Uses of this Supplement
Diabetes Mellitus
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Cholestyramine Resin
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Look Up > Supplements > Fiber
Dietary Sources
Commercial Preparations
Therapeutic Uses
Dosage Ranges and Duration of Administration
Side Effects/Toxicology


Dietary fiber is the nonstarch polysaccharide portion of plant cell walls that is resistant to hydrolysis by digestive enzymes. The major polysaccharides in fiber include cellulose, beta-glucans, hemicelluloses, pectins, and gums. Lignin, phytic acid, and isoflavones are noncarbohydrate components of fiber. Dietary fiber is typically classified as soluble (pectins, gums, some hemicelluloses) or insoluble fiber (cellulose, hemicellulose).

Soluble and insoluble fibers have different chemical properties that influence their physiological effects in the body. Soluble fibers have a high solubility and water-holding capacity that enables them to form gels, slow postprandial gastric empyting, and slow absorption of starch and glucose from the small intestines. These effects have been shown to improve glucose tolerance in diabetics. The high water-holding capacity in soluble fibers enhances microbial growth, fermentation, and production of short-chain fatty acids (SCFAs) in the GI tract. Fermentation results in a decreased colonic pH and a proliferation of ceco-colonic epithelial cells. SCFAs nourish the cells of the large intestine, inhibit growth of yeasts and disease-causing bacteria, stimulate healing, and may reduce the development of cancer. SCFAs from soluble fiber fermentation enter the portal circulation, where they decrease the liver's production of cholesterol. In the gastrointestinal tract, soluble fibers sequester bile salts and reduce lipid absorption. The combined effect of these mechanisms is a significant reduction in serum cholesterol levels over time.

Insoluble fibers that are not as extensively fermented by colonic bacteria exert primarily a softening and bulking effect that shortens the transit time and increases the fecal mass. Softer, bulkier stools help prevent constipation and the development of hemorrhoids. Insoluble fiber exercises the muscles of the digestive tract, helping them retain their tone to resist bulging and developing diverticulosis. High intake of insoluble fiber is associated with a decreased risk of colon and breast cancer. Butyrate, produced by bacterial fermentation of insoluble fiber, has been shown to have anticancer properties. Isoflavones, phytate, and lignin are biologically active components of fiber that may also contribute to its anticancer effects.

Fiber consumption tends to be highest in the underdeveloped countries where there is more emphasis on eating plant-based foods. High-fiber intake in these countries has been associated with a reduced incidence of diseases of the colon such as colitis, diverticulitis, and colon cancer. Most Americans consume between 11 to 13 grams of dietary fiber per day, less than half the amount recommended by health experts.

Dietary Sources

Dietary fiber is found in all plant-derived foods—vegetables, fruits, grains, and legumes. Whole foods contain a mixture of soluble and insoluble fiber.

A predominance of soluble fiber is found in the following foods.

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

A predominance of 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

  • Cellulose 
  • Beta-glucans 
  • Hemicelluloses 
  • Pectins 
  • Gums
  • Lignin
  • Isoflavones 
  • Phytic acid 

Commercial Preparations
  • Fiber tablets, capsules, and powders 
  • Bulk fiber laxatives (e.g., Metamucil) 

Therapeutic Uses

Weight control: Fiber contributes to a feeling of fullness and satiety and displaces fat and sugar calories that may contribute to weight gain.

Heart disease and stroke: Fiber may work to reduce heart attack and stroke risk by lowering blood pressure, improving sugar metabolism, and lowering blood fats besides cholesterol, such as triglycerides. Soluble fibers found in oatmeal, oat bran, and psyllium 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 be protective against colon and breast cancer.

Diabetes: Increased intake of soluble fiber can reduce the glycemic response, reduce insulin requirements, and improve diabetic control in some patients.

Gastrointestinal health: High intake of insoluble fibers may help prevent diverticulosis, constipation, hemorrhoids, and other related conditions. Short-chain fatty acids nourish the gastrointestinal tract and help maintain the integrity of the intestinal lining.

Dosage Ranges and Duration of Administration
  • 25 to 35 g/day dietary fiber for general health and cancer prevention 
  • 7 g/day soluble fiber from psyllium husk in conjunction with a low-cholesterol diet, to lower cholesterol levels

Side Effects/Toxicology

Excess consumption of soluble fiber supplements may increase gut permeability and create changes in the intestinal environment that favor the development of gastric cancer. Rare cases of intestinal obstruction have occurred in individuals consuming high amounts of fiber supplements in a dry, unhydrated form, or with preexisting gastrointestinal problems.


Increase fiber intake slowly to prevent diarrhea or constipation. Drink a minimum of eight 8-oz. glasses of water a day to prevent constipation.


