Dietary Sources
Available Forms
How to Take It
Possible Interactions
Supporting Research


Dietary fiber, found in the walls of plant cells, cannot be digested by enzymes in the human digestive tract but plays an essential role in human health. It is found in all plant-based foods, with most whole foods containing a combination of the two types of fiber -- soluble and insoluble. Both types of fiber help maintain the health of the intestinal tract and promote regular elimination of stool.

Soluble fiber attracts water and forms a gel in the digestive tract. This slows digestion and lowers the rate of nutrient absorption (such as starch and sugars) from the stomach and intestine. The result is a significant reduction in cholesterol levels over time, which may help prevent heart disease and stroke. Intake of soluble fiber may also improve glucose tolerance in people with diabetes. Psyllium husk, pectin, and the soft parts of fruits and dried beans and peas are examples of soluble fiber.

Insoluble fiber, on the other hand, is the portion of plant cells that gives the wall its structural integrity. Insoluble fiber can be found in the peel of fruit, such as apples and grapes. It acts as a natural laxative that speeds the passage of foods through the stomach. It also gives stool its bulk and helps it move quickly through the gastrointestinal tract.

Dietary fiber has been shown to play a role in the treatment of conditions such as gastrointestinal disease, constipation, hemorrhoids, high cholesterol, heart disease, and diabetes. However, most Americans consume only 11 to 13 grams of dietary fiber per day -- less than half the amount recommended by health experts.


Many well-designed studies have concluded that fiber relieves constipation. Studies have used wheat bran and psyllium primarily. Fiber is believed to relieve constipation by adding bulk to stool and speeding its transit through the gastrointestinal tract.

When recommended by a healthcare practitioner, fiber can be used to relieve mild to moderate diarrhea. Soluble fiber soaks up a significant amount of water in the digestive tract, thereby making stool firmer and slower to pass.

Irritable Bowel Syndrome (IBS)
Several well-designed studies have found that soluble fiber helps regulate stool frequency and consistency in people with IBS.

Soluble fiber may be recommended by a physician to help soften stool and reduce the pain associated with hemorrhoids.

Inflammatory Bowel Disease (IBD)
In a study of people with ulcerative colitis (a type of IBD), psyllium seeds were as effective as the prescription drug mesalamine in reducing recurrences of the disease. In addition, a physician may recommend the use of fiber as a bulking agent for mild to moderate cases of diarrhea from either ulcerative colitis or Crohn's disease (another important type of IBD).

High Cholesterol

Soluble fibers such as those in psyllium husk, guar gum, and oat bran have a cholesterol-lowering effect when added to a low-fat, low-cholesterol diet. Studies have shown psyllium, in particular, to be quite effective in lowering total as well as LDL ("bad") cholesterol levels.

Studies suggest that a high-fiber diet may help prevent type 2 diabetes, lower insulin and blood sugar levels, and improve cholesterol and triglyceride levels in people with diabetes. In addition, one well-designed study suggests that pregnant women with type 1 diabetes are able to lower the amount of insulin they use if they eat a high fiber diet.

Studies and clinical reports suggest that soluble fiber (such as psyllium, pectin, and guar gum) may enhance the sensation of fullness and reduce hunger cravings. For these reasons, incorporating soluble fiber into the diet may aid weight loss.

High Blood Pressure
Although not entirely conclusive, the addition of fiber (namely, 12 grams of soluble fiber per day) may help lower blood pressure.

Heart Disease
Incorporating high-fiber foods (such as oatmeal, oat bran, psyllium, and legumes) into the diet may help lower heart disease risk.

Colon Cancer
Although initial reports were promising, studies investigating the value of a high-fiber diet for colorectal cancer have been conflicting. While some studies evaluating groups of people have suggested that fiber protects against the development of colorectal cancer, most large, better-designed studies have found only a minimal association between fiber intake and colorectal cancer risk. In addition, fiber does not appear to protect against the recurrence of colorectal cancer in people who have already been treated for the condition.

Other types of Cancer
Preliminary evidence suggests that a diet high in fiber (in conjunction with lifestyle changes and conventional medication) may help protect against the development of certain types of cancer such as prostate, breast, and lining of the uterus. Further studies are needed to confirm these findings, however.

Dietary Sources

Soluble fiber is found in dried beans and peas, oats, barley, fruits, and psyllium seed husks.

Insoluble fiber is found primarily in fruits and vegetables, whole-grain products, whole grain cereals, and wheat and corn bran.

Available Forms

Dietary fiber is available as a supplement in several forms.

  • Fiber tablets, capsules, and powders
  • Bulk fiber laxatives

How to Take It


There is no specific dietary amount established for pediatric fiber intake. Children with specific fiber needs should increase the daily intake of fibrous foods slowly over a period of days. Fiber supplements should only be taken under the guidance of a qualified healthcare practitioner.


Recommended adult doses of fiber vary depending on the health condition being treated. For general health, 25 to 35 g per day dietary fiber is suggested.


