Digestion and Absorption of Carbohydrate Lecture for 1st yr MBBS by Dr Waseem

By,Dr Mohammad waseem kausar

Assistant professor,Biochemistry

IMDC

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Digestion and Absorption of carbohydrates

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Carbohydrates

• Sugars, Starches, Fibers

• Major food sources: plants

– Formed during photosynthesis

Photo © PhotoDisc

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Objectives

• Provide an overview of the structures, mechanisms, and processes of the GI tract.

• Understand how food is digested and absorbed within the body.

• Explain how foods are transported after they are digested and absorbed.

• Describe how the body keeps the complex processes of digestion, absorption, and nutrient transport running smoothly.

• Explain various influences on digestion and absorption along with various nutrition and GI disorders.

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The Gastrointestinal Tract

• Organization– Mouth anus– Accessory organs

• Salivary glands, liver, pancreas, gallbladder

• Functions– Ingestion– Transport– Secretion– Digestion– Absorption– Elimination

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Overview of Digestion

• Physical movement– Peristalsis– Segmentation

• Chemical breakdown– Enzymes– Other secretions

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Overview of Absorption

Types of absorptive

processes

• Passive Diffusion

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Facilitated Diffusion

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ActiveTransport

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Diet

Liquid , solid and semisolidPolysaccharides = starch and glycogen Disaccharides = sucrose, lactose and

maltoseMonosaccharides = fructose and pentoses.

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Digestion in Mouth Saliva and Enzymes

• Salivary amylase(ptyline) acts on starch• Moistens food for swallowing• α- amylase requires . Optimum pH and Cl- ions

• 6.6 -6.8

• It hydrolysis α-1→4 linkages at random deep inside of p/s molecule like starch, glycogen, and dextrin, producing smaller molecules maltose, glucose and trisaccharide.

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Digestion in the stomach

No action on meal no enzymes

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The Pyloric ValveThe Pyloric Valve

Small Intestine Stomach

Small intestine (Duodenum)

Pancreatic juicePancreatic amylase completes starch digestion Requirs α- amylase requires . Optimum pH(7.1)

and Cl- ionsEnd products of carbohydrate digestion

Glucose, fructose, galactose Absorbed into bloodstream

Fibers are not digested, excreted in feces

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Small IntestinePancreatic enzymes-amylase

G G GG G

GG G G

G G GGGG G

amylose

amylopectin

G G G G G

amylase

+

GG G

G G

maltotriose maltose

Limit dextrins

G

Small intestine Brush border enzymes digest disaccharides

Intestinal amylase hydrolyzes terminal α-1→4 linkages liberating free glucose.

Lactase (β-galactosidase) = pH 5.4 – 6.0

Isomaltase = hydrolysis α 1 – 6 = linkages splitting α - limit dextrin producing maltose and glucose

Maltase =Sucrase =

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Oligosaccharide digestion..cont

GG G

G GG

G

GG GG

G

Glucoamylase (maltase) or

-dextrinase

G GG

GG

-dextrinase

G GG

G

G G

Gmaltase

sucrase

Limit dextrins G

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AbsorptionSmall intestine

Absorption• Folds, villi, microvilli expand absorptive surface• Most nutrients absorbed here• Fat-soluble nutrients go into lymph• Other nutrients into blood

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Transit TimesTransit Times

*Assuming 24 hour Transit Time

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Signaling Systems: Command, Control, Defense

• Nervous system– Regulates GI activity

• Local system of nerves• Central nervous system

• Hormonal system– Increases or decreases GI activity

• Immune system– Identifies and attacks foreign invaders

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Nutrition and GI Disorders

• Constipation– Hard, dry, infrequent stools– Reduced by high fiber, fluid intake,

exercise

• Diarrhea– Loose, watery, frequent stools– Symptom of diseases/infections– Can cause dehydration

• Diverticulosis– Pouches along colon– High fiber diet reduces formation

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Nutrition and GI Disorders

• Gastroesophageal Reflux Disease (GERD)– Reduced by smaller meals, less fat

• Irritable Bowel Syndrome (IBS)• Colon cancer

– Antioxidants may reduce risk

• Gas• Ulcers

– Bacterial cause; helicobacter pylori

• Functional dyspepsia

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Bacteria in the mouth ferment sugars and in the process produce an acid that dissolves tooth enamel. Sticky foods stay on the teeth longer and keep yielding acid longer than unsticky foods.

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Lactose Intolerance

• Lactose in milk and other dairy foods

• Lactase enzyme

• Symptoms

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• Sucrase deficiency

• malabsorption

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Carbohydrates in Your Diet

• Recommended carbohydrate intake– 55-60% of kilocalories– Daily Value (for 2,000 kcal) = 300 grams– Dietary Guidelines

• Moderate sugar intake• Variety of grains, fruits, vegetables

– Fiber intake of 25 grams/day

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Carbohydrates in Your Diet

• Increasing complex carbohydrate intake– Grains,

especially whole grains

– Legumes– Vegetables

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Carbohydrates in Your Diet

• Reducing sugar intake– Use less added sugar– Limit soft drinks, sugary cereals,

candy– Choose fresh fruits or those

cannedin water or juice

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