(Reuters) - Women who exercise moderately may be less likely than their inactive peers to develop breast cancer after menopause, according to a U.S. study.
Researchers, whose study was published in the journal Cancer, found that of more than 3,000 women with and without breast cancer, those who'd exercised during their childbearing years were less likely to develop the cancer after menopause.
The same was true when women took up exercise after menopause, said the group, led by Lauren McCullough at the University of North Carolina, Chapel Hill.
"What we can say is, exercise is good for you," McCullough said.
"It's never too late to start. Our evidence suggests that if you start after menopause, you can still help yourself."
The findings add to a number of past studies tying regular exercise to lower breast cancer rates. But all the studies only point to a correlation and don't prove that exercise itself is what reduces women's breast cancer risk.
There are reasons, though, to believe it can, said McCullough.
One possible way is indirectly, by cutting body fat, she said. Excess body fat is related to higher levels of certain hormones, including estrogen, as well as substances known as growth factors which can feed tumor development.
But exercise might also have direct effects by boosting the immune system or the body's ability to clear cell-damaging "free radicals."
In the study, which included 1,500 women with breast cancer and 1,550 cancer-free women of the same age, all were interviewed about their lifetime exercise habits and other lifestyle factors, like smoking and drinking.
The researchers found a connection between exercise and breast cancer risk only among women who had already gone through menopause.
Those who'd exercised for 10 to 19 hours a week in their "reproductive years" - the years between having their first child and going through menopause - were one-third less likely to have breast cancer than women who'd been sedentary during that time.
Women who'd started exercising after menopause also had a lower risk. If they averaged 9 to 17 hours a week, they were 30 percent less likely to have breast cancer than their inactive peers.
Of course, women who exercise can be different from sedentary women in many ways. So the researchers accounted for differences in education, income, smoking and certain other factors. Exercise was still linked to lower breast cancer risk.
Then the researchers took a closer look at body weight.
They found that among relatively lighter women, exercise was linked to lower breast cancer risks. And for obese women, it may have mitigated the increased breast cancer risk tied to their excess weight.
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. That may be because breast cancer earlier in life has different causes.
The study had a number of limitations, including relying on women's memories of their exercise habits over a lifetime. In addition, any study like this can only look at broad patterns.
For now, McCullough said her findings support what's already recommended for good health - to get exercise. SOURCE: http://bit.ly/Mv5I72
(Reporting from New York by Amy Norton at Reuters Health; editing by Elaine Lies and Bob Tourtellotte)