Eating lots of fiber was linked with a lower risk of breast cancer in a systematic review and meta-analysis published in the journal Cancer in April 2020. It included data from 20 observational studies involving nearly 2 million women, about 68,000 of whom developed breast cancer.
Those who reported consuming the most total fiber were less likely to develop breast cancer either before or after menopause than women who consumed little or no fiber. When the researchers looked at the results by type of fiber, they found that soluble fiber—inoats, legumes, psyllium, seeds, and some fruits and vegetables—was independently associated with lower breast cancer risk.
The pooled studies ranged in length from two to 20 years. It’s hypothesized that fiber may decrease breast cancer incidence by helping to control blood sugar (elevated blood sugar is a risk factor for many cancers, including of the breast) and reducing circulating levels of estrogen. Previous observational research has linked higher fiber intake with a lower risk of several other cancers, notably colorectal cancer.