CARBOHYDRATE DIGESTION AND

ABSORPTIONLourdes L. Balcueva, M.D.

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

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

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

CHO Digestion:

• Steps in digestion and absorption: 1. conversion of polymers to simpler, soluble forms 2. transported across intestinal walls 3. delivery to tissues

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)

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

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

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

Brush Border

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

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

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

Absorption of Other Sugars

• Galactose – absorbed the same way as glucose Fructose – enters and leaves epithelial cells by facilitated diffusion utilizing GluT5

Absorption of Glucose

Absorption of Glucose

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.

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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