Dietary fibre is the nonstarch polysaccharide portion of plant cell walls that is resistant to hydrolysis by digestive enzymes. The major polysaccharides in fibre include cellulose, beta-glucans, hemicelluloses, pectins, and gums. Lignin, phytic acid, and isoflavones are noncarbohydrate components of fibre. Dietary fibre is typically classified as soluble (pectins, gums, some hemicelluloses) or insoluble fibre (cellulose, hemicellulose).
Soluble and insoluble fibres have different chemical properties that influence their physiological effects in the body. Soluble fibres have a high solubility and water-holding capacity that enables them to form gels, slow postprandial gastric emptying, 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 fibres 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 fibre fermentation enter the portal circulation, where they decrease the liver's production of cholesterol. In the gastrointestinal tract, soluble fibres sequester bile salts and reduce lipid absorption. The combined effect of these mechanisms is a significant reduction in serum cholesterol levels over time.
Insoluble fibres that are not as extensively fermented by colonic bacteria exert primarily a softening and bulking effect that shortens the transit time and increases the faecal mass. Softer, bulkier stools help prevent constipation and the development of haemorrhoids. Insoluble fibre exercises the muscles of the digestive tract, helping them retain their tone to resist bulging and developing diverticulosis. High intake of insoluble fibre is associated with a decreased risk of colon and breast cancer. Butyrate, produced by bacterial fermentation of insoluble fibre, has been shown to have anticancer properties. Isoflavones, phytate, and lignin are biologically active components of fibre that may also contribute to its anticancer effects.
Fibre consumption tends to be highest in the underdeveloped countries where there is more emphasis on eating plant-based foods. High-fibre 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 fibre per day, less than half the amount recommended by health experts.
Dietary Sources
Dietary fibre is found in all plant-derived foods—vegetables, fruits, grains, and legumes. Whole foods contain a mixture of soluble and insoluble fibre.
A predominance of soluble fibre is found in the following foods - dried beans, peas, oats, barley, legumes, apples, citrus fruits, carrots, psyllium seed husks
A predominance of insoluble fibre 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
Therapeutic Uses
Weight control
Fibre contributes to a feeling of fullness and satiety and displaces fat and sugar calories that may contribute to weight gain.
Heart disease and stroke
Fibre 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 fibres 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 fibres and noncarbohydrate components of fibre, such as phytic acid, may be protective against colon and breast cancer.
Diabetes
Increased intake of soluble fibre can reduce the glycaemic response, reduce insulin requirements, and improve diabetic control in some patients.
Gastrointestinal health
High intake of insoluble fibres may help prevent diverticulosis, constipation, haemorrhoids, and other related conditions. Short-chain fatty acids nourish the gastrointestinal tract and help maintain the integrity of the intestinal lining.
The association between rectal cancer and plant food and fibre intake was examined using data from 952 incident cases of rectal cancer, compared with data from 1205 population-based controls (Slattery 2004). Rectal cancer was inversely associated with intakes of vegetables, fruit, and whole-grain products, whereas a high intake of refined-grain products was directly associated with an increased risk of rectal cancer. Similarly, relative to low fiber intakes, high intakes of dietary fibre reduced the risk of rectal cancer. The reduced risk of rectal cancer associated with vegetable, fruit, and fibre intakes was strongest for persons who received the diagnosis after age 65 y. A threshold effect at approximately 5 servings of vegetables/d was needed to see a reduced risk of rectal cancer.
Dosage Ranges and Duration of Administration
25 to 35 g/day dietary fibre for general health and cancer prevention
7 g/day soluble fibre from psyllium husk in conjunction with a low-cholesterol diet, to lower cholesterol levels
