The Ins and Outs of Fiber

Even though fiber has long been a darling of the nutrition world, we are still learning about it and the definition of fiber continues to evolve. Fiber commonly refers to the complex carbohydrate part of plants that cannot be digested by humans because we don’t produce the enzymes to break them down. (There are a few animal sources of fiber, like chitin found in the exoskeletons of insects and crustaceans and the cell walls of fungi, too.) Fiber resists digestion and makes it to the colon mostly unchanged, supplying no calories or digestible carbohydrates. There are several ways to classify fibers, and different types of fiber perform different functions and offer different health benefits.

The classic way to classify fibers is by whether they absorb water (soluble fiber) or not (insoluble fiber). Soluble fiber stimulates the peristaltic waves that move food through the colon and it binds to fats and toxins in the GI tract, carrying them out of the body. Soluble fibers increase one’s sense of fullness by expanding as they absorb water and pushing against the intestinal wall. Insoluble fibers don’t absorb water as they pass through the digestive tract, but they act like a broom that sweeps the colon. They add bulk to stool, improve transit time, and support regularity. Most foods and many supplements contain a mixture of soluble and insoluble fibers but may be higher in one or the other.

Fiber can further be classified by its viscosity and by whether or not it is fermentable by our gut bacteria. Viscous fibers have gel-forming capabilities and form a thick, viscous consistency when they come into contact with water, while non-viscous fibers do not. A subcategory of viscous fibers is mucilage, like the gooey gel that forms from flax or chia seeds. Viscous fibers slow down digestion and help balance blood sugar while also prolonging a sense of fullness. Viscous fibers also modestly reduce cholesterol.³ Some fibers are also fermentable by the bacteria that live in our large intestines. These types of fiber act as prebiotics and supply our beneficial bacteria with food, supporting their growth and/or activity. In turn, our beneficial bacteria produce by-products that benefit our health, such as short chain fatty acids (SCFAs). Because fibers that are highly fermentable lose their water-holding capacity when they are fermented in the colon, they do not have a laxative effect.

Most foods and supplements are a combination of the above characteristics, and it is through these qualities that the health benefits of fiber are achieved. For example, fibers that are soluble, viscous and fermentable, help improve blood glucose levels and lower blood cholesterol levels, but because they are fermented when they get to the colon, they lose their water holding capacity and do not influence stool formation, whereas fibers that are soluble, viscous, and non-fermentable do retain their water holding capacities into the colon so they help control blood glucose, lower cholesterol, and normalize stool formation.

To reap the benefits of fiber, start by increasing the fiber in your diet. High fiber foods have the added benefit of being rich in other beneficial nutrients like vitamins, minerals, and phytonutrients. Fiber is found in many foods, so there are plenty of ways to add it to your diet.

• Start by increasing your intake of vegetables and fruits. Try to consume 7 to 10 servings of vegetables each day as well as 1 to 3 servings of fresh fruit. (A serving of vegetables is ½ cup cooked or raw vegetables or 1 cup raw leafy greens; a serving of fruit is ½ cup of raw fruit or a piece of fruit about the size of a small computer mouse.)
• Sprinkle nuts and seeds over dishes, such as salads, casseroles, and stir-fries.
• Add flaxseed meal and/or chia to smoothies, muffins, breads, and casserole recipes.
• If legumes (beans) are a part of your diet eat, them regularly.
• If you choose to eat grains, choose high-fiber, whole grains instead of processed white varieties.

High Fiber Whole Foods^{4 5}

Increase your fiber intake a little at a time, slowly building up to the recommended levels. This gives your body time to adapt to the change in your diet and promotes regular digestion. When you are eating enough fiber, you should have regular, daily, and effortless bowel movements, with little to no odor. This signals a short transit time (24 hours or a little less).

If you’re struggling to increase your dietary fiber or want to use fiber for a specific therapeutic purpose, a daily fiber supplement (aka functional fiber) can help. There are numerous options to choose from, so everyone can find a supplement that matches their needs.

