Digestion and Absorption of Carbohydrate

V.S.RAVIKIRAN, MSc.

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

ASRAM Medical college, Eluru-534005.AP, India.vsravikiran2013@gmail.c

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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.

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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

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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

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• 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.

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Carbohydrate (CHO) Digestion

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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

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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)

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Carbohydrate Absorption

• Glucose, galactose enter capillaries down concentration gradient

• Fructose enters capillaries via passive carrier mediated transport

THE END

THANKS FOR YOUR ATTENTION