Studies have not established the effect of oats on breast cancer
Oats are a good source of soluble dietary fiber (especially β-glucan) and iron, and also contain meaningful amounts of vitamin B1, vitamin B2, calcium and zinc. Avenanthramides, polyphenols found in oats, have been shown to exhibit anti-inflammatory and anti-itch activity in the skin. Consumption of oats or β-glucan has been shown to reduce cholesterol, improve colon health, offset the increased risk of upper respiratory tract infection caused by stressful exercise, and reduce high blood pressure in obese subjects. High consumption of whole grains has been associated with lower risk of colon cancer. Whole grain consumption has also been shown to be associated with greater insulin sensitivity and lower body mass index in adolescents, especially among the heaviest. A diet containing 10% oat lipids has been shown to reduce the incidence and size of carcinogen-induced liver tumors in laboratory rats.
Breast cancer-related effects of eating oats
Diets rich in soluble fiber and whole grains have been found to be protective against breast cancer in some studies and not associated with breast cancer risk in others. Oats are a source of melatonin, which has been shown to be associated with lower risk of breast cancer. A 2008 study suggested that bowel motility was inversely related to breast cancer risk because it increases estrogen excretion. Consumption of oats increases bowel motility.
Additional comments
A 2011 study found a link between increased starch intake after a diagnosis of early-stage breast cancer and a greater risk of recurrence. This suggests that those oat-based breakfast cereals that do not have a high fiber content are not a good choice.
Note that while we are continually searching for new evidence specifically concerning this food, there is not much interest in it among breast cancer researchers, so few studies are available.