Ex Nutr c5-digestion1 Gastrointestinal tract Mouth Esophagus Esophagus sphincter Stomach 3 parts: corpus (body), antrum, fundus Pyloric sphincter (pylorus)

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








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






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



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






Regulation of GI tract

Sympathetic and parasympathetic nerves Endocrine glands Paracrine glands/cells

Gastrin, secretin, cholecystokinin…


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





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



Digestion of carbohydrate


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




Digestion of fat


Various lipoproteins


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


Digestion of protein




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 38Absorption of CHO



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





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




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




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




Absorption of minerals

Minerals not very well absorbed in human intestine

Absorption rate, retention rate depend on intake, storage, and requirement




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


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





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Ex Nutr c5-digestion1 Gastrointestinal tract Mouth Esophagus Esophagus sphincter Stomach 3 parts: corpus (body), antrum, fundus Pyloric sphincter (pylorus)