Fiber - especially vegetable and fruit fiber - protects against breast cancer
What are the foods you think of when you hear the word “fiber”? Although most people probably think of whole grains, all plant foods are rich in fiber. In fact, beans contain more fiber than whole grains, and vegetables and fruits (and some seeds) contain comparable amounts – here are a few examples:
- 1 cup cooked quinoa – 5 grams fiber
- 1 cup cooked brown rice – 4 grams fiber
- 1 cup cooked kidney beans – 11 grams fiber
- 1 cup cooked broccoli – 6 grams fiber
- 1 cup blueberries – 4 grams fiber
- 1 tablespoon chia seeds – 6 grams fiber
Fiber, by definition, is resistant to digestion in the human small intestine. This means that during the digestive process, fiber arrives at the large intestine still intact. Fiber takes up space in the stomach but does not provide absorbable calories, which makes meals feel more satiating and promotes weight loss. In the colon, fiber adds bulk and accelerates movement, factors that are beneficial for colon health. Soluble fiber (primarily from legumes and oats) is effective at removing cholesterol via the digestive tract, resulting in lower blood cholesterol levels. Some types of fiber are fermented by intestinal bacteria. The fermentation products, such as butyrate, have anti-cancer effects in the colon and also serve as energy sources for colonic cells. Fermentable fiber also acts as a prebiotic in the colon, promoting the growth of beneficial bacteria. Fiber intake is associated with a multitude of health benefits, including healthy blood pressure levels and reduced risk of diabetes, heart disease and some cancers.1, 2
Fiber and breast cancer
An analysis of 10 scientific studies found that higher fiber intake is associated with lower risk of breast cancer.3 How does fiber impact one’s risk of breast cancer?
First and foremost, since animal products, refined grains, sugars and oils contain little or no fiber, fiber intake is a marker for greater intake of natural plant foods, many of which are known to have a variety of anti-cancer phytochemicals. Some breast cancer protective substances that have already been discovered include isothiocyanates from cruciferous vegetables4, organosulfur compounds from onions and garlic, aromatase inhibitors from mushrooms, flavonoids from berries, lignans from flax, chia and sesame seeds, and inositol pentakisphosphate (an angiogenesis inhibitor) from beans.
Does fiber itself have some potentially breast cancer protective actions?
High-fiber foods help to slow emptying of the stomach and absorption of sugars, which decreases the after-meal elevation in glucose. This is meaningful because elevated glucose levels lead to elevated insulin levels, which can send pro-cancer growth signals in the body, for example via insulin-like growth factor 1 (IGF-1). As such, high dietary glycemic index and glycemic load (characteristic of refined grains and processed foods) are associated with an increase in breast cancer risk.5-7 Accordingly, a study on Korean women found that higher white rice intake was associated with higher breast cancer risk.8
Increased exposure to estrogen is known to increase breast cancer risk.9-11 A woman may be exposed to estrogen via her ovaries’ own production, estrogen production by excess fat tissue, or environmental sources such as endocrine-disrupting chemicals (like BPA). Fiber can reduce circulating estrogen levels, thereby reducing breast cancer risk, because it helps to remove excess estrogen from the body via the digestive tract. Fiber binds up estrogen in the digestive tract, accelerates its removal, and prevents it from being reabsorbed into the body.12-14 In addition, soluble fiber (as shown with prunes and flaxseed) seems to alter estrogen metabolism such that a less dangerous form of estrogen is produced, whereas insoluble fiber (wheat bran) did not have the same effect.15,16 For this reason, beans, oats, chia seeds and flaxseeds may provide some extra protection due to their high soluble fiber content.
One notable case-control study looked specifically at different sources of fiber to determine the associations between vegetable fiber, fruit fiber, and grain fiber with breast cancer. Interestingly, when fiber was split up by source, only fruit fiber and vegetable fiber decreased risk; there was a 52% risk reduction for high intake of vegetable fiber, and a 46% risk reduction for fruit fiber. In contrast, there was no association between grain fiber and breast cancer risk.17 A study published in February 2013 came to a similar conclusion when analyzing the association between fiber subtypes and breast cancer risk. This study was part of the larger European Prospective Investigation into Cancer and Nutrition (EPIC) study of over 300,000 women; they found that among the fiber subtypes, only vegetable fiber was linked to decreased risk.18
Fiber itself has some breast cancer-protective properties, like limiting glycemic effects of foods and assisting in estrogen removal, but we get optimal protection when we focus on foods that are both rich in fiber and rich in phytochemicals. G-BOMBS contain numerous anti-cancer phytochemicals, and and greens, mushrooms, and flax and chia seeds in particular contain anti-estrogenic substances in addition to fiber, making them more effective breast cancer fighters than whole grains.
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17. Zhang CX, Ho SC, Cheng SZ, et al: Effect of dietary fiber intake on breast cancer risk according to estrogen and progesterone receptor status. Eur J Clin Nutr 2011, 65:929-936.
18. Ferrari P, Rinaldi S, Jenab M, et al: Dietary fiber intake and risk of hormonal receptor-defined breast cancer in the European Prospective Investigation into Cancer and Nutrition study1,2. Am J Clin Nutr 2013, 97:344-353.