Short-chain fatty acids absorption

McNeil et al measured short-chain fatty acid absorption from the human intestine. Using obese, but otherwise healthy subjects, and the dialysis bag technique of Edmonds, absorption from the rectum and subsequently from other areas of the colon was measured. Table 4 shows the initial and final concentrations of acetate, propionate and butyrate in a mixed electrolyte solution after one hour s dialysis in the human colon. It is clear that these acids are rapidly absorbed from the dialysate. In fact it would be surprising if... 

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 two important fuels for colonocytes are glutamine and short-chain fatty acids. The oxidation of both fuels provides ATP for the cells, which is important not only to maintain digestive and absorptive functions but also to maintain membrane structure and hence the physical barrier between the lumen and the blood and peritoneal cavity. This barrier normally prevents significant rates of translocation of bacteria into the peritoneal cavity and thence into the blood. If this barrier is breached, translocation of pathogens and... 

In developed conntries, fat provides about 40% of the total energy consnmed but in some individuals this percentage may be mnch higher. Almost all of this is in the form of triacylglycerol, containing mainly long-chain bnt also some short-chain fatty acids. The stractnre, digestion, absorption and eventual fate of the products of absorption are described in Chapters 4 and 5 and the metabolism of fat is discussed in Chapter 7. 

Hemicellulose and oligosaccharides are now staple items in fat replacement systems.21 They are claimed to have indirect beneficial roles in human health, because they ferment in the intestines (Tomomatsu, 1994) and large bowel where they produce short-chain fatty acids that seem to enhance electrolyte absorption and stimulate colonic muscular activity (Topping, 1994). Polydextrose is used as a low-calorie bulking agent. 

In neonate, suckling mammals, short- and medium-chain fatty acids are preferentially split at the sn-3 triacylglycerol position by oral and gastric lipases and are absorbed in the stomach, while the long-chain fatty acids are hydrolyzed at the sn- and sn-2 positions and by pancreatic lipases and are absorbed in the small intestine (50, 51). With growth, the neonate fat digestion system becomes less active, and is replaced by the small intestine-pancreatic lipase pathway. But residual oral and gastric lipase activities and direct absorption of short-chain fatty acids in the... 

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. 

The reabsorption of conjugated bile acids is mediated by ASBT, which is localized on the apical membrane of ileal enterocytes in mammals. ASBT is a drug target not only to lower plasma cholesterol level but also to improve intestinal permeability [46]. Although available monolayer cell lines do not express ASBT, it has been expressed in MDCK cells [47]. Human intestine also expresses multiple MCTisoforms [48]. These MCTs are responsible for the absorption of short-chain fatty acid. Expression of MCT in Caco-2 allows it to be an appropriate model to study short-chain fatty acid transport [9, 49, 50]. 

The large intestine extends from the ileocecal valve to the anus. It is wider than the small intestine except for the descending colon, which when empty may have the same diameter as the small intestine. Major functions of the colon are absorption of water, Na+, and other electrolytes, as well as temporary storage of excreta followed by their elimination. The colon harbors large numbers of mostly anaerobic bacteria that can cause disease if they invade tissues. These bacteria metabolize carbohydrates to lactate, short-chain fatty acids (acetate, propionate, and butyrate), and gases (CO2, CH4, and H2). Ammonia, a toxic waste product, is produced from urea and other nitrogenous compounds. Other toxic substances are also produced in the colon. Ammonia and amines (aromatic or aliphatic) are absorbed and transported to the liver via the portal blood, where the former is converted to urea (Chapter 17) and the latter is detoxified. The liver thus protects the rest of the body from toxic substances produced in the colon. Colonic bacteria can also be a source of certain vitamins (e.g., vitamin K, Chapter 36). 

In H. diminuta, two distinct carrier-mediated transport systems for fatty acid absorption have been partially characterized, one specific for short-chain fatty acids and the other for longer chain molecules. Acetate absorption in H. diminuta is mediated by the short-chain carrier at low concentrations, but passive diffusion occurs at high concentrations (15). 

Prevent constipation due to the production of short-chain fatty acids, improve absorption of calcium and other minerals, reduce risk of colon cancer... 

Psyllium has also been found helpful in binding and reducing the absorption of dietary fat, which can help with weight control (4, 31, 32). Psyllium promotes the production of short chain fatty acids, which in turn result in a more acidic colon and helps to increase the growth of friendly bacterial. This also facilitates detoxification and boosts body natural defences against fatal diseases (4,29). 

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