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Patient information: High-fiber diet (Beyond the Basics)

Arnold Wald, MD
Section Editor
Timothy O Lipman, MD
Deputy Editor
Shilpa Grover, MD, MPH
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Fiber is normally found in beans, grains, vegetables, and fruits. However, most people do not eat as much fiber as is commonly recommended. This topic discusses what fiber is, why it is helpful, and how to increase dietary fiber.


There is no single dietary "fiber." Traditionally, fiber was considered that substance found in the outer layers of grains or plants and which was not digested in the intestines. Wheat bran, the outer layer of wheat grain, fit this model. We now know that "fiber" actually consists of a number of different substances. The term "dietary fiber" includes all of these substances and is now considered a better term than just "fiber."

Most dietary fiber is not digested or absorbed, so it stays within the intestine where it modulates digestion of other foods and affects the consistency of stool. There are two types of fiber, each of which is thought to have its own benefits:

Soluble fiber consists of a group of substances that is made of carbohydrates and dissolves in water. Examples of foods that contain soluble fiber include fruits, oats, barley, and legumes (peas and beans).

Insoluble fiber comes from plant cell walls and does not dissolve in water. Examples of foods that contain insoluble fiber include wheat, rye, and other grains. The traditional fiber, wheat bran, is a type of insoluble fiber.

Dietary fiber is the sum of all soluble and insoluble fiber.


The health effects of a high-fiber may depend to some extent on the type of fiber eaten. However, the difference between the health effects of the two types of fiber are not very clear and may vary between individuals, so many providers encourage adding fiber in whatever way is easiest for the patient.


Insoluble fiber (wheat bran, and some fruits and vegetables) has been recommended to treat digestive problems such as constipation, hemorrhoids, chronic diarrhea, and fecal incontinence. Fiber bulks the stool, making it softer and easier to pass. Fiber helps the stool pass regularly, although it is not a laxative. (See "Patient information: Constipation in adults (Beyond the Basics)" and "Patient information: Hemorrhoids (Beyond the Basics)" and "Patient information: Chronic diarrhea in adults (Beyond the Basics)".)

Soluble fiber (psyllium, pectin, wheat dextrin, and oat products) can reduce the risk of coronary artery disease and stroke by 40 to 50 percent (compared to a low fiber diet) [1,2].


It is not clear if a high-fiber diet is beneficial for people with irritable bowel syndrome or diverticulosis. Fiber may be helpful for some people with these diagnoses while it may worsen symptoms in others.


The recommended amount of dietary fiber is 20 to 35 grams per day. By reading the nutrition label on packaged foods, it is possible to determine the number of grams of dietary fiber per serving (figure 1).

Dietary sources of fiber — The fiber content of many foods, including fruits and vegetables, is available in the table (table 1). Breakfast cereals can be a good source of fiber. Some fruits and vegetables are particularly helpful in treating constipation, such as prunes and prune juice.

Other sources of fiber — For those who do not like high-fiber foods such as fruits, beans, and vegetables, a good source of fiber is unprocessed wheat bran; one to two tablespoons can be mixed with food. One tablespoon of wheat bran contains approximately 1.6 grams of fiber.

In addition, a number of fiber supplements are available. Examples include psyllium, methylcellulose, wheat dextrin, and calcium polycarbophil. The dose of the fiber supplement should be increased slowly to prevent gas and cramping, and the supplement should be taken with adequate fluid. The fiber in these supplements is mostly of the soluble type.


Adding fiber to the diet can have some side effects, such as abdominal bloating or gas. This can sometimes be minimized by starting with a small amount and slowly increasing until stools become softer and more frequent.

However, many people, including those with irritable bowel syndrome, cannot tolerate fiber supplements and do better by not increasing fiber in their diet. (See "Patient information: Irritable bowel syndrome (Beyond the Basics)".)






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Patient information: Fecal incontinence (The Basics)
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Patient information: Constipation in adults (Beyond the Basics)
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Colorectal cancer: Epidemiology, risk factors, and protective factors
Diet in the treatment and prevention of hypertension
Lipid lowering with diet or dietary supplements
Healthy diet in adults
Management of chronic constipation in adults

The following organizations also provide reliable health information.

National Library of Medicine

     (www.nlm.nih.gov/medlineplus/dietaryfiber.html, available in Spanish)

National Institute on Diabetes and Digestive and Kidney Diseases


Harvard School of Public Health



Literature review current through: Jun 2016. | This topic last updated: Apr 11, 2016.
