue to ovulate during every cycle," she said. "This would give us a mechanism to explain the benefits of exercise on breast cancer risk and may help us understand how other risk factors work as well."

Chart: "Risk Factors: What We Know"

Many of the identified and possible risk factors for breast cancer are not easily modifiable, and the physical mechanisms that might be involved are not fully understood. Here is a summary of what is known and suspected now.

IDENTIFIED RISK FACTORS:

Family history. Risk is lower among those with no history of the disease among close relatives.

Age at onset of menstruation. Risk is lowest among those who start menstruating late.

Age at first pregnancy. Risk is lowest among those who have their first child by the age of 20 and who have a greater number of pregnancies.

Socioeconomic status. Higher status is associated a reduced risk?

POSSIBLE RISK FACTORS:

Breastfeeding. There may be some protective effect for those who breastfeed.

Dietary factors, like high-fat diets, high alcohol intake or a need for certain vitamins or fiber.

Environmental factors, like pollution or electromagnetic fields.

Hormones from birth-control pills or as replacement therapy.

PROBABLE RISK FACTOR MODIFIER:

Moderate regular physical activity, even two or three hours a week, is associated with a significantly lower breast cancer risk.