The study has a number of limitations. It relied on women's memories of their exercise habits over a lifetime, for one.
Exercisers as a whole were less likely than sedentary women to develop breast cancer. But no one can say what effect exercise might have on any one woman's risk of breast cancer -- if it does have an effect at all.
In the U.S., the average woman has about a 1-in-8 chance of developing breast cancer in her lifetime, and a 1-in-36 chance of dying from it, according to the American Cancer Society.
For now, McCullough said her findings support what's already recommended for good health. And they suggest that women might benefit even if they start exercising after menopause.
"It's never too late to start," she said "Our evidence suggests that if you start after menopause, you can still help yourself."
There was no link seen between exercise and breast cancer for the nearly 1,000 women in the study who developed breast cancer before menopause.
According to McCullough, that may be because breast cancer earlier in life has different causes compared with after menopause -- when most breast cancers occur.
SOURCE: http://bit.ly/Mv5I72 Cancer, online June 25, 2012.