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Gastrointestinal tract. Mouth Esophagus Esophagus sphincter Stomach 3 parts: corpus (body), antrum, fundus Pyloric sphincter (pylorus) Storage, mixing, regulation of emptying Normally ~1.5L, up to 6L. Small intestine. Duodenum, jejunum, ileum Intestinal mucosa contains folds of Kerkring
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Ex Nutr c5-digestion Gastrointestinal tract • Mouth • Esophagus • Esophagus sphincter • Stomach • 3 parts: corpus (body), antrum, fundus • Pyloric sphincter (pylorus) • Storage, mixing, regulation of emptying • Normally ~1.5L, up to 6L
Ex Nutr c5-digestion Small intestine • Duodenum, jejunum, ileum • Intestinal mucosa contains folds of Kerkring • Villi • Brush border consisting of ~600 microvilli • Increase surface area ~600 folds • Villi consists of a layer of epithelial cells • Water, water-soluble particles, electrolytes diffuse/transport into blood vessels hepatic portal vein liver • Water-insoluble particles lymphatic vessels large veins
Ex Nutr c5-digestion Mobility and transit time • Food spend 1-3 days in GI tract: transit time • Transit time in small intestine: 3-10 hr • Mobility: movement of food of GI tract • Small intestine contains 2 layers of smooth muscle: longitudinal and circular • peristalsis
Ex Nutr c5-digestion Gastrointestinal tract • Gall bladder: store, concentrate, release bile • Bile: produced by liver, contain electrolytes, bile salts, cholesterol, lecithin, bilirubin • Store 30-60 ml, release up to 1200 ml per day • Pancreas • Secrete digestive enzymes, HCO3- • Ileocecal valve • Large intestine • Ascending colon, transverse colon, descending colon, rectum, anal canal • Absorption of water and electrolytes, storage of feces
Ex Nutr c5-digestion Regulation of GI tract • Sympathetic and parasympathetic nerves • Endocrine glands • Paracrine glands/cells • Gastrin, secretin, cholecystokinin…
Ex Nutr c5-digestion Daily secretion of intestinal juices • Saliva: 1000 ml • Gastric secretions: 1500 ml • Pancreatic secretion: 1000 ml • Bile: 1000 ml • Small intestine: 2000 ml • Large intestine: 200 ml • Total: 6700 ml • Mostly reabsorbed
Ex Nutr c5-digestion Carbohydrate digestion • Saliva contains a-amylase • Also contains lysozymes and IgA, protect from invading bacteria • CHO digestion slow down in stomach • Inactivation of amylase by low pH • 30-40% CHO digested, predominantly to oligosaccharides • Pancreas secrete a-amylase to intestine • Also sucrase, lactase, maltase • Bacteria in large intestine ferment cellulose (fiber) • H2, CO2, volatile fatty acids (C2, C4), methane
Ex Nutr c5-digestion Digestion of carbohydrate
Ex Nutr c5-digestion Lipid digestion • Lingual lipase, slow, 10-30% TG digestion • Pancreatic lipase in intestine • Combine with bile 膽鹽, form micelles (disklike) 乳化 (emulsification) • Absorption through villi • Bile salts emulsify lipids into small droplets, make hydrolysis by pancreatic lipase easier • Xenical 羅氏纖
Ex Nutr c5-digestion Digestion of fat
Ex Nutr c5-digestion Various lipoproteins
Ex Nutr c5-digestion Protein digestion • Stomach: pH ~2.0 • Activate pepsin (a protease) by HCl • Denature food proteins • Intestine • Pancreatic proteases secreted as inactive precursors: trypsinogen, endopeptidases, carboxypeptidases, enteropeptidases • Activated by trypsin 胰蛋白脢 • Digest polypeptide into tripeptides, dipeptides, single amino acids
Ex Nutr c5-digestion Digestion of protein
Ex Nutr c5-digestion Absorption of carbohydrates • Monosaccharides absorbed by carrier-mediated transport in epithelial cell • Sodium cotransporter, combine with Na-K-ATPase pump • Sodium-independent facilitated-diffusion transporter with specificity for fructose (GLUT 5) • GLUT 2 in contraluminal side
Ex Nutr c5-digestion Absorption of CHO
Ex Nutr c5-digestion Absorption of fatty acids • Monoacylglycerol, FA incorporated into micelles, transported to villi • Diffusion through epithelial membrane • In epithelial cell, FA re-esterified into TG, form chylomicrons 乳糜微粒, transported through lymphatic system • Short- and medium-chain FA diffuse through contraluminal membrane into portal vein • bile salts reabsorbed
Ex Nutr c5-digestion Absorption of amino acids • AA, dipeptides, tripeptides absorbed by sodium-dependent active transport • At least 7 different brush border-specific transport proteins for different AA • Capacity of small intestine to absorb AA is very large, <1% ingested protein in feces
Ex Nutr c5-digestion Absorption of water • 99% water absorption in small intestine, by simple diffusion • Water flow toward compartment with lower water concentration (higher solute concentration) • Osmole: number of solute particles • mOsm/kg (osmolality), mOsm/L (osmolarity) • ~290 mOsm/L for most body fluids • If ingested high osmolarity, water move into gut lumen
Ex Nutr c5-digestion Absorption of vitamins • Mostly in small intestine via diffusion • Digestion free vitamins from coenzymes • Fat-soluble vitamins absorbed along with FA • Incorporated into chylomicrons • Transported through lymphatic system • Water-soluble vitamins • Not retained to any great extent in body • Additional vit K, B12, thiamine, riboflavin can be formed by bacteria in colon
