| The definition of fiber, as established by a panel of experts convened by the Institute of Medicine, made a distinction between naturally occurring fiber in plant foods (dietary fiber) and isolated or synthetic fibers that can be added to foods or used as supplements (functional fiber). If fiber is removed from its natural food source and used as a supplement it is considered to be functional fiber, not dietary fiber.
Dietary fiber: lignin, cellulose, beta-glucans, hemicelluloses, pectins, gums, inulin and oligofructose, and resistant starch.
Functional fiber: any isolated or extracted forms of the dietary fibers listed above, psyllium, chitin and chitosan, fructooligosaccharides, polydextrose and polyols, and resistant dextrins.
Total fiber is the sum of dietary fiber and functional fiber.
There are other ways of classifying fiber as well. These methods include:
Viscous and Nonviscous fiber. Viscous fibers form solutions or gels in water which helps them delay emptying of the stomach. This delayed emptying rate helps stabilize blood sugar levels, reduce cholesterol, and improve the sensation of feeling full and satisfied after a meal. Viscous fibers include pectins, beta-glucans, guar gum and mucilages (psyllium). Cellulose and lignin are nonviscous fibers.
Fermentable and Nonfermentable fiber. The bacteria in your colon can ferment fiber resulting in short-chain fatty acids such acetate, propionate and butyrate and various gases. These short-chain fatty acids can be absorbed and metabolized to produce energy. Fibers that are readily fermented include pectins, beta-glucans, guar gum inulin and oligofructose. The foods that are rich in these types of fibers include oats and barley and fruits and vegetables. Cereal fibers rich in cellulose, such as wheat bran, are relatively resistant to bacterial fermentation.
Soluble and Insoluble fiber. Soluble fibers are dispersible in water. Originally it was though that soluble fibers were more likely to form viscous gels and be fermented by bacteria, but, research has shown that solubility does not predict physiological effects. However nutritional and health care professionals, as well as the U.S. Food and Drug Administration still use these terms for nutrition labeling. Soluble fibers include beta-glucans, gums, mucilages, pectins, and some hemicelluloses. Oat products and legumes are rich sources of soluble fiber.
DIETARY FIBER
Soluble fiber – In general, soluble fibers are not effective as laxatives but, are the best fibers for health – lowering cholesterol, improving diabetes, etc. Good food sources include legumes (peas, soybeans, and other beans); oats; some fruits (apples, bananas) and berries; certain vegetables (broccoli and carrots); root vegetables (potatoes and yams – the skin has insoluble fiber); and psyllium seed (about 2/3 soluble fiber).
BETA-GLUCANS – oats (2-7.5% fiber by weight), barley (2-11%), rye (1.4 -2.6%), wheat (0.5-1%) mushrooms, yeast, bacteria and algae all contain beta-glucans. Beta-glucan is a complex carbohydrate made from individual glucose molecules. The manner in which glucose molecules are joined determines how the human body can digest or use them. For example, amylose, a common starch, is connected as shown in the diagram. Cellulose, an insoluble fiber, connects the glucose at the same site, but in a different way as shown in the diagram. Beta-glucans, which are soluble, have a mixture of either 1,4 or 1,6 bonds and 1,3 bonds connected different from either amylose or cellulose. Cereal grains are mixed 1,3 and 1,4 bonds while yeast and fungal beta-glucans are 1,3 and 1,6 bonds.
Chemical structures of amylose (a digestible carbohydrate), cellulose (a fiber that is not digestible by human enzymes) and beta-glucan (a fiber)
Pectins – are found in citrus peels, apple pulp, plums, Concord grapes, quinces, gooseberries, red currants and cranberries. Pectin is widely used as gelling agents in foods and added to jams and yogurts.
Gums – are a diverse group of fibers isolated from seeds. Guar gum is a thickening agent extracted from the guar bean or Indian locust bean. Several supplements are on the market touting weight loss. It does not work [Pittler MH, Ernst E. Guar gum for body weight reduction: meta-analysis of randomized trials. Am J Med. 2001;110(9):724-730.
Insoluble fiber – whole grains; bran; nuts and seeds; vegetables such as green beans, cauliflower, zucchini, celery; skins of some fruits, including tomatoes. In general, insoluble fibers are great at helping bowel conditions such as constipation, diverticulitis and hemorrhoids. However, they are not very effective in other health conditions such as cholesterol and diabetes.
Cellulose – is the major component of plant walls.
Hemicelluloses – are found in plant cell walls that surround cellulose.
Lignin – is found within “woody” plant cell walls bound to fibrous polysaccharides.
FUNCTIONAL FIBER
Chitin and chitosan – Both are found in the exoskeletons of arthropods (e.g., crabs and lobsters) and in the cell walls of most fungi. There is no laxative effect. Chitosan may produce a reduction of LDL. No reports of any effect on glucose are found. Numerous studies failed to show any benefit on weight loss (Pittler et al., 1999; Ho and colleagues 2001).
Inulin, Oligofructose, and Fructooligosaccharides – are naturally occurring in a variety of plants including chicory, onions, and Jerusalem artichoke. However, most commercially available products are synthesized from sucrose or extracted and purified from chicory roots. Inulin is added to replace fat in products, such as salad dressing, while sweet-tasting oligofructose is added to products, such as fruit yogurts and desserts. Of special note, both Inulins and oligofructose are classified as prebiotics because of their ability to stimulate the growth of beneficial Bifidobacteria species in the human colon. This suppresses the growth of pathogenic bacteria known to cause diarrhea.
Psyllium – is a viscous and soluble mucilage type fiber (thick glutinous substance, secreted by seed covers such as marsh mallows, flaxes, and certain seaweeds) from psyllium seed. It is a good laxative and lowers cholesterol. Psyllium and Beta-glucan are the only two fibers approved by the FDA to make the claim that it may reduce the risk of heart disease.
FORMULA FOR LIFE EASY CHOICE
Finally, no food is just one type of fiber. All the fruits and vegetables and grains have both soluble and insoluble fibers to different extents. The most important thing to learn is the types of food that emphasize the healthy soluble fibers – fruits, legumes, oat, barley, rye and certain vegetables (carrots, potato with skin, corn, beets, greens, squash, and artichoke). And, bulk up on insoluble fiber for colonic health – vegetables and whole grains.
A quick method of estimating the amount of fiber in foods is: One serving of fruit or vegetables is roughly 1.5 grams; one serving of refined grain products is about 1 gram; one serving of whole-grain products is approximately 2.5 grams.
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