A new study shows that walking briskly for less than three hours a week reduces the risk almost as much as longer workouts.
Researchers looked at 74,000 women enrolled in the ongoing Women's Health Initiative. They found that exercising a mere 1 1/4 to 2 1/2 hours each week -- walking briskly or similar exercise -- reduced the risk of breast cancer by 18% compared with inactive women. That protective effect is only slightly less than the 22% reduced risk observed in women who exercise at least 10 hours a week.
You Don't Have to Be an Athlete
"This is great news for women who don't want to be athletes," says lead researcher Anne McTiernan, MD, PhD, of the Fred Hutchinson Cancer Research Center in Seattle. "Even modest exercise can reduce their risk of breast cancer."
The benefits of exercise on breast cancer are well established, with more than 30 studies showing that women who regularly engage in physical activity have significantly less breast cancer. But most of those studies show protection resulting from more vigorous levels of activity -- typically starting at the often-recommended 30 minutes per day.
But this new study, published in this week's edition of TheJournal of the American Medical Association, shows that even moderate intensity exercise done regularly -- say, walking at a 20-minute-per-mile pace -- may offer significant breast cancer protection.
"Some of the older studies made it look like women had to do vigorous activity in order to reduce their risk," McTiernan tells WebMD. "But it looks like light, brisk walking done a few days a week may also offer significant benefit."
Even for Those at Highest Risk
There is a caveat, though: This lower risk is more likely to occur in women who are thin, average, or only slightly overweight -- especially as they reach middle age and beyond. Modest or even more intense exercise alone plays little, if any, role in reducing cancer risk in the obese, she says.
"Exercise works in reducing breast cancer risk by reducing body fat, which itself is a risk factor," says McTiernan, a longtime breast cancer researcher who authored Breast Fitness: An Optimal Exercise and Health Plan for Reducing Your Risk of Breast Cancer. "But if body fat levels are not low enough, even if women are overcompensating with exercise, they're not getting the same benefit. They also need to diet in order to lose that body fat."
The good news is that McTiernan finds even these modest levels of regular exercise offer significant protection even in women considered to be at higher risk -- such as those with a family history of breast cancer.
"In this particular study, they show that if you exercised more, you didn't necessarily get more benefits in terms of protection against breast cancer," says I-Min Lee, MBBS, ScD, of Harvard Medical School, who wrote an accompanying editorial. "But there are other studies that show that the more exercise you do, the greater reduction in risk you get. Still, even lower amounts of exercise are helpful."
One reason: Women who exercise are typically leaner.
It's an Estrogen Thing
"Leanness affects estrogen metabolism, and higher levels of estrogen increases breast cancer risk," says Grace Wyshak, PhD, of Harvard's School of Public Health, who has also studied the effects of exercise on breast cancer.
In one 1985 study on nearly 4,000 women, Wyshak noted that women who engaged in regular physical activity during their college years or earlier could decrease their risk of breast cancer by 17%. In a follow-up study done 15 years later, she found that some protective effect lingered, even when their activity levels waned.
Despite past activity levels, it's never too late to reap these cancer-fighting rewards. And staying trim plays an especially important role as women age.
"In middle and older women, reducing body fat will reduce the level of estrogen, because after menopause, the main source of estrogen is body fat," says McTiernan. "However, exercise can affect other hormones, like insulin, that can also promote tumors. Exercise also improves immune function, which can also reduce risk."