A fresh look at the medical evidence shows women who eat more fiber are less likely to get breast cancer.
Chinese researchers found those who ate the most of the healthy plant components were 11 percent less likely to develop breast cancer than women who ate the least.
Their findings don't prove fiber itself lowers cancer risk, however, because women who consume a lot of it might be healthier overall than those who don't.
The results "can identify associations but cannot tell us what will happen if people change their behavior," said John Pierce, a cancer research at the University of California, San Diego, who was not involved in the work.
While earlier research has yielded mixed conclusions on the link between cancer and fiber, it would make scientific sense: According to the Chinese researchers, people who eat high-fiber diets have lower levels of estrogen, which is a risk factor for breast tumors.
So to get more clarity, the researchers combined 10 earlier studies that looked at women's diets and followed them over seven to 18 years to see who developed cancer.
Of more than 710,000 women, 2.4 percent ended up with breast cancer. And those in the top fifth of fiber intake were 11 percent less likely to do so than women in the bottom fifth.
That was after accounting for differences in risk factors like alcohol drinking, weight, hormone replacement therapy and family members with the disease.
Still, it's impossible to rule out that big fiber eaters had healthier habits overall that would cut their risk, Jia-Yi Dong of Soochow University in Suzhou and his colleagues write in the American Journal of Clinical Nutrition.
And the potential effect was "very small," Dr. Eleni Linos of Stanford University, who wasn't involved in the research, told Reuters Health in an email.
About one in eight American women get breast cancer at some point, with less than a quarter of them dying from it.
Although the connection between breast cancer risk and fiber is a small one, fiber is "something that we know is healthy for you anyway," said Christina Clarke, a research scientist at the Cancer Prevention Institute of California in Fremont.
Known benefits of a high-fiber diet include lower cholesterol and weight loss. If it turns out to cut cancer risk as well, that would be an extra bonus, Clarke said.
Fruits, vegetables, beans, and whole grains are all high in fiber.
According to the U.S. Department of Agriculture's 2010 Dietary Guidelines, most Americans don't get enough fiber. The guidelines recommend that women eat 25 grams of fiber per day and men eat 38 grams, while the average Americans gets just 15 grams a day.
"Increasing dietary fiber intake in the general public is of great public health significance," the Chinese team concludes.