CARBOHYDRATES

CARBOHYDRATES

CARBOHYDRATES Dr. Madushani Silva(MBBS)North Colombo Teaching Hospital Ragama

CarbohydratesCarbohydrates are called carbohydrates because they are essentially hydrates of carbon (i.e. they are composed of carbon and water and have a composition of (CH2O)n.

The major nutritional role of carbohydrates is to provide energydigestible carbohydrates provide 4 kilocalories per gram. No single carbohydrate is essentialPhotosynthesis: Suns energy becomes part of glucose moleculeenergy

Carbon dioxideWaterChlorophyllGLUCOSE6 CO2 + 6 H20 + energy (sun)C6H12O6 + 6 O2

120 grams of glucose / day = 480 calories6Types of Carbohydrates Monosacchrides

DisaccharidesContain 2 monosacchride unitsPolysacchrides Contain many monosacchride units

Simple Sugars -

7Glucose basic unit of polysaccharidesGalactose found as part of lactose in milkFructose found in fruits, vegetables and honey (consumption on the rise.High fructose corn syrup)8Monosacchrides Three Carbons = Triose

Four Carbons = Tetrose

Five Carbons = Pentose

Six Carbons = Hexose9Glucose

13Fructose

14Galactose D-galactose

Disaccharides

LACTOSE INTOLERANCE: Many healthy people, adults and children have the inability to digest lactose.It maybe inherited or acquired and results from a deficiency of the enzyme LACTASE.Lactase is necessary to breakdown lactose to galactose and glucose.

LactuloseHeated milk (small amounts), mainly syntheticNot hydrolyzed by intestinal enzymes, but fermented by intestinal bacteria; used as a mild osmotic laxative

Complex carbohydratesOligosaccharides

Polysaccharides

PolysaccharidesHomopolysaccrarides Heteropolysaccarides Pure Congugated

HomopolysaccraridesStarchGlycogenDietary fiberHeteropolysaccaridesPure Pectin Gums GlycosaminoglycansConjugated Protein Proteoglycans (Carbo. constitute a major portion ) Glycoproteins (Proteins constitute a major portion) Lipids Glycolipids

StarchMajor storage carbohydrate in higher plants

StarchAmylose long straight glucose chains (a1-4)Amylopectin branched every 24-30 glc residues (a 1-6)Provides 80% of dietary calories in humans worldwide

GlycogenGGGGGGGGa 1-4 linkGGGGGa 1-6 linkGGGGGGMajor storage carbohydrate in animalsalso known as animal starch

stored in muscle and liver

complete hydrolysis yields glucoseGGGGGGGGa 1-4 linkGGGGGa 1-6 linkGGGGGGLong straight glucose chains (a1-4)Branched every 4-8 glc residues (a 1-6)More branched than starchLess osmotic pressureEasily mobilized Linear polymers of repeating disaccharidesComplex carbohydrates containing amino sugars and uronic acids.Glycosaminoglycans tend to be negatively charged, because of the prevalence of acidic groups.

Glycosaminoglycans (mucopolysaccharides)Hyaluronic acid (Glass): lubricants in synovial fluid , eye, cartilage and tendons; hyaluronidase secreted by bacteria bacteria invasion. Similar enzyme for sperm to penetrate ovum.Chondroitin sulfate (Cartilage): tensile strength pf cartilage, tendons and ligament, aorta.Dermatan sulfate (Skin): skin, blood vessel and heart valves. Pliability of skin.

GlycosaminoglycansGlycosaminoglycansKeratan sulfates (horn): cornea, horn, hair, hoof, nails, claws, no uronic acid.Heparin (liver): made in mast cell- a anticoagulant with highest negative charge density, release to blood, inhibit blood clotting by binding to antithrombin III - bind to and inhibit thrombin, a protease essential to blood clotting. ProteoglycanThey may be attached to a protein molecule to form a proteoglycan.

glycosaminoglycans that are covalently linked to serine residues of specific core proteins.The glycosaminoglycan chain is synthesized by sequential addition of sugar residues to the core protein. Proteoglycan

Dietary fibers

FibersFound in food derived from plantsIncludes polysaccharides such as cellulose, hemicellulose, pectins, gums and mucilagesAlso includes non-polysaccharides such as lignin, cutins and tannins

Fibers are not a source of energy because Human digestive enzymes cannot break down fibers

The bacteria in human GI tract can breakdown some fibers. FibersSoluble fibers

Insoluble fibersclassification of fibers based on their solubilities in water40classification of fibers based on their solubilities in water(1)Soluble fibers: includes gum,pectin, some hemicellulose and mucilages found in fruits, oats, barley and legumes .

