A fresh look at the medical evidence shows that women who eat more fibre are less likely to get breast cancer.
Chinese researchers found that those who ate the most of the healthy plant components were 11% less likely to develop breast cancer than women who ate the least. Their findings don't prove fibre 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 behaviour," said John Pierce, a cancer researcher 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 fibre, it would make scientific sense: According to the Chinese researchers, people who eat high-fibre diets have lower levels of oestrogen, which is a risk factor for breast tumours.
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.
Figures combined from the research
Of more than 710,000 women, 2.4% ended up with breast cancer. And those in the top fifth of fibre intake were 11 % 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 fibre 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, said.
Although the connection between breast cancer risk and fibre is a small one, fibre 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-fibre 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 fibre.
According to the US Department of Agriculture's 2010 Dietary Guidelines, most Americans don't get enough fibre. The guidelines recommend that women eat 25 g of fibre per day and men eat 38 grams, while the average person gets just 15 grams a day.
"Increasing dietary fibre intake by the general public is of great public health significance," the Chinese team concludes.
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