![]() In rats receiving TPN, for example, infusion of D-glucose into the small intestine increases specific transport capacity for glucose, while substituting protein isocalorically for carbohydrate in the diet increases amino acid transport capacity and reduces the transport capacity for galactose. In addition, adaptive changes in small intestinal mucosal mass are generally associated with parallel changes in segmental absorptive function, but the magnitude of induction of individual transport processes can be selectively affected by the specific nature of the nutrients within the lumen. Luminal nutrient also stimulates intestinal mucosal growth indirectly by releasing gut hormones from the distal small intestine, colon and pancreas. Although the exact mechanism underlying this process remains unclear, it is likely that the workload of absorption determines the gut mucosal growth response. ![]() There is an increasing body of evidence showing that luminal nutrients stimulate gut mucosal growth by local direct effect at their site of absorption, and this direct action does not result from the use of the nutrients as sources of energy by mucosal cells, since nonmetabolizable absorbed substrates, such as galactose and 3- O-methyl- D-glucose, also promote mucosal cell proliferation. This inhibition in the GI mucosal growth occurs even when the overall nutritional state of the animals are maintained by total parenteral nutrition (TPN). During starvation, the small intestinal mucosa atrophies rapidly, with a reduction in cell proliferation being noted within hours of food withdrawal. As befitting a role requiring a high degree of tact and dexterity, the GI epithelium incorporates an array of strategies to facilitate peaceful communication between luminal contents, including nutrients and microbes and the mucosal renewal system, thereby preserving its tissue homeostasis. This interface acts as a peace keeping force positioned between two opposing armies, and it must placate the local environment to prevent conflict from erupting. The GI epithelium is critically located at the interface between the body and environment.
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