The latest major research report on diet’s link to breast cancer risk concluded that data is too limited to allow any conclusions about dietary fiber. Since then, a few more studies have added to the picture. In a recent combined analysis of ten population studies, involving more than 712,000 women, those who consumed the most dietary fiber were less likely to develop breast cancer than those who consumed the least. Is this enough to justify any change in steps we already take to reduce breast cancer risk?
In this new summary of 10 studies, risk of breast cancer was 11% lower in women consuming the most dietary fiber than in those with the lowest intake. These studies can’t prove that fiber itself was responsible, however. So what this new analysis really does is identify more potential benefit in doing what we already should be doing, but mostly aren’t.
Why could fiber affect breast cancer risk?
- Weight: Most foods providing dietary fiber provide relatively few calories in a substantial portion. Fiber’s bulk may lead you to feel full on fewer calories, thus helping you reach and maintain a healthy weight. Since excess body fat and weight gain are strongly tied to postmenopausal breast cancer risk, that could partially explain a link between higher fiber consumption and lower breast cancer risk. However, almost all of the studies in this recent analysis adjusted for weight status and calorie consumption. That suggests that among women at equally healthful weights, those with higher fiber intake would have lower risk of breast cancer.
- Hormones: Dietary fiber might also act by changing hormone levels. Most breast cancer in the U.S. is estrogen receptor-positive (ER+), meaning estrogen fuels its growth. Some studies suggest that fiber might bind to estrogen secreted in the gut, thus reducing levels of estrogen circulating in the blood (leaving less available to promote growth of estrogen-sensitive breast cancer). Dietary fiber also slows the absorption of carbohydrate we eat, resulting in a slower rise in blood sugar. This can mean lower insulin levels. That could be significant, because insulin seems to act as a growth factor promoting development of ER+ and ER- breast cancer.
- Inflammation: Finally, ongoing research is studying whether dietary fiber might reduce chronic, low-grade inflammation that promotes development of heart disease and cancer. In the HEAL study of breast cancer survivors, women who consumed more fiber had almost 50 percent lower odds of elevation in a marker of inflammation compared to women consuming very low levels of fiber.
Yet was the fiber itself responsible? The plant foods that supply dietary fiber contain thousands of natural plant compounds and nutrients that may put the brakes on inflammation and several different points in the process of cancer development. Eighty percent of the antioxidants in grains are found in the bran and germ layers, which also provide most of the fiber. Don’t assume refined grains plus some isolated source of fiber are the same as whole grains.
Should women increase fiber consumption?
For most women, the answer is yes, but not necessarily to reduce breast cancer risk. Women in these breast cancer studies consuming the least amount of fiber generally took in about 12 to 16 grams per day. That’s comparable to average U.S. adult fiber consumption, and far below levels associated with overall health and lower colon cancer risk. Women rated as high fiber consumers weren’t at an extreme level. They generally ate 26 to 34 grams of fiber per day. That means they at least met the minimum recommendation for good health, and in some cases reached slightly higher levels consistent with a diet that lowers overall cancer risk.
What would it take to get that amount of fiber?
* Simply eating the bare minimum of 5 servings of vegetables and fruits plus 3 servings of whole grains should easily get you to 21 grams of dietary fiber.
* Get to 25 to 35 grams of fiber/day with some combination of additional steps such as:
- Work up to 7 to 10 servings of vegetables and fruits
- Replace additional refined grains with whole grains
- Choose cereal with 5 or more grams of dietary fiber per serving
- Include dried beans (like kidney beans, garbanzo beans, and lentils)
- Snack on a handful of nuts instead of low-fiber chips or crackers
As you aim for lower breast cancer risk, remember the key role of weight control for postmenopausal women. That means using higher fiber foods as replacements for other foods is key. Don’t let fiber content of unhealthy, high-calorie food distract you with a “health halo”.
Whether or not eating more fiber reduces breast cancer risk, most Americans need more fiber. Here’s a brochure from the American Institute for Cancer Research that provides the scoop.
For detailed listing of fiber content in foods, check the USDA National Nutrient Database
You can see steps that research most strongly supports to reduce breast cancer risk in this brief summary of the Continuous Update Project Report 2011 from the American Institute for Cancer Research/World Cancer Research Fund.
Jia-Yi Dong et al. Dietary fiber intake and risk of breast cancer: a meta-analysis of prospective cohort studies. Am J Clin Nutr September, 2011.
Villaseñor et al. Dietary fiber is associated with circulating concentrations of C-reactive protein in breast cancer survivors: the HEAL study. Breast Cancer Res Treat, 2011.