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Even for Those at Highest Risk continued...

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."

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