Dietary fiber is the collective name for a group of indigestible carbohydrate-based compounds found in plants. They are the materials that give the plant rigidity and structure. Two types of fiber are important to human health: insoluble fiber and soluble fiber.

In 2016, the U.S. Food and Drug Administration defined dietary fiber as “nondigestible soluble and insoluble carbohydrates (with three or more monomeric units), and lignin that are intrinsic and intact in plants; isolated or synthetic nondigestible carbohydrates (with three or more monomeric units) determined by FDA to have physiological effects that are beneficial to human health.” This definition was created because food manufacturers had previously been able to declare any nondigestible carbohydrate to be fiber.

Sources of fiber.

Sources of fiber.


Many large, well-designed, long-term studies have been done on the health effects of a diet high in fiber. The almost universally accepted result is that health benefits result when individuals meet or exceed the dietary reference intakes (DRIs) for fiber for their age group. This concept is so well-accepted that it has become the official position of the National Institutes of Health and other U.S. government agencies charged with improving the health of the nation. Increased fiber intake was included in the government-promoted goals for Healthy People 2020.

Food sources of fiber

Soluble liber

Insoluble liber


Apples (with skin)



Black beans

Bran and bran cereals

Black-eyed peas

Brown rice







Brussels sprouts



Couscous, whole wheat

Citrus fruit (oranges, grapefruit)


Kidney beans

Green beans


Pears (with skin)

Navy beans


Northern beans

Vegetables, raw

Nuts and seeds

Wheat bran

Oat bran

Whole grain cereals

Oatmeal and foods made with oats

Whole grains


Whole wheat breads


Whole wheat pasta

Peas, dried


Pinto beans





Fiber is found only in foods of plant origin. It occurs in the skins, seeds, leaves, and roots of fruits and vegetables and in the germ and bran layers of grains. Pectins, lignans, cellulose, gums, and mucilages are all different forms of fiber found in plant foods. Humans lack the digestive enzymes to break down fiber, and so it passes through the digestive tract largely unchanged.

Because insoluble dietary fiber is not digested, it does not provide energy (calories). Instead, fiber adds bulk to the waste (stool or feces) in the large intestine (colon). Increased bulk causes the walls of the intestine to contract rhythmically (peristalsis) so that waste moves through the large intestine more rapidly. In the colon, most of the water in digested food is reabsorbed into the body, and then the solid waste is eliminated. By passing through the colon more rapidly, less water is reabsorbed from the waste. The stool remains soft and moist, and is easy to expel without straining.

Good sources of insoluble fiber include:



Recommended intakes (g/day)

Children 1 < yr.

Not established

Children 1-3 yrs.


Children 4-8 yrs.


Boys 9-13 yrs.


Girls 9-13 yrs.


Boys 14-18 yrs.


Girls 14-18 yrs.


Men 19-50 yrs.


Women 19-50 yrs.


Men 50> yrs.


Women 50> yrs.


Pregnant women


Breastfeeding women


SOURCE: Institute of Medicine. Dietary Reference Intakes for Energy, Carbohydrate, Fiber, Fat, Fatty Acids, Cholesterol, Protein, and Amino Acids. Washington, DC: National Academy of Sciences, 2005.

Foods rich in soluble fiber often are recommended to help improve blood glucose and cholesterol levels, whereas diets containing high amounts of insoluble fiber contribute to bowel regularity and may help prevent diverticular disease. Because high-fiber diets tend to be satisfying but relatively low in calories, they are often promoted for weight management.

High-fiber foods tend to be high in substances known as FODMAPS: fermentable oligosaccharides, disaccharides, monosacharides, and polyols. FODMAPs are short-chain carbohydrates and sugar alcohols. Humans lack the enzymes to digest these carbohydrates, so they are not fully absorbed in the small intestine and instead drag water into the intestine where it can ferment and generate gases. Foods high in FODMAPs include wheat, rye, barley, onion, garlic, cruciferous vegetables such as cabbage and brussels sprouts, cauliflower, asparagus, beans, many fruits, and artificial sweeteners such as sorbitol, xylitol, mannitol, and isomalt.


Some doctors have begun to recommend that people pay particular attention to consuming prebiotic fibers in an effort to nurture intestinal microorganisms. Prebiotic fiber, also called resistant starch, is indigestible fiber that functions as food for the beneficial bacteria that live in the gut and perform essential roles in digestion. Adequate prebiotic fiber intake can help maintain the balance and diversity of intestinal bacteria. Dr. William Davis recommends that people avoid the consumption of cellulose, such as bran, because it does nothing for bowel flora, and instead consume fiber that intestinal bacteria can consume. This includes legumes, inulin (which is found in leeks and onions, asparagus, garlic, and chicory root, and is also sold in powder form as a dietary supplement), green bananas or plaintains, and potatoes, either cooked and cooled or eaten raw.

