ex nutr c5-digestion1 gastrointestinal tract mouth esophagus esophagus sphincter stomach 3 parts:...
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Ex Nutr c5-digestion 1
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
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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
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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
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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
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Regulation of GI tract
Sympathetic and parasympathetic nerves Endocrine glands Paracrine glands/cells
Gastrin, secretin, cholecystokinin…
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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
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Carbohydrate digestion
Saliva contains -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 -amylase to intestine
Also sucrase, lactase, maltase Bacteria in large intestine ferment cellulose (fiber)
H2, CO2, volatile fatty acids (C2, C4), methane
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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
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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
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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
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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
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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
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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
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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
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Absorption of minerals
Minerals not very well absorbed in human intestine
Absorption rate, retention rate depend on intake, storage, and requirement
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Regulation of gastric emptying
Gastric motility and secretion mostly automatic
Stomach signals: usually positive feedback Nervous signals from stretching and extension of
stomach wall, relax pyloric sphincter Release of gastrin
Intestinal signals: usually negative feedback Duodenum contains receptors for pH, distension
of duodenum, osmolarity
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Factors for gastric emptying Fluid volume in stomach, exponential Not affected by exercise up to 80% VO2max
Short- or long-term high intensity exercise reduce gastric emptying
High osmolarity reduce gastric emptying Not important in 200-400 mOsm/L, typical for
most sport drinks High energy density reduce gastric emptying Temperature of food usually no effect Psychological stress reduce gastric emptying