Because supplements may have side effects or interact with medications, they should be taken only under the supervision of a knowledgeable healthcare provider.

In general, fiber supplements may reduce or delay the absorption of certain medications. For this reason, it is best to refrain from taking fiber supplements at the same time as other medications. Instead, medications should be taken at least one hour before or between two and four hours after taking fiber.

Fiber should always be taken with a full 8 oz glass of water. It is also important to drink at least 6 to 8 full glasses of water throughout the day or constipation may develop. Taking fiber supplements without adequate liquids may cause it to swell and, in extreme cases, cause choking. Do not take this product if you have difficulty swallowing. People with esophageal stricture (narrowing of the esophagus) or any other narrowing or obstruction of the gastrointestinal tract should not take fiber supplements.

If you experience chest pain, vomiting, or difficulty swallowing or breathing after taking fiber supplements, seek immediate medical attention.

A potential side effect from any fiber product is gas and bloating.

Although very uncommon, allergic reactions (even anaphylaxis) to soluble fiber may develop in people who consume these types of supplements over a long period of time.

Possible Interactions

If you are currently being treated with any of the following medications, you should not use fiber supplements without first talking to your healthcare provider.

Antidepressant Medications, Tricyclics
Dietary fiber has been shown to lower the blood levels and effectiveness of tricyclic antidepressant medications such as amitriptyline, doxepin, and imipramine in three patients. Reduced dietary fiber intake increased the blood levels and improved symptoms in these patients. Individuals currently taking tricyclic medications should consult a healthcare provider before increasing fiber intake.

Diabetes Medications
While fiber supplements may help to regulate blood sugar levels, they may also interfere with the absorption of anti-diabetic medications, specifically glyburide and metformin. Therefore, fiber supplements should not be taken at the same time as this medication.

Taking soluble fiber such as psyllium with carbamazepine, a medication used to treat seizure disorders, may decrease the absorption and effectiveness of carbamazepine. If taking both soluble fiber and carbamazepine, blood levels of carbamazepine should be monitored closely by a healthcare provider.

Cholesterol-lowering Medications
Combining psyllium or other soluble fibers with cholestyramine or colestipol (two types of cholesterol-lowering medications known as bile acid sequestrants) may be beneficial in lowering cholesterol levels. Individuals taking these medications should consult a healthcare practitioner to determine whether psyllium is safe and appropriate.

On the other hand, taking fiber in the form of pectin (from fruit) and oat bran reportedly reduces the body's ability to absorb cholesterol-lowering medications known as "statins" (such as lovastatin and atorvastatin), and could lead to decreased effectiveness of this class of medications.

Fiber supplements may reduce the body's ability to absorb digoxin, a medication used to regulate heart function. Therefore, fiber supplements should not be taken at the same time as this medication.

Reports suggest that psyllium or other soluble fibers may lower lithium levels in the blood, reducing the effectiveness of this medication. Lithium levels should be monitored very closely by a healthcare provider, particularly if there is any significant change in fiber intake.

In one study, the fiber supplement guar gum reduced blood levels of penicillin. Therefore, it would be best not to take penicillin at the same time as fiber supplements.

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Review Date: April 2002
Reviewed By: Participants in the review process include: Ruth DeBusk, RD, PhD, Editor, Nutrition in Complementary Care, Tallahassee, FL; Jacqueline A. Hart, MD, Department of Internal Medicine, Newton-Wellesley Hospital, Harvard University and Senior Medical Editor Integrative Medicine, Boston, MA; Gary Kracoff, RPh (Pediatric Dosing section February 2001), Johnson Drugs, Natick, Ma; Steven Ottariono, RPh (Pediatric Dosing section February 2001), Veteran's Administrative Hospital, Londonderry, NH. All interaction sections have also been reviewed by a team of experts including Joseph Lamb, MD (July 2000), The Integrative Medicine Works, Alexandria, VA;Enrico Liva, ND, RPh (August 2000), Vital Nutrients, Middletown, CT; Brian T Sanderoff, PD, BS in Pharmacy (March 2000), Clinical Assistant Professor, University of Maryland School of Pharmacy; President, Your Prescription for Health, Owings Mills, MD; Ira Zunin, MD, MPH, MBA (July 2000), President and Chairman, Hawaii State Consortium for Integrative Medicine, Honolulu, HI.

Copyright © 2004 A.D.A.M., Inc

The publisher does not accept any responsibility for the accuracy of the information or the consequences arising from the application, use, or misuse of any of the information contained herein, including any injury and/or damage to any person or property as a matter of product liability, negligence, or otherwise. No warranty, expressed or implied, is made in regard to the contents of this material. No claims or endorsements are made for any drugs or compounds currently marketed or in investigative use. This material is not intended as a guide to self-medication. The reader is advised to discuss the information provided here with a doctor, pharmacist, nurse, or other authorized healthcare practitioner and to check product information (including package inserts) regarding dosage, precautions, warnings, interactions, and contraindications before administering any drug, herb, or supplement discussed herein.

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