Soluble fibersActions on body:(i) Delay GI transit(benefits digestive disorders)(ii) Delay glucose absorption (benefits diabetes)(iii) Lowers blood cholesterol(benefits heart disease)

classification of fibers based on their solubilities in water(2) Insoluble fibers: includes cellulose, many hemicellulose, lignin found in wheat bran , corn bran, whole grain bread, cereals and vegetables (carrot, cabbage)

Insoluble fibersActions in body: (i)Accelerates GI transit and increases fecal weight(promotes bowel movement)(ii) Slows starch hydrolysis and delays glucose absorption(Benefits diabetes)

2005 Dietary Guidelines for Americans Recommends

Consume 3 or more servings (or oz. equivalents) of whole-grain products per day, with the rest of the recommended grains coming from enriched or whole-grain products. In general, at least half the grains should come from whole grains.

46ExerciseComment on the importance of dietary fibers in the example shown by the picture below

Dietary carbohydrates

Starch Sucrose Glucose and fructose Lactose Cellulose Other plant polysaccharidesDigestible

Non-digestibleby humans

49Only monosaccharides are absorbed into the bloodstreamfrom the gut.

Digestion of carbohydrates involves their hydrolysis into monosaccharidesDigestive EnzymesEnzymes for carbohydrate digestionEnzymeSourceSubstrateProducts

-AmylaseSalivary glandStarch, glycogenOligosaccharidesPancreasDextrinaseSmall intestineOligosaccharidesGlucoseIsomaltaseSmall intestine-1,6-glucosidesGlucoseMaltaseSmall intestineMaltoseGlucoseLactaseSmall intestineLactoseGalactose, glucoseSucraseSmall intestineSucroseFructose, glucoseLactase deficiency produces lactose intolerance51Absorption of monosaccharides by intestinal mucosal cellsMajor monosaccharidesGlucose, galactose, fructose

Entry into mucosal cells from intestinal lumenActive transport of glucose and galactose with a concurrentuptake of Na+ ions

52Facilitated transport of fructose via transporter proteinGLUT-5

Entry into the portal circulation from mucosal cellsFacilitated transport via transporter protein GLUT-2

Blood glucose concentrationsMeasured in mmol/L = mM or in mg/dL

Conversion factor: 1 mM = 18 mg/dL

Normal plasma glucose concentrations roughly3.9 8.3 mM

Hypoglycemia: < 2.2 mM Diabetes: > 7.0 mM (fasting) 54

glycaemic indexTheglycaemic index (GI) provides a measure of how quicklyblood sugarlevels (i.e., levels ofglucosein the blood) rise after eating a particular type of food. Dietary Carbohydrates and Blood GlucoseSome carbohydrate-containing foods produce a rapid rise followed by a steep fall in blood glucose concentration, whereas others result in a gradual rise followed by a slow decline. Food with a low glycemic index tends to create a sense of satiety over a longer period of time, and may be helpful in limiting caloric intake.

Dietary Carbohydrates and Blood Glucose

Glycemic index

PotatoInstant oatmealWhite breadWatermelon

Basmati riceStoneground whole wheat breadRaisinsPineapple

Kidney beansChocolate ice creamOatmeal made with steel-cut oatsSpaghetti, al dente

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The glycaemic index is a ranking of foods from 0 to 100 based on their effect on blood glucose levelsHigh GI eg: White breadLow GI eg: legumesTimeBlood glucose level

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Blood glucose concentrationsMeasured in mmol/L = mM or in mg/dLConversion factor: 1 mM = 18 mg/dLNormal plasma glucose concentrations roughly3.9 8.3 mMHypoglycemia: < 2.2 mM Diabetes: > 7.0 mM (fasting) > 11.1 mM 2 h after ingestion of 75 g glucose68Requirements for carbohydrate

Carbohydrates are not essential nutrients, because the carbon skeletons of amino acids can be converted into glucose .

However, the absence of dietary carbohydrate leads to ketone body production ,and degradation of body protein whose constituent amino acids provide carbon skeletons for gluconeogenesis .

Questions?Thank you

cytosol

heparan sulfate glycosaminoglycan

core protein

transmembrane -helix