digestion and absorption of carbohydrates for medical school
TRANSCRIPT
V.S.RAVIKIRAN, MSc., Department of Biochemistry,
ASRAM Medical college, Eluru-534005.AP, [email protected]
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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 )
starcholigosaccharides glucose
pancreatic amylaseBrush borderenzymes
Intestinal lumen Intestinalepithelialcells
blood
glucose glucose
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