By Michael Greger, M.D., NutritionalFacts.Org.
A recent editorial in the journal of the American Association for Cancer Research starts out “There are far too many breast cancer survivors,” by which she means it’s great that women with breast cancer are living longer, but lamenting the fact that the number of women getting breast cancer in the first place isn’t going down. ‘A million women every year. As with any other epidemic, identification and aggressive reduction of any reversible risk factors must become an immediate priority.
One such risk factor appears to be inadequate fiber consumption. For example, this new study out of Yale. Among pre-menopausal women, higher intake of soluble fiber (highest versus lowest quartile of intake) was associated with a significantly reduced risk of breast cancer, 62% lower odds. And when they just looked at younger women with the hardest to treat tumors, the estrogen receptor negative tumors, then those eating the most fiber appeared to have 85% lower odds of breast cancer. This is what’s called case-control study, where you compare women who already have disease to those that don’t and you ask both to tell you what they used to eat. And so how they get these statistics is that the breast cancer patients were significantly less likely to report eating lots of plant foods, the only natural place fiber is found. The reason it’s important to understand how they arrived at their conclusion is that maybe it’s not the fiber at all that’s what’s so protective. The reduced risk of breast cancer associated with dietary fiber intake observed in this study may in fact indirectly reflect the effects from other dietary nutrients, and thus dietary fiber here may simply act as a marker for other exposures which have been linked to a reduced risk of human cancer as well, such as folate, phytochemicals, carotenoids, vitamin C and E which are also like dietary fiber found in plant foods, such as vegetables, fruits, and pulses (legumes, beans peas lentils soy), as well as in grains. And look, if you’re eating more plants, you may be eating fewer animals. An increased consumption of fiber from foods of plant origin (such as vegetables, fruits, and grains) may reflect a reduced consumption of foods of animal origin.
A combined analysis of a dozen such studies published in the Journal of the National Cancer Institute show they all found pretty much the same thing, a consistent, statistically significant, association between breast cancer risk and saturated fat intake, which is mostly from cheese and chicken, and consistent protective effect for a number of markers of fruit and vegetable intake was demonstrated; such as, which like fiber, is basically only found in plant founds. Every 20 grams of fiber a day was associated with a 15% drop in breast cancer risk.
Case control studies are susceptible to something called recall bias, though, since they rely on people’s memory. If people with cancer are more likely to selectively remember all the bad things they ate, since they may be feeling responsible for their condition, it could artificially inflate the correlation; so prospective cohort studies may provide stronger evidence.That’s where you take a bunch of healthy women and follow them and their diets over time to see who gets cancer and who doesn’t. By 2011, 10 such studies had been done, and the same thing was found. Every 10-g/d increment in dietary fiber intake was associated with a significant 7% reduction in breast cancer risk. Pretty much the same the other studies found, remember, 15% for every 20 grams? This has important public health implications. That was 2011. By 2012 we were up to 16 prospective, or forward-looking studies on dietary fiber and breast cancer, and they found the same thing. But for the first time it showed a nonlinear response.
The more fiber you eat the more benefit you appear to get. American women eat a little under 15 grams of fiber a day, less than half the minimum daily recommendation. Maybe that’s why vegetarian women may have lower breast cancer rates, more plant foods equals more fiber. But vegetarians only seem to be averaging about 20 grams a day. So one might really have to venture out into vegan territory, off the chart at 47 grams a day, or a really healthy vegan diet (59), or eat lots of vegan thai food averaging 68.7.
About Michael Greger M.D.
Michael Greger, M.D., is a physician, author, and internationally recognized professional speaker on a number of important public health issues. Dr. Greger has lectured at the Conference on World Affairs, the National Institutes of Health, and the International Bird Flu Summit, testified before Congress, appeared on The Dr. Oz Show and The Colbert Report, and was invited as an expert witness in defense of Oprah Winfrey at the infamous “meat defamation” trial. Currently Dr. Greger proudly serves as the Director of Public Health and Animal Agriculture at the Humane Society of the United States.