Nutrient absorption carrier

The small intestine contains a wide variety of transporters (amino acid transporters, oligopeptide transporters, glucose transporters, lactic acid transporters etc.) on the apical membrane of the epithelial cells, which serve as carriers to facilitate nutrient absorption by the intestine. On the basolateral membrane, the presence of amino acid and oligopeptide transporters has been demonstrated. Active transport mechanisms for di- and tri-peptides have also been demonstrated in the nasal and buccal epithelia. 

The mucosa of the GIT represents an interface between the external and internal environments. The expansive surface area is necessary for the efficient hydrolysis of foodstuffs and the absorption of energy and nutrients. The mucosa also influences the systemic availability of non-nutrient compounds in the diet, both beneficial and detrimental. Digestion and absorption of glucosinolates are critical determinants of health benefits (see Chapter 4) Similarly, the bioavailability and health benefits of phytoestrogens, such as genistein (see Chapters 5 and 10) are at least partly dependent on the carrier-mediated processes of absorption associated with the GIT (Oitate et al, 2001). Moreover, the metabolic activities of the mucosa can influence the systemic concentrations and forms of dietary phytochemicals, as exemplified by research with soy isoflavones (Andlauer et al., 2000). 

Although a portion of the nutrients released from feedstuff s is absorbed by diffusing across the apical membrane of enterocytes or through the junctional complexes of adjacent enterocytes (paracellular absorption), the majority of nutrients are absorbed from the lumen of the GIT by carrier proteins that are inserted into the apical membrane of enterocytes and colonocytes. 

Historically, the absorption of lipid-soluble nutrients has been considered to be carrier-independent, with solutes diffusing into enterocytes down concentration gradients. This is true for some lipid-soluble components of plants (e.g. the hydroxytyrosol in olive oil Manna et al., 2000). However, transporters have been reported for several lipid-soluble nutrients. For example, absorption of cholesterol is partly dependent on a carrier-mediated process that is inhibited by tea polyphenols (Dawson and Rudel, 1999) and other phytochemicals (Park et al., 2002). A portion of the decreased absorption caused by tea polyphenols may be due to precipitation of the cholesterol associated with micelles (Ikeda et al., 1992). Alternatively, plant stanols and other phytochemicals may compete with cholesterol for transporter sites (Plat and Mensink, 2002). It is likely that transporters for other lipid-soluble nutrients are also affected by phytochemicals, although this has not been adequately investigated. 

Since many essential nutrients (e.g., monosaccharides, amino acids, and vitamins) are water-soluble, they have low oil/water partition coefficients, which would suggest poor absorption from the GIT. However, to ensure adequate uptake of these materials from food, the intestine has developed specialized absorption mechanisms that depend on membrane participation and require the compound to have a specific chemical structure. Since these processes are discussed in Chapter 4, we will not dwell on them here. This carrier transport mechanism is illustrated in Fig. 9C. Absorption by a specialized carrier mechanism (from the rat intestine) has been shown to exist for several agents used in cancer chemotherapy (5-fluorouracil and 5-bromouracil) [37,38], which may be considered false nutrients in that their chemical structures are very similar to essential nutrients for which the intestine has a specialized transport mechanism. It would be instructive to examine some studies concerned with riboflavin and ascorbic acid absorption in humans, as these illustrate how one may treat urine data to explore the mechanism of absorption. If a compound is... 

However, substances with physico-chemical characteristics that are not favorable for absorption could still reach the systemic circulation because specific mechanisms exist to enable, e.g., dietary fats and electrolytes to be absorbed. Occasionally a substance may be sufficiently similar to a nutrient substance to compete with that nutrient for a carrier mediated or active transport... 

Most food absorption takes place in the small intestine. The gastrointestinal tract possesses specialized carrier systems for certain nutrients such as carbohydrates, amino acids, calcium, and sodium. Some xenobiotics use these routes of passage through the cells, while others enter through passive diffusion. 

Facilitated diffusion is very similar to passive diffusion with the difference that transfer across membranes is assisted by the participation of carrier proteins embedded in the membrane bilayer. Again, the direction of passage will be from the side of the membrane with high concentration of a chemical to the side with low concentration this also occurs without energy expenditure by the cell. Such a process is somewhat specific in the sense that it applies to molecules that are able to bind to a carrier protein. Absorption of nutrients such as glucose and amino acids across the epithelial membrane of the gastrointestinal tract occurs by facilitated diffusion. Since a finite number of carriers are available for transport, the process is saturable at high concentrations of the transported molecules and competition for transport may occur between molecules of similar structure. 

Theoretically, a lipophilic drug may pass through the cell or go around it. If drug has a low molecular weight and is lipophilic, the lipid cell membrane is not a barrier to drug diffusion and absorption. In the intestine, molecules smaller than 500 MW may be absorbed by paracellular drug absorption. Numerous specialized carrier-mediated transport systems are present in the body especially in the intestine for the absorption of ions and nutrients required by the body. 

Dietary fat depresses gastric secretion, slows gastric emptying and stimulates biliary and pancreatic flow, thereby facilitating the digestive process. Often, the acceptance of food and its palatability depends upon flavor and aroma. Although TAG in the pure state are relatively tasteless, they absorb and retain flavor of food. Furthermore, in combination with other nutrients, lipids provide a texture that enhances palatability and contributes to a feeling of satiety. Finally, dietary lipids serve as carriers for fat-soluble vitamins A, D, E, and K, and as an aid to then-absorption in the intestine. 

These membranes differ by the presence of a specific carrier, to effect facilitated diffusion (also called mediated transport). As a result, transport is faster than can be calculated for simple diffusion, but at equilibrium no difference exists between the concentrations inside and outside the cell (contrast with Type 3 membranes). Type 2 membranes exist not only to hasten the absorption of a limited number of vital nutrients, but also to effect absorption of those metabolites which could not be expected to penetrate a Type 1 membrane because of ionization or an excessively hydrophilic structure. 

Absorption of nutrients occurs mainly in the small intestine. Carbohydrates are absorbed as monosaccharides by active transport, a process involving carrier proteins. Amino acids and fatty acids are also absorbed by active transport, but emulsified triglycerides are absorbed by passive diffusion. Large molecules, especially the immunoglobulins present in colostrum, are absorbed by a process known as pinocytosis. Many minerals and vitamins require special processes of absorption. 

Absorption of nutrients in the small intestine may happen by passive transport (or dilfusion) or active transport (involving a protein carrier). 

All chemical and mechanical phases of digestion are directed toward changing the ingested material (food or xenobiotics) into absorptive forms that can pass through mucosal cells into blood and lymphatic vessels. The process of absorption occurs via passive diffusion (para and transcellular) and carrier-mediated process (facilitated or active transport). About 90 % of absorption of nutrients occurs in the small intestine. 

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