Most Americans consume 11 to 13 grams of dietary fiber per day, less than half the amount recommended by health experts. Dietary fiber is the portion of plant cell walls that cannot be digested and absorbed to produce energy. Yet fiber is essential to good health. It is credited with helping to fight serious conditions such as gastrointestinal disease, high cholesterol, cancer, heart disease, stroke, and diabetes. It also can relieve constipation and reduce hemorrhoid flare-ups.

Dietary fiber is found in all plant-based foods, with most whole foods containing a combination of the two types of fiber—soluble and insoluble. As it travels through the body, soluble fiber combines with water and forms a gel. This process serves to slow the emptying of the stomach and the absorption of starch and sugars from the small intestines after eating. 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.

Insoluble fiber, which is not easily broken down during the digestive process, is a natural laxative. It gives stool its bulk, helping it move more quickly through the gastrointestinal tract. This may help prevent colorectal cancer.

Dietary fiber offers a variety of potential therapeutic uses.

• Weight control: Fiber contributes to a feeling of fullness and displaces fat and sugar calories that may contribute to weight gain.
• Heart disease and stroke: Soluble fibers found in oatmeal, oat bran, and psyllium (derived from the seed coats or husks of various plantain plants) can significantly lower blood cholesterol levels and reduce the risk of heart disease and stroke.
• Cancer prevention: Insoluble fibers and noncarbohydrate components of fiber, such as phytic acid, may offer protection against colon and breast cancer.
• Diabetes: Increased intake of soluble fiber can reduce insulin requirements and improve blood sugar control in some people with diabetes.
• Gastrointestinal health: High intake of insoluble fibers may help prevent diverticulosis, constipation, hemorrhoids, and other related conditions.

Soluble fiber is found in the following foods.

• Dried beans, peas
• Oats, barley, legumes
• Apples, citrus fruits, carrots
• Psyllium seed husks

Insoluble fiber is found in the following foods.

• Whole-grain products, wheat and corn bran, whole grain cereals
• Cauliflower, green beans, potatoes, skins of fruits and vegetables

Dietary fiber is available as a supplement in several forms.

• Fiber tablets, capsules, and powders
• Bulk fiber laxatives (for example, Metamucil)

Pediatric

• There is no specific dietary amount established for fiber intake.
• If the child is thought to need more fiber, increase the daily dietary intake of fibrous foods slowly over a period of days.

Adult

Recommended adult doses of fiber vary depending on the health condition being treated.

• 25 to 35 g per day dietary fiber for general health and cancer prevention
• 7 g per day soluble fiber from psyllium husk along with a low-cholesterol diet, to lower cholesterol levels

Check with your health care professional before taking dietary fiber supplements. Increase fiber intake slowly to prevent gas, diarrhea, or constipation. Drink at least eight 8 oz glasses of water a day to prevent constipation. Taking too much soluble fiber supplements can result in the development of stomach cancer. Rare cases of intestinal obstruction have occurred in people with preexisting gastrointestinal problems and in individuals consuming high amounts of fiber supplements in a dry, unhydrated form.

Taking too much of soluble fiber supplements, such as guar gum, may cause a vitamin B₁₂ deficiency in people whose diets lack enough of that vitamin. Individuals following a diet that is not rich in minerals may experience health problems if they consume large amounts of fiber supplements (which are lacking in minerals). However, this is not the case with fiber that comes from foods.

Taking psyllium with carbamazepine, which is used to treat seizure disorders, may decrease the absorption and effectiveness of this medication. If you use psyllium while on carbamazepine, your blood levels of this medication should be monitored closely. Consult with your healthcare provider before adding any new supplements to your existing medication regimen.

Combining psyllium with cholestyramine or colestipol may be beneficial in lowering cholesterol levels. Consult with your healthcare practitioner about whether this may be an option for you. However, taking fiber in the form of pectin (from fruit) and oat bran reportedly reduces the body's ability to absorb lovastatin, a cholesterol-lowering medication, and could lead to decreased effectiveness of this medication.

Psyllium may lower lithium levels in the blood, reducing the effectiveness of the medication. Consult your healthcare provider before using psyllium with lithium. If both are used, take them at least one hour apart, preferably two. Lithium levels should be monitored very closely, particularly with any change in intake of fiber.

Fiber supplements may reduce the body's ability to absorb digoxin, a medication used to regulate heart function. Therefore, you should not take fiber supplements at the same time as this medication; it would be best to take them either two hours before or after your dose of digoxin.

While fiber supplements may help to regulate blood sugar levels, they may also interfere with the absorption of your antidiabetic medications, specifically glyburide and metformin. Therefore, you should not take fiber supplements at the same time as your antidiabetic medications. Your blood sugar levels should be monitored closely.

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

Similarly, increasing dietary fiber led to decreased 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 the conditions of these patients. If you are currently taking these medications, it would be best not to increase your fiber intake, but if you do, consult with your healthcare provider so that the blood levels of these medications can be monitored closely.

In general, taking psyllium or other fiber supplements with any medications may reduce or delay the absorption of these medications. It is best to refrain from taking these supplements at the same time as these other medications; take them at least 1 to 2 hours apart.

Anderson JW, et al. Oat-bran cereal lowers serum total and LDL cholesterol in hypercholesterolemic men. Am J Clin Nutr. 1990;52:495-499.

