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CARBOHYDRATE DIGESTION AND
ABSORPTIONLourdes L. Balcueva, M.D.
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OBJECTIVES• 1. To be able to describe digestion of carbohydrates in the GIT 2. To discuss absorption of carbohydrates in the enterocytes 3. To describe carbohydrate transport from the enterocyte to the blood
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Digestion- the change of food from a complex to a simple form; from an insoluble to a soluble state in the GIT
Mechanical digestion- involves chewing of food into smaller particles to increase surface area incontact with digestive enzymes;propelled through GIT by peristalsis
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Chemical digestion-• - hydrolysis• - addition of water and digestive enzyme• - chemical breakdown of food particles
Metabolic digestion: - nutrients used at cellular level for basic life processes - liver plays a central role reassembling digested nutrients back into complex molecules
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CHO Digestion:
• Steps in digestion and absorption: 1. conversion of polymers to simpler, soluble forms 2. transported across intestinal walls 3. delivery to tissues
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CHO Digestion
1.Mouth – saliva contains lingual amylase; catalyzes a (14) glycoside bonds; inactivated by strong pH in the stomach 2. Stomach - acid hydrolysis – continues the degradation; chyme (mixture of food, saliva and gastric secretion) goes to small intestines
3. Small intestines – pancreatic amylase brush borders – disaccharidases ( sucrase, maltase and lactase)
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Polysaccharide Digestion• - Hydration of polysaccharides thru mastication is essential for action of amylase. - Enzyme amylase is specific for internal a 14 glycosidic linkages; inactive to a 16 linkages and to 14 linkages of branching units of glycosyl residues - cleaved units are trisaccharides (maltotriose, disaccharide maltose and oligosaccharide w/ one or more a 16 branches called a-limit dextrin
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Polysaccharide Digestion
in
- Sucrase-isomaltase complex-hydrolytic cleavage of a-16 linkage- Oligosaccharidases and glucosidases- act on other products of digestion of starch- Final product of digestion- glucose- Amylase – occurs free in the intestinal lumen-- a-glucosidase and isomaltase – attached to enterocyte mucosal membrane
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Disaccharidases
• -Disaccharidases are attached to small intestinal brush-border membrane• All disaccharidases are inducible EXCEPT LACTASE- Rate limiting factor in absorption of disaccharides
(except lactase) is the transport of the resulting monomeric sugar - Rate limiting factor of lactose absorption is the
hydrolysis of lactose itself
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Absorption of Carbohydrates• - Only monosaccharides are absorbed by the intestines - Absorption rate of galactose is more that of glucose; fructose is absorbed at lesser rate than glucose - Carbohydrates are polar- they cannot diffuse thru lipid bilayer of cell membrane
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Transport Mechanism-• SGluT- ( Sodium Dependent Glucose Transporter) absorption from lumen to intestinal cell is by co- transport mechanism (secondary active transport – indirect
because mech only get the driving force from the pre-established gradient,in this case, Na gradient)
- glucose and galactose are transported from lower concentration to higher concentration; - coupled to movement of Na from higher concentration to lower conc.; drives glucose molecules
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Glucose Transport
• - Sodium later expelled by sodium- potassium ATPase - Glucose Transporter Type 2 (GluT2)- not Na dependent; releases glucose into the blood; behaves as gated pore thru a process called ping-pong mechanism
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Absorption of Other Sugars
• Galactose – absorbed the same way as glucose Fructose – enters and leaves epithelial cells by facilitated diffusion utilizing GluT5
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Absorption of Glucose
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Absorption of Glucose
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Transporter Major Sitesof Expression Characteristics
SGLUT 1 Intestinal mucosa, kidney tubules
Cotransports one molecule of glucose or galactose along with two sodium ions. Does not transport fructose.
GLUT-1 Brain, erythrocyte, endothelial cells, fetal tissues
Transports glucose (high affinity) and galactose, not fructose. Expressed in many cells.
GLUT-2 Liver, pancreatic beta cell, small intestine, kidney.
Tranports glucose, galactose and fructose. A low affinity, high capacity glucose transporter; serves as a "glucose sensor" in pancreatic beta cells.
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Glucose Transporters
GLUT-3 Brain, placenta and testes
Transports glucose (high affinity) and galactose, not fructose. The primary glucose transporter for neurons.
GLUT-4 Skeletal and cardiac muscle, adipocytes
The insulin-responsive glucose transporter. High affinity for glucose.
GLUT-5 Small intestine, sperm
Transports fructose, but not glucose or galactose. Present also in brain, kidney, adipocytes and muscle.
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