Fiber, dietary digestive tract

Historically, dietary fiber referred to iasoluble plant cell wall material, primarily polysaccharides, not digested by the endogenous enzymes of the human digestive tract. This definition has been extended to iaclude other nondigestible polysaccharides, from plants and other sources, that are iacorporated iato processed foods. Cellulose [9004-34-6] (qv) is fibrous however, lignin [9005-53-2] (qv) and many other polysaccharides ia food do not have fiberlike stmctures (see also Carbohydrates). 

Fiber components are the principal energy source for colonic bacteria with a further contribution from digestive tract mucosal polysaccharides. Rate of fermentation varies with the chemical nature of the fiber components. Short-chain fatty acids generated by bacterial action are partiaUy absorbed through the colon waU and provide a supplementary energy source to the host. Therefore, dietary fiber is partiaUy caloric. The short-chain fatty acids also promote reabsorption of sodium and water from the colon and stimulate colonic blood flow and pancreatic secretions. Butyrate has added health benefits. Butyric acid is the preferred energy source for the colonocytes and has been shown to promote normal colonic epitheUal ceU differentiation. Butyric acid may inhibit colonic polyps and tumors. The relationships of intestinal microflora to health and disease have been reviewed (10). 

The primary fate of dietary fibers is digestion and catabolism by the gut microflora to short-chain fatty acids and carbon dioxide. The major products of this microbiai mctaboiism — acetic, propionic, and butyTic acid — are important sources of energy for ruminants (sheeps cows). Dietary fiber is retained in a chamber of their gastrointeslinai tracts, calied the rumen, where it is converted to short-chain fatty acids by the gut micro flora. The fatty acids produced may supply 3 75 y<> of the energy requirement of the ruminant. 

Chlorella containing chlorophyll and dietary fiber seems to protect against health disorders caused by dioxin exposure by capturing dioxin in the digestive tract by diminishing the amount of dioxin absorption. Thus,... 

Data have been presented emphasizing the variability of dietary fiber sources. Water holding capacity estimates differed by technique as well as source. The most Important advantage of the osmotic suction technique is that the measurement Includes the water soluble dietary fiber components. This is a more realistic model of the digestive tract. The human fecal batch in... 

Additional experiments are needed to relate not only the monosaccharide composition of the fibers fed to fecal output but also the structures of those fibers and how they may have been modified In the digestive tract even If not fermented. The effect of fiber particle size and pretreatment should be studied. Free sugar, starch, cellulose and uronlc acid measurements should be made in order to obtain a more complete picture of what survives and what is metabolized. The effect of dietary fibers on the digestion and utilization of other polysaccharides and other food components should be studied. This Information, together with fermentation data. Including gas and VFA production, will provide a better understanding of the role and value of different dietary fibers and their effects on nutrient bloavallablllty. 

Cellulose is the main constituent of plant cell walls, where it usually occurs together with hemi-celluloses, pectin and lignin. Since cellulase enzymes are absent in the human digestive tract, cellulose, together with some other inert polysaccharides, constitute the indigestible carbohydrate of plant food (vegetables, fruits or cereals), referred to as dietary fiber. Cellulases are also absent in the digestive tract of animals, but herbivorous an-... 

In conclusion, the insoluble fraction, which is by far the main part of cereal dietary fiber, does not bind measurable amounts of ash after digestion with proteolytic and amylolytic enzymes under conditions similar to those in the human gastrointestinal tract. All the binding capacity seems to be due to the small soluble fraction in which the polysaccharides but also most of the phytic acid is recovered. 

Dietary fiber, composed principally of polysaccharides, cannot be digested by human enzymes in the intestinal tract. In the colon, dietary fiber and other nondi-gested carbohydrates may be converted to gases (H2, CO2, and methane) and short-chain fatty acids (principally acetic acid, propionic acid, and butyric acid) by bacteria in the colon. 

Dietary fiber is only one of several possible pharmacologically active substances found in foods, and present in high concentrations in leguminous seeds which may be responsible for the different rates of digestion and blood glucose responses of different meals. Enzyme inhibitors, lectins and saponins are other so called antinutritional factors, also associated with dietary fiber which are able to alter small intestinal function (48). The gastrointestinal tract evolved to deal with these constituents in foods so that while toxic in large amounts (as in uncooked beans) small amounts may have beneficial effects. 

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