Acacia

Acacia (aka gum arabic) is extracted from the stems and branches of various species of the Acacia genus. It is rich in soluble fiber, has a neutral taste, and mixes well with liquids. It is also approved as an additive in foods for properties such as thickening, stabilizing, and emulsifying.⁶

Chia seeds

Chia seeds are mostly insoluble fiber, but due to their high mucilage content, they have a high water-holding capacity that forms a viscous gel and acts like a soluble fiber.⁷ Chia seeds have prebiotic properties. They are also high in the omega-3 fatty acid alpha-linolenic acid and antioxidants.

Flaxseed

Flaxseed fiber is about one-third soluble, mostly coming from the mucilage seed coat, and two-thirds insoluble.⁸ Lignan is one of the insoluble fibers and of which flax is the highest known source. Lignans from plant sources like flax are converted to mammalian lignans by bacteria in the colon. Lignans have phytoestrogenic properties that help balance estrogen by exerting either a weak estrogenic or antiestrogenic effect. They may also benefit cardiovascular health, blood sugar balance, and be protective against cancer.⁹

Glucomannan

Glucomannan is derived from the root of Amorphophallus konjac, aka, konjac root. It is soluble and fermentable and one of the most viscous dietary fibers.^{10 11}

Guar gum

Guar gum, from the Indian guar or cluster bean, is a soluble, viscous, and fermentable fiber. It is often used as a thickener or emulsifier in processed foods.¹²

Oat bran

Oat bran is the outer part of the whole oat groat. It is 50-60% insoluble fiber and is a rich source of the soluble fiber beta-glucans, which are also found in barley, fungi, and algae.^{13 14} The beta-glucans from oats are highly viscous and fermentable and have been widely researched for their ability to lower LDL cholesterol and total cholesterol. They also support healthy blood sugar balance and have immunomodulatory effects.   

Pectin

Pectin is mainly found in fruits, with apples and citrus fruits being particularly good sources. Pectin is a viscous soluble fiber that is readily fermented.¹⁵

Prebiotics

Prebiotics, such as inulin, fructooligosaccharides (FOS), galactoologosaccharides (GOS), and maltooligosaccharides (MOS), have only recently been included in the definition of fiber. These fibers are soluble, but not viscous, so they don’t have a strong effect on blood sugar, sense of fullness, or cholesterol levels. They are highly fermentable though and are well recognized to feed beneficial gut bacteria so those bacteria can proliferate and produce beneficial substances like SCFAs that also benefit us.¹⁶

Psyllium

Psyllium comes from the Plantago ovata plant's seeds and is probably the best-known supplemental fiber in the United States. It is a rich source of soluble fiber and highly viscous. Psyllium has been well researched to help balance blood sugar and modestly lower cholesterol. It also normalizes stool so it can be used in cases of constipation, diarrhea, and fecal incontinence.^{17 18}

Resistant starch

Resistant starch is a specific type of starch that is only found in certain foods, like cooked and cooled potatoes, cooked and cooled rice, legumes, pumpernickel bread, and green bananas. In supplements you’ll see resistant dextrin, which is derived from corn. This type of starch resists digestion and is highly fermentable by our gut bacteria, making it a strong prebiotic fiber.¹⁹

To get the most out of your fiber supplement start slowly and work your way up to the desired dose. Be sure to drink plenty of water both when taking the supplement and throughout the day. If you’re using a fiber supplement to feel full or to balance blood sugar, take it just before meals. For most other uses, you can take it whenever is most convenient for you, although fiber supplements should be taken away from other supplements and medications (~one hour before or two hours after), since some fibers can interfere with their absorption.  