In a study with 4 healthy male volunteers, the bioavailability of carbamazepine was reduced by 1.22 mg/mL when carbamazepine (200 mg po) was coadministered with ispaghula husk (3.5 g) (Etman 1995). Coadminstration delayed achievement of maximum carbamazepine plasma concentrations. The decrease in carbamazepine absorption due to fiber consumption could lead to subclinical concentrations of the drug. However, there were no reports of adverse effects associated with the use of psyllium hydrophilic mucilloid (3.4 g bid) for constipation in a patient on carbamazepine therapy (1 g/day) (Ettinger et al. 1992).

Cholestyramine Resin; Colestipol

Preclinical and clinical data suggests that psyllium powder can be combined with bile acid sequestrants for the treatment of hyperlipidemias. In a study of cholesterol-fed hamsters, the combination of cholestyramine and psyllium (4% of diet) was almost as effective as cholestyramine monotherapy (3% of diet) in lowering blood and liver cholesterol levels (Turley et al. 1996). However, the combination therapy was more effective in promoting fecal bile acid excretion and inhibiting intestinal cholesterol absorption than the resin alone. In a randomized, double-blind controlled trial involving 121 patients with moderate hypercholesterolemia, treatment with a combination of colestipol (2.5 g tid) and psyllium (5 g tid) for 10 weeks was found to be better tolerated than colestipol monotherapy and as effective as either agent alone (Spence et al. 1995).


In one study, administration of digoxin tablets (0.75 mg) with a high fiber meal (5 g crude fiber) markedly decreased the absorption and urinary excretion of the drug in 12 healthy volunteers (Brown et al. 1978). However, in a double-blind study with 10 healthy volunteers, guar gum did not significantly alter urinary digoxin concentrations at 24 hours in spite of an initial reduction in drug absorption (Huupponen et al. 1984). Digoxin absorption is not affected significantly when the drug is taken as a capsule rather than as a tablet (Johnson et al. 1987). Although administration of two capsules of digoxin (0.4 mg/day) with breakfast supplemented with bran fiber (11 g) reduced serum digoxin levels by 6 to 7% compared to controls, the differences were not deemed to be clinically significant. The effects of high fiber intake on the absorption of digoxin from capsules were less than those reported for tablets.


Coadministration of glibenclamide (2.5 mg glyburide) and glucomannan (3.9 mg), a dietary fiber, reduced drug absorption and plasma levels in 9 healthy volunteers (Shima et al. 1983). However, mean plasma glucose levels were lower in the group treated with both glibenclamide and fiber compared to glibenclamide alone.


Psyllium mucilage administered to six healthy male volunteers reduced lithium sulfate (12 mEq in 100 mL water/day) absorption by approximately 14% (Toutoungi et al. 1990). Clinically, a single case of a potential interaction between psyllium and lithium has been reported (Perlman 1990). In spite of increasing oral doses of lithium, blood levels remained sub-therapeutic until psyllium (1 tsp. bid) was discontinued; blood lithium levels increased promptly when the psyllium was withdrawn. Individuals taking lithium should wait at least one hour before taking psyllium mucilage. Lithium levels should be frequently monitored whenever psyllium is introduced or withdrawn.


In one study, patients taking lovastatin (80 mg/day) with pectin (15 g/day) or oat bran (50 to 100 g/day) and a lipid-lowering diet experienced a dramatic increase in LDL-cholesterol (Richter et al. 1991). Discontinuation of the pectin and oat bran caused LDL-cholesterol values to fall to pretreatment levels. Both forms of fiber may reduce lovastatin absorption. Concentrated intake of soluble fiber should be avoided while on lovastatin therapy because it may reduce the efficacy of the drug.


Coadministration of guar gum with metformin (1700 mg) reduced absorption rates for the drug (Gin et al. 1989). Fiber therapy may diminish the hypoglycemic effects of metformin.


In a double-blind study with 10 healthy volunteers, coadministration of guar gum with penicillin significantly reduced serum concentrations of the drug (Huupponen et al. 1984).

Tricyclic Antidepressants (TCAs)

An increase in dietary fiber intake caused three patients to become refractory to previously effective treatment with tricyclic antidepressants (Stewart 1992). Commencement of a high-fiber diet decreased serum levels of the drugs. Reduced dietary fiber increased serum concentrations and improved the clinical outcome.


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.

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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.

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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.

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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.

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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.

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Copyright © 2000 Integrative Medicine Communications

This publication contains information relating to general principles of medical care that should not in any event be construed as specific instructions for individual patients. 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. The reader is advised to check product information (including package inserts) for changes and new information regarding dosage, precautions, warnings, interactions, and contraindications before administering any drug, herb, or supplement discussed herein.