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Postmenopausal women can reduce their risk of breast cancer simply by taking a brisk, half-hour walk five days a week, according to a study out today.
''It's not too late to start exercising,'' says lead author Anne McTiernan, an internist and epidemiologist at Fred Hutchinson Cancer Research Center in Seattle.
McTiernan's team studied nearly 75,000 volunteers, ages 50 to 79, in the government-sponsored Women's Health Initiative, or WHI. The WHI is best known for reporting in July 2002 that the risks of estrogen plus progestin outweigh the benefits. That conclusion arose from a study in which women were assigned to take either hormones or placebo pills.
The new findings, in the Journal of the American Medical Association, come from the WHI's observational study, in which participants decided on their own whether to go on hormone therapy or not. Researchers are following the women over time to identify factors, such as exercise, that predict disease development. At the start of the study, all of the women were healthy and about half were on hormones.
Researchers asked the women whether they exercised hard enough to break a sweat and speed up their heartbeat at least three times a week. The scientists were interested in the women's current exercise habits as well as at three specific, earlier ages: 18, 35 and 50.
Over a follow-up period of nearly five years, 1,780 participants were diagnosed with breast cancer. Women's current exercise habits had the biggest effect on breast cancer risk, followed closely by their habits at age 35. Overall, women who currently walked briskly -- or engaged in similarly strenuous exercise -- for 1 to 2 hours a week were 18% less likely to develop breast cancer than women who were currently sedentary. Women who had engaged regularly in strenuous exercise at age 35 had a 14% lower risk than women who were sedentary at that age. Exercise habits at age 18 or age 50 had only a marginal effect.
The effect of exercise was most pronounced in underweight women but was also seen in normal weight or somewhat overweight women. Doing the equivalent of 10 hours of brisk walking each week cut breast cancer risk by more than 30% in those women.
But researchers did not find a benefit in obese women. Other studies have suggested that obese women tend to over-report how much they exercise, McTiernan says. ''That's not something we could prove,'' she says, adding that the finding was not an excuse for obese women to forgo exercise.
Scientists speculate that exercise protects against breast cancer by lowering body weight. They believe that fatty tissue produces hormones and growth factors, such as estrogen and insulin, that may promote tumor growth.
The new study ''provides further evidence that no pain, no gain is an outdated notion,'' says WHI scientist JoAnn Manson, chief of preventive medicine at Boston's Brigham and Women's Hospital. ''You don't need to exercise strenuously and for very long durations in order to reap health benefits,'' says Manson, who was not a co-author on McTiernan's paper.
The best exercise, McTiernan says, is one you'll stick with.
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