Researchers 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.
And it did not take a vigorous workout; regular exercise at any intensity level was linked to a lower breast cancer risk, the researchers say.
The findings, reported in the journal Cancer, add to a number of past studies tying regular exercise to lower breast cancer odds.
But like those past studies, this latest one can only point to a correlation: It does not prove that exercise, itself, is what cut women's breast cancer risk.
Still, there are reasons to believe it can, said lead researcher Lauren McCullough, of the University of North Carolina, Chapel Hill.
One possible way is indirectly, by reducing body fat, McCullough said in an interview. 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, McCullough said -- by boosting the immune system or the body's ability to clear cell-damaging "free radicals."
That is all speculation for now.
But, McCullough said, the findings do support the general health recommendation that adults stay active throughout their lives.
"What we can say is, exercise is good for you," she told Reuters Health. "We don't know what it's going to do for any one woman."
DIFFERENCE SEEN AFTER MENOPAUSE
The study included 1,500 Long Island women with breast cancer and 1,550 cancer-free women 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'd 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 McCullough's team 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 among relatively lighter women, exercise was linked to lower breast cancer. And for obese women, it may have mitigated the increased breast cancer risk tied to their excess pounds.
The study has a number of limitations. It relied on women's memories of their exercise habits over a lifetime, for one.
And any study like this can only look at broad patterns, McCullough said.
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. (Reuters)
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