Digestion dietary fibers

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). 

Not all of the starch in a food ends up being digested. The starch that is not absorbed by the body is called resistant starch, and it is considered dietary fiber. It is also a source of nutrition for intestinal flora, which make important vitamins (and intestinal gas). 

Various mechanisms have been proposed to explain the hypocholesterolemic effect of GA (Annison et al., 1995 Tiss et al., 2001). Some studies have suggested that the viscosity of fermentable dietary fiber contributes substantially to the reduction of lipids in animals and humans (Gallaher et al., 1993 Moundras et al., 1994). However, other studies suggested that this property is not related to plasma lipids (Evans et al., 1992). The mechanism involved is clearly linked to increased bile acid excretion and fecal neutral sterol or a modification of digestion and absorption of lipids (Moundras et al., 1994). 

After absorption in the small bowel, remaining undigested food passes from the ileum through the ileocecal valve to the colon. A major role of the colon is absorption of fluid. Some of the water and sodium absorption achieved by the colon is facilitated by short-chain fatty acids (SFCAs) formed from digestion of certain dietary fibers by colonic bacterial enzymes. 

Later it was observed that butyric acid is a naturally occurring dietary HD AC inhibitor. It is formed upon dietary fiber fermentation within our digestive system. Although butyrate does not normally reach high enough levels for cancer treatment, it has been proposed that daily consumption over an individual s lifetime may provide chemoprevenfive effects. ... 

Relevance to potatoes Dietary fiber includes cell wall NSP and RS, as both are digestion-resistant polysaccharides and both enter the colon where they may contribute to colonic health. NSP is low in potatoes but with RS formation NSP + RS (= dietary fiber) in potatoes can reach levels that allow the nutrient claim high in dietary fiber. ... 

Little agreement has been reached as to which dietary components or which food processes physiologially affect mineral availability. Many plant foods contain phytic acid, oxalic acid or other dietary fiber components that can be shown to chelate minerals. The effect of these dietary substances upon the final bioavailability of the mineral in question will depend upon the digestibility of the chelate (106). 

Inicimn in particular, are readily converted into a nondigesltble or slowly digestible physical form ander certain food processing conditions. These resistant starches are readily fermented by colonic bacteria. Small amounts of waxes, cutin. and minerals in fruits and vegetables contribute to total dietary fiber values but may be physiologically inert. 

Dietary fiber has a pronounced effect on the characteristics of the fecal mass and on the rate of passage of digest through the G1 trad, High fiber diets also play a role in the excretion of bile acids and cholesterol. 

Metabolism of pectin. Pectin has only recently come, to be considered a part of the dietary fiber complex. Previously it was excluded because 1) it is not fibrous (except at the molecular level), 2) it escapes detection in standard fiber tests owing to its solubility, and 3) it usually does not survive intestinal passage. In a reassessment of which dietary components should be considered fiber, Trowell (49) proposed that dietary fiber include those constituents of food resistant to hydrolysis by man s alimentary enzymes. Spiller (50, 51) suggested that confusion surrounding the term "fiber" be avoided by using the term "plantix" to denote those plant materials of polymeric nature not attacked by human digestive enzymes. 

Carbohydrates which are digestible appear to have no dramatic effect on metabolism of xenobiotics however, the nondigestible carbohydrates can mediate the toxicity of a xenobiotic. Dietary fiber can reduce nutrient availability by absorbtion of certain minerals and/or vitamins. Inclusion in the diet of fiber has been associated with a lower incidence of colon cancer, and this... 

Dietary fiber has been suggested to play a protective role against chemically-induced toxicity (1) and against colon cancer (2). However, the mechanism(s) by which dietary fiber modulates chemical toxicity or colon cancer has not been well studied. The fiber fraction of the diet is resistant to mammalian digestive enzymes and consequently dietary fiber is not absorbed from the small intestine (3). However, certain types of dietary fiber specifically fermentable fibers, including the pectic substances and hemicelluloses, are readily digested by the intestinal microflora (4,5). Pectic... 

Jerusalem artichoke has beneficial effects on digestion. It is a good source of dietary fiber, for instance, which helps to bulk food and reduce constipation. However, there can be digestive downsides too. 

The degradation of plant cell walls requires fra/is-p-coumaroyl esterase, an important enzyme in the digestion of forages and dietary fiber. The trans-p-coumaric acid released by enzymatic hydrolysis was assayed by reversed-phase HPLC. 

Most aspects t>f carbohydrate nutrition are simpler than those of other nutrients, (For example, fat nutrition is complicated by the fact that the metabolism of fats requires bile salts to maintain solubility during digestion and lipoproteins and albumin during distribution in the body.) On the other hand, the nutrition of the carbohydrates that take the form of dietary fibers is very complicated. This complexity . > due to the fact that they arc metabolized by enzymes of the gut microflora. 

Oasses, Sources, and Binchemistry Fuel for the Host Dietary Fiber and Flealth of the Small and Large fntestines Mictoorganisins and Digestion Gut Microflora Microbe Host Interactions Malabsorption Syndromes Summary References BibLiography... 

---