Dietary Fiber
What is Fiber?
Fiber is considered an indigestible carbohydrate. Instead of being broken down into sugar molecules as other sources of carbohydrates are, fiber passes through the digestive system relatively intact since human digestive enzymes cannot break them down. Fiber is mainly found in fruits, vegetables, whole grains, legumes, nuts and seeds. Since fiber is not digested and absorbed like the proteins, fats, and carbohydrates, it passes into the colon and serves as a food source for beneficial bacteria in the gut microbiome which improves the overall health. Many people in the U.S. do not eat enough fiber and consume under the Dietary Reference Intake (DRI) amount. In the table below you will find the DRI for fiber intake for adults:
DRI for Daily Fiber Intake:
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Ages 19-50: 25 grams/day
Ages 50+: 21 grams/day
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Ages 19-50: 38 grams/day
Ages 50+: 30 grams/day
Soluble vs. Insoluble Fiber
Fiber is classified as either soluble or insoluble.
Soluble fiber dissolves in water, can be fermented by the gut microbiome (food for bacteria in the gut microbiome), softens stool and reduces constipation.
Insoluble fiber does not dissolve in water, is not as fermentable as soluble fiber, increases the bulk of the stool, promotes motility in the digestive tract, and also reduces constipation.
Both soluble and insoluble fiber are beneficial to our digestive health. It’s important to remember that when eating fiber-rich foods to consume at least 8 ounces of water or more!
Examples of Soluble & Insoluble Fiber:
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Oats, peas, apples, carrots, sweet potatoes, Brussel sprouts, black beans, flaxseeds
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Beans, lentils, whole grains, quinoa, spinach, almonds, bran cereals, wheat bran, brown rice, almonds, and walnuts
Benefits of Fiber for IBS-C & Functional Constipation
Irritable Bowel Syndrome-Constipation (IBS-C) and Functional Constipation (FC) are two common disorders of the digestive system. Increasing soluble fiber intake to meet the DRI for daily consumption can improve motility, prevent discomfort by softening stool, and promote regular bowel movements without increasing the painful symptoms of IBS (i.e. bloating, gas, stomach cramps). Foods such as figs, prunes, kiwi, are considered functional, low-FODMAP food sources of fiber that are beneficial for people with IBS-C and FC. Increasing fermentable fiber allows for more food for the “good” or beneficial bacteria in out gut to grow. The fermentation of fiber in the colon also creates a more acidic environment which keeps the “bad” bacteria from growing and causing disease. This prevents inflammation of the bowel and protects against colon cancer as well.
Potential Concerns of Excess Fiber Intake
There are many benefits to consuming a high fiber diet that meets close to or at the DRI amount, but consuming an excess amount of fiber can be harmful. Fiber binds to minerals in the body like zinc, calcium, magnesium and iron and consuming too much fiber can reduce the absorption of these essential minerals leading to deficiencies and other health problems. Too much fiber can also cause diarrhea, bloating, pain and gas to form due to rapid fermentation in the colon.
References
Smolin, L. & Grosvenor, M. Nutrition: Science and Applications. 4th ed. Wiley; 2019.
Turner ND, Lupton JR. Dietary fiber. Advances in Nutrition. 2021. doi:10.1093/
Guan ZW, Yu EZ, Feng Q. Soluble Dietary Fiber, One of the Most Important Nutrients for the Gut Microbiota. Molecules. 2021;26(22):6802. Published 2021 Nov 11. doi:10.3390/molecules26226802
Singh P, Tuck C, Gibson PR, Chey WD. The Role of Food in the Treatment of Bowel Disorders: Focus on Irritable Bowel Syndrome and Functional Constipation. Am J Gastroenterol. 2022;117(6):947-957. doi:10.14309/ajg.0000000000001767
