March 13, 2000 (Cambridge, Mass.) -- Becky Boock has always been a devoted athlete, reveling in the thrill of the race. A former competitive runner and swimmer, the 19-year-old Canadian races in at least three triathlons each summer. Now she has an additional reason to keep moving: Boock recently lost her mother to breast cancer.
"Exercise is both an outlet and a way of prevention for me," she says. "I can only hope my healthy lifestyle will help me stay safe." Boock isn't alone in this hope.
Even women who haven't lost a relative to breast cancer often fear this disease most. And until recently, experts haven't been able to offer solid evidence to those wondering whether lifestyle factors such as exercise might reduce their risk.
But now that's changing. After years of conflicting findings, a new consensus is emerging. It's very good news for women who already work out, as well as for anyone who's looking for a new reason to get motivated: Regular exercise, it seems, really can cut a woman's chances of getting breast cancer.
The latest study, published in the January 19 issue of the Journal of the National Cancer Institute, found that active women were about 30% less likely to get the disease. Last October, Harvard University researchers published findings from the large-scale Nurses' Health Study showing that regular exercisers cut their risk by 20%. "If you take all the data on balance, there is a moderate reduction of risk," says Beverly Rockhill, Ph.D., lead author of the Harvard study.
The data haven't always pointed in this direction. Indeed, in an earlier analysis of Nurses' Study data, Rockhill and her colleagues were unable to show that exercise offered any shield. "We found no protective effect whatsoever," Rockhill said at the time. Other researchers cited in a review paper published in the January 21, 1998 issue of the Journal of the National Cancer Institute found that exercise reduced risk before -- but not after -- menopause. Still others cited in that paper found precisely the opposite. And at least one study reviewed there showed that exercise raised risk.
Why all the back-and-forth? Mainly, researchers say, because it's tough to pinpoint exactly how much a woman has been exercising during her lifetime. Many studies have asked women how much they exercised at a particular time, then extrapolated that amount over several years. "It's vitally important that women be asked about their lifetime history of exercise," says Leslie Bernstein, PhD, an epidemiologist at the University of Southern California.
Bernstein pioneered a way of measuring lifelong activity that relies on detailed interviewing. In one important study that used this method, published in the September 21, 1994 issue of the Journal of the National Cancer Institute, Bernstein found that a regular exercise habit cut the risk of breast cancer by a whopping 40%. Bernstein's methods give her study more weight than many of the others, and support the case for exercise's protective effect, says Marilie Gammon, PhD, an epidemiologist at the University of North Carolina.
Biology Bolsters the Case
There are also plausible physiological reasons why exercise ought to help, say researchers who have studied the issue. With each menstrual period, the hormone estrogen courses through a woman's body, prompting her breast cells to divide. Experts suspect that the more of these estrogen surges a woman experiences, the higher her risk of breast cancer.
In a number of ways, exercise can reduce the amount of estrogen that the body produces. Young girls who are very active can stave off the onset of menstruation. Adult women who exercise even moderately are likely to ovulate less regularly, even though their periods may continue. Postmenopausal women whose ovaries no longer pump out the hormone still are exposed to a version of estrogen produced by fat stores; exercising enough to hold weight down and convert fat into muscle ought to reduce older women's estrogen exposure, and hence their risk, as well.
What remains a bit murky is exactly how much exercise a woman needs to do and at what time in her life she needs to do it. More seems to be better, but some is better than none. Bernstein's study, for instance, found that women who exercised four hours per week got more protection than those who worked out for only two.
For now, most experts suggest that women hoping to lessen their chances of breast cancer follow the Surgeon General's recommendation of at least 30 minutes of moderate activity -- such as brisk walking -- per day. And, of course, they should continue to get regular breast exams and mammograms, which are still the best protection of all.
As for Becky Boock, she'll be participating this spring in a hometown race that means more to her than any other: A 10K Mother's Day relay that will raise money for breast cancer research. Organizers have dedicated the race to Boock's mother.
Tinker Ready is a freelance health and science writer based in Cambridge, Mass. Her work appears regularly in the Boston Globe and Nature Medicine.