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NME2.20: CARBOHYDRATE ABSORPTION
20/02/08
LEARNING OUTCOMES
Describe the functional microanatomy of the small intestine
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The mucosa of the small intestine has extensive villi and crypts lined by simple columnar epithelia
o Most villous epithelial cells are mucosal goblet cells or absorptive cells with microvilli
o Crypt epithelial cells include secretory endocrine cells and phagocytic paneth cells
Describe the digestion and absorption of dietary carbohydrates
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Carbohydrates are absorbed in the (primarily proximal) small intestine as monosaccharides
o β1-4 glycosidic links cannot be degraded by mammalian enzymes
o Undigested carbohydrates are usually broken down by colonic bacteria
o Cellulose (dietary fibre) is degraded by colonic bacteria into short-chain fatty acids
Ingested monosaccharides do not require enzymatic degradation and can be absorbed directly
Lactose is the primary disaccharide carbohydrate in the typical Western diet (200g/day)
o Degraded by lactase into its constituent monosaccharides glucose and galactose
o Other enzymes include maltase and sucrase (also handles maltose)
Starch is the primary polysaccharide carbohydrate in the typical Western diet (200g/day)
o Starch comes in two forms: amylose (unbranched) and amylopectin (branched, α1-6)
o Amylase is capable of hydrolysing α1-4 glycosidic bonds only
o Amylose and glycogen are degraded into oligo-, di- and eventually monosaccharides
o Amylopectin is degraded into ‘branched fragments’ called α-limit dextrins
α-limit dextrins are finally broken down into glucose by sucrase-isomaltase
Explain the role of secondary active and facilitative transport processes in absorption of carbohydrates
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SGLT1 sodium-coupled transporters along the apical membrane of small intestinal epithelia extract
glucose and galactose
GLUT5 directly transports fructose from the intestinal lumen into absorptive enterocytes
GLUT2 then transports all of these substances across the basolateral membrane into the blood
Absorption of glucose is coupled with sodium absorption which drives water absorption
o Glucose solutions are used in oral rehydration therapy to promote GI water re-absorption
Outline how gastrointestinal dysfunctions may lead to mal-absorption
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Limited amounts of certain indigestible carbohydrates normally reach the colon
Mal-absorption of carbohydrates results in their increased colonic bacterial digestion which releases
hydrogen gas
o Increased breath H2 levels are a good clinical indicator of carbohydrate mal-absorption
Lactase deficiency results in an inability to digest lactose causing a lactose intolerance
o Symptoms include osmotic diarrhoea, cramps, flatulence
Glucose-galactose mal-absorption occurs with defective SGLT1 transporters with similar symptoms
Sucrase-isomaltase deficiency prevents α-limit dextrins from being absorbed with similar symptoms