Patient information: High fiber diet

HIGH FIBER DIET OVERVIEW

Eating a diet that is high in fiber has many potential health benefits, including a decreased risk of heart disease, stroke, and type 2 diabetes. Because high fiber foods may be healthy for reasons other than their fiber content, the research has not always been able to determine if fiber is the healthful component. A high fiber diet is a commonly recommended treatment for digestive problems, such as constipation, diarrhea, and hemorrhoids, although individual results vary widely, and the scientific evidence supporting these recommendations is weak.

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.

WHAT IS 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 cells 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.

BENEFITS OF A HIGH FIBER DIET

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 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.

There are several potential benefits of eating a diet with a high fiber content:

• 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" and "Patient information: Hemorrhoids" and "Patient information: Chronic diarrhea in adults".)

• 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].
• Soluble fiber can also reduce the risk of developing type 2 diabetes. In people who have diabetes (type 1 and 2), soluble fiber can help to control blood glucose levels.
• 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.

HOW MUCH FIBER DO I NEED?

The recommended amount of dietary fiber is 25 grams (for women) to 30 grams (for men) of fiber 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 table 1 (table 1A-C). 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 contains 12 grams of fiber).

In addition, a number of fiber supplements are available. Examples include psyllium, methylcellulose, wheat dextrin, and calcium polycarbophil (table 2). 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.

FIBER SIDE EFFECTS

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".)

WHERE TO GET MORE INFORMATION

Your healthcare provider is the best source of information for questions and concerns related to your medical problem. Because no two people are exactly alike and recommendations can vary from one person to another, it is important to seek guidance from a provider who is familiar with your individual situation.

This discussion will be updated as needed every four months on our web site (www.uptodate.com/patients). Additional topics as well as selected discussions written for healthcare professionals are also available for those who would like more detailed information.

Some of the most pertinent include:

Patient Level Information:
Patient information: Constipation in adults
Patient information: Hemorrhoids
Patient information: Chronic diarrhea in adults
Patient information: Irritable bowel syndrome

Professional Level Information:
Colorectal cancer: Epidemiology, risk factors, and protective factors
Diet in the treatment and prevention of hypertension
Lipid lowering with diet or dietary supplements
Prudent diet
Treatment of constipation in adults

A number of web sites have information about medical problems and treatments, although it can be difficult to know which sites are reputable. Information provided by the National Institutes of Health, national medical societies and some other well-established organizations are often reliable sources of information, although the frequency with which they are updated is variable.

• National Library of Medicine

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

• National Institute on Diabetes and Digestive and Kidney Diseases

      (www.niddk.nih.gov)

• Harvard School of Public Health

      (www.hsph.harvard.edu/nutritionsource/what-should-you-eat/fiber/index.html)

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