Intestinal absorption tract

Absorption of mannitol (209), sorbitol (210), and xyfltol (4) from the intestinal tract is relatively slow, compared to that of glucose. In humans, approximately 65% of orally adrninistered mannitol is absorbed in the dose range of 40—100 g. About one-third of the absorbed mannitol is excreted in the urine. The remainder is oxidized to carbon dioxide (211). 

A variety of therapies for thallium poisoning have been suggested by neutralising thallium in the intestinal tract, hastening excretion after resorption, or decreasing absorption. Berlin-Blue (fertihexacyanate) and sodium iodide in a 1 wt % solution have been recommended. Forced diuresis hemoperfusion and hemodialysis in combination results in the elimination of up to 40% of the resorbed thaHous sulfate (39). 

When a-glucosidase activity is inhibited, carbohydrate digestion is prolonged and takes place further along the intestinal tract. This in turn delays and spreads the period of glucose absorption, which reduces the extent of the postprandial rise in blood glucose concentrations. The effectiveness of a-glucosidase inhibitors is dq en-dent on the consumption of a meal rich in complex carbohydrate. 

The exact site of anthocyanin absorption is not fully known recently it was suggested that the stomach is one of the preferential sites for the process in humans. Interestingly, anthocyanin glycosides are efhciently absorbed in rats (up to 25%) from both stomach " and small intestine, suggesting that anthocyanins have more than one site of absorption along the intestinal tract. 

The strategy of the preceding sections was based on predicting the permeabilities of drug compounds in the human jejunum. The rest of the intestinal tract has higher pH, and this needs to be factored in when considering models to predict not human permeabilities, but human absorption (see Fig. 2.3 and Table 7.2). 

The preceding section can be further generalized, to properly account for absorption of nonionized molecules. The selection of the maximum Pe for HIA prediction implicitly recognized that only a fraction of the small intestine is available for the maximum absorption of acids (with pKa near 4) and bases (with pKa near 9). But when this approach is applied to nonionizable molecules, then the absorption may be underestimated, since absorption should be uniform across the whole intestinal tract. The remedy is to sum the two Pe values. This is roughly equivalent to... 

ABR Thomson, JM Dietschy. (1981). Intestinal lipid absorption Major extracellular and intracellular events. In LR Johnson, J Christensen, SL Schultz, eds. Physiology of the Gastrointestinal Tract. New York Raven Press, p 1146. 

GE Amidon, NFH Ho, AB Freeh, WI Higuchi. Predicted absorption rates with simultaneous bulk fluid flow in the intestinal tract. J Theor Biol 89 195-210, 1981. 

Penicillamine is well absorbed (40-70%) from the gastro-intestinal tract and, therefore, has a decided advantage over many chelating agents. Food, antacids, and iron reduce its absorption, so it should be taken on an empty stomach. Preferably, the... 

Most pathology of cholera is thought to result from an enterotoxin that increases cyclic adenosine monophosphate-mediated secretion of chloride ion into the intestinal lumen, which results in isotonic secretion (primarily in the small intestine) exceeding the absorptive capacity of the intestinal tract (primarily the colon). 

Hypomagnesemia is usually associated with disorders of the intestinal tract or kidneys. Drugs (e.g., aminoglycosides, amphotericin B, cyclosporine, diuretics, digitalis, cisplatin) or conditions that interfere with intestinal absorption or increase renal excretion of magnesium can cause hypomagnesemia. 

Let us consider in brief the nitrate biogeochemical provinces, where a significant growth of cancer incidents has been observed. It is known that the absorption of nitrate and nitrite from water in the intestinal tract is 2 times as much as from food products. This means that the accumulation of nitrate in drinking water sources should be of special concern during consideration of food webs in any relevant biogeochemical province. 

Theory and computational aspects of intestinal permeability have been reviewed in detail by Egan and Lauri [27], Briefly, a drug must be somewhat permeable through the membrane of the intestinal tract if it is to be administered orally and achieve systemic exposure. The rate of membrane permeability is strongly related to the lipophilicity and hydrophilicity of the molecule. Thus, models with a small number of descriptors related to those two properties can provide useful predictions of drug absorption. 

---