Short chain fatty acids, dietary fiber

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). 

During the in vitro fermention the amount of macromolecular pectin was diminished continuously. On the other hand the fraction of OligoGalA was increased at first and diminished later. The content of short chain fatty acids, which are typical end products of fermentation of dietary fibers rised permanently (Figure 5). Low-esterified pectins were fermented in vitro faster by human faecal flora than the high-esterified pectins. 

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. 

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. 

Dietary Fiber or Short-Chain Fatty Acids Are Needed for Health of Enterocytes... 

Whether dietary fiber is required for the health of the colonocylcs has not been proven, although evidence suggests such a requirement Absorption of salts and water is a major function of the large intestine. Short-chain fatly adds stimulate the absorption of sodium, chloride, and water in the colon (Hoverstad, 1986). in the absence of short-chain fatty acids, the mucosa of the colon may become inflamed or atrophied. 

Short-chain fatty acids, especially butyrate, are a preferred source of energy for the colonic epithelium. There is evidence to suggest that butyrate enemas are effective in the treatment of ulcerative cohtis. The seeds of Plantago ovata (a source of fermentable dietary fiber) increase fecal concentrations of butyrate and acetate. In a randomized, open, parallel-group, multicenter study in 105 patients with ulcerative cohtis, P. ovata seeds 10 mg bd were as effective as mesalazine 500 mg tds in maintaining remission over 12 months (10). Adverse effects were similar in the two groups, and included constipation, flatulence, nausea, and diarrhea. 

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. 

Highly-fermentable fiber residues, like resistant starch, oat bran, and pectin are transformed by colonic bacteria into short-chain fatty acids including butyrate. One study found that resistant starch consistently produces more butyrate than other types of dietary fiber. 

Increased fiber intake and the consumption of fruits and vegetables are well-known chemopreventive dietary strategies against colon carcinogenesis. It is reported that fiber intake reduces colonic transit time and increases the production of short chain fatty acids such as butyric acid. The latter is produced in the mammalian colon in mM concentrations as a by-product of anaerobic bacterial fermentation of dietary fiber. Butyric acid has been recognized as one candidates chemopreventive active against colon carcinogenesis because it induces apoptosis and differentiation in colon cancer cell lines. 

An increase in long-chain fatty acids in the diet increases their secretion in the milk and inhibits synthesis of short- and medium-chain fatty acids in mammary tissue. Added dietary fat and whole cottonseed decrease the protein content of milk by about 0.1%. Dry fats —calcium salts of fatty acids that are 82% fat content—and prilled (beadlet-form) fats have been introduced in recent years. These products are free-flowing and easy to mix with the ration, do not coat the fiber, and are not biohydrogenated in the rumen. It has been suggested that cows can be fed up to 15% of their requirements as fat, the equivalent of 6-7% of total dry matter (9). 

A short-chain fatty add thiokinase catalyzes the ATP-dependent conversion of propionic acid to propionyl-CoA. Which cells of the gastrointestinal tract might be expected to have high levels of short-chain thiokinase and of methylmalonyl-CoA mutase. Hint see the section on Dietary Fiber in Chapter 3. 

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