Water and dietary fiber( DF) are two components in any healthy diet and both of them are considered non-nutrients from conventional considerations. While water is absorbed by the body since it is the medium for all metabolic reactions, DF is not digested by the GI enzymes, contributing neither calorie nor any other major nutrient. Still both of them need to be present in food for normal health. From a relatively obscure position in the proximate composition tables, food fibers have captured center stage attention within a matter of two decades because of their sterling role in protecting human health. Most of the DF are non-starch polysaccharides(NSP) and are part of plant kingdom. Humans do not possess the necessary enzyme system to break them into sugar and therefore they pass through the GI tract without much change except for microbial action. If they do not have any metabolic role in human body why are they considered essential for good health?
By definition a fiber is a class of materials which are continuous filaments or indiscreet elongated pieces. Natural fibers hold tissues together in plants and animals. There can be vegetable fibers based on an arrangement of cellulose and lignin like cotton, hemp, jute, flax, sisal etc. Wood fibers are from tree sources which are delignified to make paper. Animal fibers include silk, wool and furs which are proteinacious in nature. Mineral fibers like asbestos, fiber glass etc are of industrial importance. Man made fibers are based on petrochemicals like polymers or cellulose based like rayon. Though the plant based fibers that are edible go through the GI tract without digestion, they can be acted upon by the gut microbes through fermentation, often producing nutrients like short chain fatty acids. DF with low fermentability include cellulose, hemicellulose, lignan, waxes and resistant starches. On the other hand beta glucans, pectin, natural gums, inulin, oligosaccharides and resistant dextrins are of high fermentability in the gut.
Potential advantages of DF include production of health promoting components during fermentation of soluble fibers and the property of of insoluble fibers to increase bulk, soften stool and shorten fecal transit time in the GI tract. A daily intake of 25 gm of DF is recommended for an adult consuming a 2000 kC diet. DF does not bind minerals or vitamins, as is assumed by many; on the other hand increased absorption of minerals has been reported when fermentable fibers are consumed. Probably phytates, not considered as a real DF do interfere with mineral absorption to some extent. Whole grains, bran from wheat and corn, nuts and seeds, lignans and vegetables are important sources of insoluble fiber while legumes like peas, soybeans, beans and others, oats ans rye, some fruits, some vegetables like broccoli, jerusalem artichokes and carrot, root vegetables like potato, plant gums like guar gum and acacia gum and psyllum seed husk contain significant levels of soluble fiber. High fiber containing foods include legumes (15-19 gm per serving), wheat bran (17gm per serving), prunes (12 gm per serving) Asian peas (10gm per serving). The exotic palmberry Acai is reported to be the richest source of DF, about 25-44% in its freeze dried version.
Importance of DF in human health has spawned a multibillion dollar industry and there are hundreds of products claiming to be rich in this 'non-nutritious' nutrient. Generally foods claiming to be high in DF must contain not less tha 5 gm per serving, between 2.5 and 4.9 gm qualifying for the 'good source' tag and at least 2.5 gm fiber to be incorporated during processing from external sources for claiming as 'more added fiber'. Such products are used to treat many disorders like hemorrhoids, diverticulosis, crohn's disease irritable bowel syndrome, abdominal discomforts etc. Proactive consumers take DF preparations to lower cholesterol levels in blood, reduce risks associated with cancers, lose weight and ensure regular bowel movement. Though modern food technology provides processing solutions for removing fiber from raw foods, it is only wise on the part of the consumer to shun regular consumption of such refined foods in the interest of his own health.