Exercise Seen to Cut Breast Cancer Risk
Exercise is one of the few things a woman can do to reduce her risk of breast cancer, many researchers now believe.
They’re still unsure about how much exercise and when in a woman’s life it would be most valuable. But a new review of research and a meeting of scientists indicate an emerging consensus.
“I see it as good news, because there is little that people can do,” said researcher Marilie D. Gammon of the Columbia School of Public Health in New York City.
Biological risk factors that are virtually out of a woman’s hands include an early age for the start of menstruation and a late age for the start of menopause. However, weight gain and alcohol use can be controlled, and the experts think exercise may be added to this list.
In Gammon’s review of 16 studies on recreational exercise, 11 showed a risk reduction ranging from 12% to 60%. Her report was published in the Journal of the National Cancer Institute.
And a research task force recently concluded that physical activity may prevent breast cancer. The issue was discussed by a working group of the National Action Plan on Breast Cancer, a coalition of government and private organizations. However, workshop attendees agreed that the evidence is not yet conclusive.
The fact that a sizable minority of studies found no clear benefit could be blamed on problems in the research, Gammon said. The studies asked women to recall how much they exercised, and many could have forgotten, she said.
However, recent studies increasingly support a benefit.
One such study in Norway followed women who filled out physical activity questionnaires in the 1970s and early 1980s. The report in the New England Journal of Medicine in May 1997 found that, among women who exercised at least four hours a week, the risk of breast cancer was about one-third lower.
A study at USC, published in the Journal of the National Cancer Institute in 1994, found a reduction of 40% with 3.7 hours a week of exercise.
Both were cited in Gammon’s review.
None of the lead authors in the three studies know how much exercise is needed to produce a benefit. But they agree that it doesn’t have to be a lot.
A brisk walk may do some good, said Dr. Inger Thune of Norway’s University of Tromso, who worked on the New England Journal study. “The only thing I can tell you is it doesn’t have to be a marathon,” Gammon said.
Similarly, researchers don’t know the best time in a woman’s life to exercise. Some think exercise has to start in the teen years; others think it can begin later.
A big reason for the uncertainty is that no one is sure yet of the mechanism by which exercise could reduce risk. Some researchers think the exercise might control weight, which could affect production of estrogen.
A high estrogen level is a breast cancer risk factor. And in post-menopausal women, estrogen levels naturally are lower. But fat cells can turn another hormone into estrogen, so fatter women would have more estrogen, said researcher Leslie Bernstein, lead author of the USC study. “It’s more than your body expects to see when you are older, and in that way you have an increased risk of breast cancer,” she said.
Researchers at the Harvard School of Public Health reported in the Journal of the American Medical Assn. in November that women who gained weight had a higher breast cancer risk. But the study did not have enough cases of weight loss as a result of exercise to know if this would reduce the risk, said Dr. Zhiping Huang, a research fellow.