Because fiber is so important in the diet, the amount of fiber in canned goods, frozen foods, and other processed foods sold commercially in the United States must be shown on the label. A food that is labeled “high in fiber” contains 5 or more grams of fiber per serving. As of 2012, manufacturers were required to show only the total amount fiber in each serving of food. In 2016, the FDA required food manufacturers to break down the fiber content listed on food labels into soluble and insoluble fiber.

Low-fiber diets for intestinal diseases

Doctors recommend a low-fiber diet for people with certain medical problems such as diverticulitis or irritable bowel disease. The idea is that eating small amounts of fiber allows the intestines to rest and heal.

A waxy substance made by the liver and also acquired through diet. High levels of cholesterol in the blood may increase the risk of cardiovascular disease.
Having fewer than three bowel movements a week or having difficulty passing stools that are often hard, small, and dry.
A condition in which the colon (large intestine) develops a number of outpouchings or sacs (diverticula).
Composed of fermentable oligosaccharides, disaccharides, monosacharides, and polyols, FODMAPs are short-chain carbohydrates and sugar alcohols.
A seed used for medicinal purposes taken from the common fleawort, Plantago psyllium, and related species.

In the 2010s, experts found that restricting (FODMAPs) was an effective method of treating IBS. There have been a number of good controlled studies assessing low-FODMAP diets in the treatment of IBS, and they seem to be effective in many cases. Devising a low-FODMAP diet is personal to a patient, so patient and practitioner should be prepared to spend some time on the process. Foods high in FODMAPs are prevalent on the lists of high-fiber foods historically approved for consumption by IBS patients, which may lead to some confusion.

Recommended dosage

The average American consumes only 14 grams of fiber each day, despite extensive research that shows that higher levels of fiber provide increased health benefits. In 2016, the U.S. Food and Drug Administration increased the daily recommended fiber consumption for adults from 25 to 28 grams.


Fiber should be increased in the diet gradually. If fiber intake increases suddenly, abdominal pain, gas, and diarrhea may result. Also, when eating a high-fiber diet, it is important to drink at least eight glasses (64 ounces or 2 liters) of water or other fluids daily. People whose fluid intake must be restricted for medical reasons should avoid a high-fiber diet.


Fiber supplements such as psyllium may reduce the absorption of certain medications when taken at the same time. In general, medications should be taken at least one hour before or two hours after fiber supplements.


Fiber and high-fiber diets have traditionally been thought to help prevent diverticulosis, a disease of the large intestine, but a study published in the February 2012 issue of Gastroenterology challenged this notion. Doctors previously believed that diverticula formed as a result of chronic constipation, but participants in the study were found to be less likely to develop diverticulosis when eating diets lower in fiber. While researchers will need to revisit the question of how and why diverticula are formed, the study does not undermine the overall importance of fiber in the diet.

Parental concerns

Despite great attention to research on dietary fiber, as of 2018 few studies had been conducted to assess its effects in children. Nevertheless, experts suggest increasing children's dietary fiber consumption. They believe all the health benefits adults experience by increasing their fiber intake should be the same for children, such as lower risks for obesity, diabetes, and constipation.

See also Constipation ; Crohn's disease ; Detoxification diets ; Diabetic diet ; Diverticular disease diet ; Hemorrhoids ; High-fiber diet ; Inflammatory bowel disease ; Irritable bowel syndrome ; Irritable bowel syndrome diet ; Prebiotics and probiotics ; Whole grains .



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Frazier, Karen. The Quick & Easy IBS Relief Cookbook: Over 120 Low-FODMAP Recipes to Soothe Irritable Bowel Syndrome Symptoms. Emeryville, CA: Rock-ridge, 2017.

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Zou, Jun, Benoit Chassaing, Vishal Singh, et al. “Fiber-Mediated Nourishment of Gut Microbiota Protects against Diet-Induced Obesity by Restoring IL-22Mediated Colonic Health.” Cell Host and Microbe 23, no. 1 (January 10, 2018): 41–53.


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Academy of Nutrition and Dietetics, 120 S. Riverside Plaza, Ste. 2190, Chicago, IL, 60606-6995, (312) 899-0040, (800) 877-1600,, .

American College of Gastroenterology (ACG), 6400 Goldsboro Rd., Ste. 200, Bethesda, MD, 20817, (301) 263-9000,, .

American Gastroenterological Association (AGA), 4930 Del Ray Ave., Bethesda, MD, 20814, (301) 654-2055, Fax: (301) 654-5920, .

National Digestive Diseases Information Clearinghouse (NDDIC), 2 Information Way, Bethesda, MD, 20892-3570, (800) 891-5389, TTY: (866) 569-1162, Fax: (703) 738-4929,, .

Marie Fortin, MEd, RD
Revised by Amy Hackney Blackwell, PhD

  This information is not a tool for self-diagnosis or a substitute for professional care.