References

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2. U.S. Department of Agriculture and U.S. Department of Health and Human Services. (2020, December). Dietary Guidelines for Americans, 2020-2025. 9th Edition. Retrieved from https://www.dietaryguidelines.gov/sites/default/files/2020-12/Dietary_Guidelines_for_Americans_2020-2025.pdf
3. McRorie, J.W., McKeown, N.M. (2017). Understanding the physics of functional fibers in the gastrointestinal tract: an evidence-based approach to resolving enduring misconceptions about insoluble and soluble fiber. Journal of the Academy of Nutrition and Dietetics.  DOI:https://doi.org/10.1016/j.jand.2016.09.021
4. Mateljan, G. (2007). The World’s Healthiest Foods. George Mateljan Foundation.
5. n.d. (2012). Food sources of soluble fibre. Dietitians of Canada. Retrieved from https://carleton.ca/healthy-workplace/wp-content/uploads/soluble-fibre.pdf
6. Title 21, § 172.780 Acacia (gum arabic). (2023). https://www.ecfr.gov/current/title-21/chapter-I/subchapter-B/part-172/subpart-H/section-172.780
7. Vázqyez-Ovando, A., Rubio, J.G. R., Guerrero, L.C., Betancure, D. (2009). Physiochemical properties of a fibrous fraction from chia (Salvia hispanica L.). Lebensmittel-Wissenchaft und-Technologie, 42(1), 168-173.  DOI:10.1016/j.lwt.2008.05.012
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9. Adolphe, J.L., Whiting, S.J., Jurrlink, B.H.H., Thorpe, L.U., Alcorn, J. (2009). Health effects with consumption of the flax lignan secoisolariciresinol diglucoside. British Journal of Nutrition, 103(7), 929-938. doi:10.1017/S0007114509992753
10. Ho, H. V. T., Jovanovski, E., Zurbau, A., Blanco Mejia, S., Sievenpiper, J. L., Au-Yeung, F., Jenkins, A. L., Duvnjak, L., Leiter, L., & Vuksan, V. (2017). A systematic review and meta-analysis of randomized controlled trials of the effect of konjac glucomannan, a viscous soluble fiber, on LDL cholesterol and the new lipid targets non-HDL cholesterol and apolipoprotein B. The American journal of clinical nutrition, 105(5), 1239–1247. https://doi.org/10.3945/ajcn.116.142158
11. Chiu, Y. T., & Stewart, M. (2012). Comparison of konjac glucomannan digestibility and fermentability with other dietary fibers in vitro. Journal of medicinal food, 15(2), 120–125. https://doi.org/10.1089/jmf.2011.0084
12. Higdon, J. (2019, March). Fiber. Linus Pauling Institute. https://lpi.oregonstate.edu/mic/other-nutrients/fiber   
13. Chen, H.L., Haack, V.S., Janecky, C.W., Vollendorf, N.W., Marlett, J.A. (1998). Mechanisms by which wheat bran and oat bran increase stool weight in humans. Am J Clin Nutr, 68(3), 711-719. https://doi.org/10.1093/ajcn/68.3.711
14. Fisher, J.J. (2002). Food scientist’s guide to dietary fiber [unpublished master’s thesis]. University of Minnesota. Retrieved from:  https://krex.k-state.edu/handle/2097/1454
15. Higdon, J. (2019, March). Fiber. Linus Pauling Institute. https://lpi.oregonstate.edu/mic/other-nutrients/fiber   
16. Petre, A. (2022, Apr 4). What are oligosaccharides? All you need to know. Healthline.com. Retrieved from https://www.healthline.com/nutrition/oligosaccharides
17. Surdea-Blaga, T., Cozma-Petrut, A., & Dumitraşcu, D. L. (2021). Dietary interventions and irritable bowel syndrome - what really works?. Current opinion in gastroenterology, 37(2), 152–157. https://doi.org/10.1097/MOG.0000000000000706
18. Bliss, D.Z., Savik, K., Jung, H-J, G., Whitebird, R., Sheng, X. (2014). Dietary fiber supplementation for fecal incontinence: a randomized clinical trial. Res Nurs Health, 37(5), 367-378. doi: 10.1002/nur.21616
19. Christensen, L. (2020, Aug 13). The many types of fiber: your guide to dietary fiber, prebiotics, and starches. ChrisKresser.com. Retrieved from https://chriskresser.com/types-of-dietary-fiber/