Can fiber reduce breast cancer risk? 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. However, the report’s analysis of observational cohort studies shows that women who consumed more dietary fiber were less likely to develop breast cancer than those who consumed less. Is this enough to justify any change in steps we already take to reduce breast cancer risk?
*Note: Originally published in 2011, this post has been updated in 2019 based on results of new studies.
In this analysis of 16 studies, risk of breast cancer was 5% lower for each 10 grams of dietary fiber. This type of study can’t prove that fiber itself was responsible. So what this analysis really does is give us more reason to meet recommendations for healthy eating. Right now, most women aren’t.
How could fiber reduce 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. That could help 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 viscous types of fiber might bind to estrogen secreted in the gut, thus reducing levels of estrogen circulating in the blood. (That would leave less estrogen available to promote growth of estrogen-sensitive breast cancer.) Moreover, this type of dietary fiber slows the absorption of carbohydrate you 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. Other research suggests that fiber’s influence on inflammation might vary among different people and may relate mainly to specific fibers that can be fermented by gut bacteria to form protective compounds.
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. The vast majority of grains’ natural plant compounds and nutrients that support antioxidant defenses 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, and not only to reduce breast cancer risk. Women in these breast cancer studies consuming the least amount of fiber took in from 10 to 16 grams per day. Since U.S. women average 16 grams of fiber daily, that means many fall in this range. And that’s well below levels associated with overall good health and lower colon cancer risk.
Fiber intake doesn’t need to be extreme. Women with higher-fiber diets in these studies generally ate 25 to 35 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 your overall diet to 18 to 21 grams of dietary fiber.
* Get to 30 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 some refined grains with additional 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 strong role of a healthy weight 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”.
For detailed listing of fiber content in foods, check the USDA Nutrient Database. From the list of nutrients to search, you can check total fiber, or soluble and insoluble fiber. Just remember that not all soluble fiber is the same in viscosity or fermentability. And make sure you’re looking at an appropriate portion size.
Check here for a brief summary of the American Institute for Cancer Research/World Cancer Research Fund, Third Expert Report highlighting the steps that research most strongly supports as ways to reduce breast cancer risk.
For a more detailed summary of the AICR/WCRF Third Expert Report section on breast cancer, check here, where you will also find a link to the full chapter on breast cancer risk.
Chen S, Chen Y, Ma S, et al. Dietary fibre intake and risk of breast cancer: A systematic review and meta-analysis of epidemiological studies. Oncotarget. 2016;7(49):80980–80989. doi:10.18632/oncotarget.13140
*Note that the analysis of prospective cohort studies shows a significant effect that has less heterogeneity than analysis that includes case-control studies, which are a methodologically weaker type of study.
Key TJ, Angela B, Bradbury KE, et al. Foods, macronutrients and breast cancer risk in postmenopausal women: a large UK cohort [published correction appears in Int J Epidemiol. 2018 Dec 5]. Int J Epidemiol. 2018;48(2):489–500. doi:10.1093/ije/dyy238
Thompson SV, Hannon BA, An R, Holscher HD, Effects of isolated soluble fiber supplementation on body weight, glycemia, and insulinemia in adults with overweight and obesity: a systematic review and meta-analysis of randomized controlled trials. Am J Clin Nutr. 2017 Dec;106(6):1514-1528
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; 129(2):485-94.