08 Carbohdrates - Digestion_Absorption_Transport_Distribution

08 Carbohdrates - Digestion_Absorption_Transport_Distribution

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Carbohydrates: Digestion, Absorption, Transport and Distribution HNFE 3025 Fall 2009
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Dietary Carbohydrates ~250 g (~1,000 kcal) in typical US diet Consists of Disaccharides (sucrose & lactose) Oligosaccharides (as in corn syrup) Polysaccharides (60-70%) – Digestible (starch) – Indigestible (dietary fiber such as cellulose). Most of our dietary CHO is starch. Average American diet ~50 g/day of ‘added sugars’ in soda, ice cream, candy and fruit- flavored drinks. Dietary fiber is ~15 g/d some broken down by intestinal bacteria producing short chain fatty acids, others resistant to breakdown. 2
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Carbohydrate Digestion Polysaccharide digestion begins in the mouth Salivary α -amylase A glycosidase that hydrolyses α 1-4 glycosidic bonds – β1-4 of cellulose, β1-4 of lactose, and α 1-6 of branch points are resistant to this enzyme Low pH of gastric juice in the stomach inhibits activity This stage produces few monosaccharides – produces dextrins (short chain polysaccharides) and maltoses Completed in the small intestine Pacreatic α -amylase Resumption of digestion of dextrins in duodenum Bicarbonate increases pH to favor enzyme activity Brush border enzymes 3
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4 Fig. 3-7, p. 70
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Starch Digestion α-amylases break α1-4 bonds in the interior of amylose molecules to produce oligosaccharides then to maltose and maltotriose Salivary α-amylase brief effect Pancreatic α-amylase works in the small intestine From amylopectins also get maltose, maltotriose and limit dextrins 5 α1-4 glycosidic bonds α1,-6 glycosidic bond
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Digestion of Disaccharides Four brush border enzymes Maltase and sucrase: cleave α 1-4 bonds of maltose and maltotriose to glucose Isomaltase ( α -dextrinase): cleaves the α 1-6 bond of limit dextrins Sucrase: cleaves sucrose to produce fructose and glucose Lactase: cleaves the β 1-4 bond between galactose and glucose in lactose Activity is high in infants and decreases after weaning, which may lead to lactose intolerance Lactase deficiency is very common – Caucasian Americans – 20% – Blacks and American Indians – 70-80% – Asians – 100% 6
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Macronutrient Absorption The wall of the small intestine is composed of absorptive musocal and muscus-secreting goblet cells Line the villi and project into lumen Have hair-like projections call microvilli (brush
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08 Carbohdrates - Digestion_Absorption_Transport_Distribution

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