This could be the simplest bit of health advice ever: Exercise reduces women's risk of breast cancer, no matter what kind of exercise they do, how old they are, how much they weigh, or when they get started.
Researchers in France looked at studies that involved more than 4 million women around the world who participated in prospective studies from 1987 to 2013. They found that the more active a woman is, the better her odds of avoiding breast cancer. Women who were most active, with more than an hour a day of vigorous activity, got the most benefits, lowering their cancer risk by 12 percent.
But women weren't as active saw reduced risk, too, notes Mathieu Boniol, research director at the Strathclyde Institute for Global Public Health in Lyon, France. More activity was better, but anything was better than nothing. He presented the data Thursday at the European Breast Cancer Conference in Glasgow.
"This decrease is the same whatever the country, whatever the age, whatever the menopausal status," Boniol told Shots. And it didn't matter if women were active in work, activities of daily living, or sports. "It's very good news."
Scientists don't know why physical activity reduces breast cancer risk. There's been speculation about exercise's effect on hormones and inflammation, but no one knows for sure.
Other studies have found breast cancer risk reductions as high as 25 percent from physical activity, but Boniol says that because of the huge number of women included in this analysis, the 12 percent reduction is more accurate.
Women who were overweight or obese benefited a little less, but still lowered their risk by 10 percent overall.
And women who got moving after menopause also saw benefits from exercise. "It's not something to say, 'Oh, I've never done sports why do that right now?' " Boniol says. "We now have evidence that it could still be beneficial. And it's cheap. It's a very cheap way to do prevention of breast cancer."