Digestion and absorption of carbohydrates for Medical School

Digestion and Absorption of Carbohydrate

V.S.RAVIKIRAN, MSc.

V.S.RAVIKIRAN, MSc., Department of Biochemistry,

ASRAM Medical college, Eluru-534005.AP, [email protected]

om

Digestion and Absorption of

Carbohydrate

Digestion and Absorption of Carbohydrate

- Most digestible dietary carbohydrate is starch.

- The starch digestion begins in the mouth by salivary amylase.

- But fully digestion of starch occurs in the small intestines.

5

Carbohydrate Digestion and Absorption

• Mouth– Salivary amylase begins digestion of starch

• Small intestine– Pancreatic amylase completes starch digestion– Brush border enzymes digest disaccharides

• End products of carbohydrate digestion– Glucose, fructose, galactose– Absorbed into bloodstream

• Fibers are not digested, excreted in feces

6

Simple Sugars:Mono and Disaccharides

• Monosaccharides – single sugar unit– Glucose

• Found in fruits, vegetables, honey• “blood sugar” – used for energy

– Fructose• “fruit sugar”• Found in fruits, honey, corn syrup

– Galactose• Found as part of lactose in milk

7

8

• Disaccharides – two linked sugar units– Sucrose: glucose + fructose

• “table sugar”

• Made from sugar cane and sugar beets

– Lactose: glucose + galactose

• “milk sugar”

• Found in milk and dairy products

– Maltose: glucose + glucose

• Found in germinating cereal grains

• Product of starch breakdown

NUTRITION and GI DISORDERS

LACTOSE INTOLERANCECAUSES:• Inability to digest lactose, caused by a shortage of lactase• Symptoms: nausea, cramps, bloating, gas, and diarrhea

• TREATMENT• Don’t eat dairy products! Or eat less, and more processed types (ex.

Yogurt vs. milk )

- Starch is digested to oligosaccharides (3-8 glucose residues), disaccharide maltose, and glucose.

SF Biology II 11

Carbohydrate (CHO) Digestion

SF Biology II 12

starcholigosaccharides glucose

pancreatic amylaseBrush borderenzymes

Intestinal lumen Intestinalepithelialcells

blood

glucose glucose

Glucose is absorbed by: - sodium-dependent glucose transporter (SGLT).

- solvent drag

Carbohydrate digestion and absorption

In lumen• Starch maltose + maltriose + α-

limit dextrins– Amylase

At brush-border• Maltose/maltriose glucose

– Glucoamylase (maltase)– Sucrase-isomaltase

• α-limit dextrins glucose– Sucrase-isomaltase

• Sucrose glucose + fructose– Sucrase-isomaltase

• Lactose glucose + galactose– Lactase

SGLT-1• Na+ coupled(Na/K ATPase for gradient)• D-hexoses w/ pyranose ring

apical basolateral

GLUT-5• Fructose absorption• Jejunum• Facilitated diffusion

GLUT-2

Transporter Present in PropertiesGlu T1 RBC, brain, kidney, colon,

retina, placentaGlucose uptake in most of cells

Glu T2 Serosal surface of intestinal cells, liver, beta cell pancreas

Low affinity; glucose uptake in liver ; glucose sensor in beta cells

Glu T3 Neurons, brain High affinity; glucose into brain cells

Glu T4 Skeletal, heart, muscle, adipose tissue

Insulin mediated glucose uptake

Glu T5 Small intestine, testis, sperms, kidney

Fructose transporter; poor ability to transport glucose

Glu T7 Liver endoplasmic reticulum Glucose from ER to cytosol

SF Biology II 18

CHO Digestion — Summary

• Polysaccharides to disaccharides (gut amylases)

• Disaccharides to monosaccharides (brush border)

– Glucose, galactose enter cells by

energy-dependent secondary active transport

• i.e. ‘piggy-back on Na+ gradient (active)

SF Biology II 19

Carbohydrate Absorption

• Glucose, galactose enter capillaries down concentration gradient

• Fructose enters capillaries via passive carrier mediated transport

THE END

THANKS FOR YOUR ATTENTION