Research has shown that very active women are less likely to develop breast cancer than their sedentary peers, but little has been known about the effects of moderate exercise and whether less active women who start exercising get similar results.
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.
There are many risk factors for breast cancer that women have no control over, such as family history of the disease or the age at which they began menstruating, Eliassen told Reuters Health.
But physical activity “is one of the few breast cancer risk factors that women can do something about. And it’s never too late.”
Over the course of the study, the women reported how active they were and what types of ecercise 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.
Overall, five in 100 women got breast cancer over 20 years.
Brisk walking appeared to be the most protective against breast cancer, outweighing swimming, hiking or jogging. But Eliassen noted that walking was also the most common form of exercise, which may have skewed the results.
Regular exercise was associated with lower risk even after removing the potential influence of drinking and weight.
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.
While research has already shown that exercise reduces the amount of estrogen in the blood, this study also showed that exercise also reduced the risk of breast cancers that aren’t influenced by estrogen.
“Physical activity also has an effect on chronic inflammation, and insulin sensitivity. So there are ways in which physical activity could be acting,” she added.