Anderson JW, et al. Hypocholesterolemic effects of different bulk-forming hydrophilic fibers as adjuncts to dietary therapy in mild to moderate hypercholesterolemia. Arch Intern Med. 1991;151:1597-1602.

Bridges SR, et al. Oat bran increases serum acetate of hypercholesteroemic men. Am J Clin Nutr. 1992;56:455-459.

Brodribb AJ, Humphreys DM. Diverticular disease: three studies. Part I: Relation to other disorders and fibre intake. Br Med J. 1976;1:424-425.

Brown DD, Juhl RP, Warner SL. Decreased bioavailability of digoxin due to hypocholesterolemic interventions. Circ. 1978;58(1):164-172.

Cohen LA. Dietary fiber and breast cancer. Anticancer Res. 1999;19(5A):3685-3688.

Compher CW, et al. Wheat bran decreases aberrant crypt foci, preserves normal proliferation, and increases intraluminal butyrate levels in experimental colon cancer. JPEN. Sep-Oct 1999;23(5):269-277.

Elsenhaus B, Caspary WF. Differential changes in the urinary excretion of two orally administered polyethylene glycol markers (PEG 900 and PEG 4000) in rats after feeding various carbohydrate gelling agents. J Nutr. 1989;119:380-387.

Etman MA. Effect of a bulk forming laxative on the bioavailability of carbamazepine in man. Drug Dev Ind Pharm. 1995;21(16):1901-1906.

Ettinger AB, Shinnar S, Sinnett MJ, Moshe SL. Carbamazepine-induced constipation. J Epilepsy. 1992;5(3):191-193.

Fermentable fibers and vitamin B₁₂ dependency. Nutr Rev. 1991;49:119-120.

Galland L. The Four Pillars of Healing. New York: Random House, Inc.; 1997:195-198.

Gin H, Orgerie MB, Aubertin J. The influence of guar gum on absorption of metformin from the gut in healthy volunteers. Horm Metab Res. 1989;21(2):81-83.

Graedon J, Graedon T. Deadly Drug Interactions: The People's Pharmacy Guide. New York: St. Martin's Griffin; 1995:4, 51.

Huupponen R, Seppala P, Iisalo E. Effect of guar gum, a fibre preparation, on digoxin and penicillin absorption in man. Eur J Clin Pharmacol. 1984;26(2):279-281.

Jenkins DJ, Jenkins AL. Dietary fiber and the glycemic response. Proc Soc Exp Biol Med. Dec 1985;180(3):422-431.

Johnson BF, Rodin SM, Hoch K, Shakar V. The effect of dietary fiber on the bioavailability of digoxin in capsules. J Clin Pharmacol. 1987;27(7):487-490.

Kenji T, et al. Dietary factors and prevention of colon cancer. Nippon Geka Gakkai Zasshi. 1999;(6):368-372. In Japanese.

Kritchevsky D. Dietary fibre and cancer. Eur J Cancer Prev. 1997;6:435-441.

Lanza E, et al. Dietary fiber intake in the US population. Am J Clin Nutr. 1987;46:790-797.

Lupton JR, Jacobs LR. Fiber supplementation results in expanded proliferative zones in rat gastric mucosa. Am J Clin Nutr. 1987;46:980-984.

Marlett JA, et al. Mechanism of serum cholesterol reduction by oat bran. Hepatology. 1994;20:1450-1457.

Perlman BB. Interaction between lithium salts and ispaghula husk [letter]. Lancet. 1990;355:416.

Richter WO, Jacob BG, Schwandt P. Interaction between fibre and lovastatin. Lancet. 1991;338;706.

Shils ME, Olson JA, Shike M, eds. Modern Nutrition in Health and Disease. 8th ed. Media, Pa: Williams & Wilkins; 1994:89, 92-98.

Shima K, Tanaka A, Ikegami H, et al. Effect of dietary fiber, glucomannan, on absorption of sulfonylurea in man. Horm Metab Res. 1983;15(1):1-3.

Spence JD, Huff MW, Heidenheim P et al. Combination therapy with colestipol and psyllium mucilloid in patients with hyperlipidemia. Ann Intern Med. 1995;123:493-499.

Stewart DE. High-fiber diet and serum tricyclic antidepressant levels. J.Clin. Psychopharmacol. 1992;12:438-440.

Thomas CL, ed. Taber's Cyclopedic Medical Dictionary. 15th ed. Philadelphia, Pa: F.A. Davis Company; 1985:615.

Toutoungi M, Schulz P, Widmer J, Tissot R. Probable interaction entre le psyllium et le lithium. Therapie. 1990;45(4):358.

Turley SD, Daggy BP, Dietschy JM. Effect of feeding psyllium and cholestyramine in combination on low density lipoprotein metabolism and fecal bile acid excretion in hamsters with dietary-induced hypercholesterolemia. J Cardiovasc Pharmacol. 1996;27:71-79.

Turnbull WH, Thomas HG. The effect of plantago ovata seed containing preparation on appetite variables, nutrient and energy intake. International J Obesity. 1995;19:338-342.

Walker AR, Segal I. Epidemiology of noninfective intestinal diseases in various ethnic groups in South Africa. Isr J Med Sci. 1979;15:309-313.

Whitney EN, Hamiliton EN. Understanding Nutrition. 3rd ed. New York, NY: West Publishing Company; 1979:69.