"In a study reported in the Archives of Internal Medicine, A. Heather Eliassen and a research team at Harvard reviewed data from nearly 100,000 postmenopausal women, following them for 20 years. . .Over the course of the study, the women reported how active they were and what types of exercise they got.

Those who scheduled at least an hour of brisk walking per day, or an equivalent amount of activity, were 15 percent less likely to get breast cancer than those who walked less than one hour per week.

Brisk walking was defined as roughly three to four miles per hour, at a speed at which it's harder to hold a conversation than when casually strolling.

Even those who got little exercise before menopause but boosted their activity afterwards were 10 percent less likely to develop breast cancer than those who stayed inactive.

The design of the study did not allow for proof that exercise caused less breast cancer because there may also have been other factors common to the women in the study that also prevented breast cancer. But it was "suggestive," even though it is unclear why exercise would have that impact, Eliassen said."
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