1 Carbohydrate Digestion and Metabolism

2 Carbohydrates Carbohydrates are composed of carbon and water and have a composition of (CH2O)n. The major nutritional role of carbohydrates is to provide energy and digestible carbohydrates provide 4 kilocalories per gram. energy Carbon dioxide Water Chlorophyll GLUCOSE 6 CO2 + 6 H20 + energy (sun) C6H12O6 + 6 O2

4 Simple Sugars - Glucose – basic unit of polysaccharidesGalactose – found as part of lactose in milk Fructose – found in fruits, vegetables and honey (consumption on the rise….High fructose corn syrup)

5 Disaccharides

6 Complex carbohydratesOligosaccharides Polysaccharides Starch Glycogen Dietary fiber (Dr. Firkins)

7 Starch Major storage carbohydrate in higher plantsAmylose – long straight glucose chains (α1-4) Amylopectin – branched every glc residues (α 1-6) Provides 80% of dietary calories in humans worldwide

8 Glycogen Major storage carbohydrate in animalsa 1-4 link G a 1-6 link Major storage carbohydrate in animals Long straight glucose chains (α 1-4) Branched every 4-8 glc residues (α 1-6) More branched than starch Less osmotic pressure Easily mobilized

9 Digestion Pre-stomach – Salivary amylase : a 1-4 endoglycosidasea 1-4 link a 1-6 link maltose isomaltose amylase maltotriose a Limit dextrins Key- We must breakdown these very large oligosaccharides into monosaccharides in order to absorb them. Alpha amylase. – Cannot attack a1-4 linkase close to 1-6 branch points.

10 Small Intestine + Pancreatic enzymes a-amylase a amylase maltotriosemaltose + G G G G G G G G G G a amylase amylose G G G G G G G G G G G G G G G G G amylopectin a Limit dextrins

11 Oligosaccharide digestion..conta Limit dextrins G G G G sucrase G G G G G maltase G G Glucoamylase (maltase) or a-dextrinase G G G Maltase – specifically removes a single glucose from the nonreducing end of a linear a1-4 glucose chain…breaking down maltose into glucose. (exosaccharidases) Alpha dextinase – cleaves 1,6-alpha glucosidic linkages a-dextrinase G G G G G G G G G G G

12 Small intestine Portal for transport of virtually all nutrientsWater and electrolyte balance Enzymes associated with intestinal surface membranes Sucrase a dextrinase Glucoamylase (maltase) Lactase peptidases

13 Carbohydrate absorptionHexose transporter Monosaccharides are still too large for passive diffusion across brush border membrane. We use facilitated diffusion to absorb these molecules. Glucose and galactose use a sodium-glucose symport (SGLUT1) while fructose uses the glut5 We must transport Na out of the cell to maintain proper electrochemical gradient (sodium potassium pump) Water will also follow sodium into enterocyte. This is critical to maintain proper water balance. apical basolateral

14 Digestion of CarbohydratesMonosaccharides Do not need hydrolysis before absorption Very little (if any) in most feeds Di- and poly-saccharides Relatively large molecules Must be hydrolyzed prior to absorption Hydrolyzed to monosaccharides Only monosaccharides can be absorbed

15 Non-Ruminant Carbohydrate DigestionMouth Salivary amylase Breaks starches down to maltose Plays only a small role in breakdown because of the short time food is in the mouth Ruminants do not have this enzyme Not all monogastrics secrete it in saliva

16 Carbohydrate DigestionPancreas Pancreatic amylase Hydrolyzes alpha 1-4 linkages Produces monosaccharides, disaccharides, and polysaccharides Major importance in hydrolyzing starch and glycogen to maltose Amylase Polysaccharides Disaccharides

17 Digestion in Small IntestineDigestion mediated by enzymes synthesized by cells lining the small intestine (brush border) Brush Border Enzymes Disaccharides Monosaccharides * Exception is β-1,4 bonds in cellulose

18 Digestion in Small IntestineSucrase Sucrose Glucose + Fructose * Ruminants do not have sucrase Maltase Maltose Glucose + Glucose Lactase Lactose Glucose + Galactose * Poultry do not have lactase

19 Overview Monogastric Carbohydrate DigestionLocation Enzymes Form of Dietary CHO Mouth Salivary Amylase Starch Maltose Sucrose Lactose Stomach (amylase from saliva) Dextrin→Maltose Small Intestine Pancreatic Amylase Maltose Brush Border Enzymes Glucose Fructose Galactose Glucose Glucose Glucose Large Intestine None Bacterial Microflora Ferment Cellulose

20 Carbohydrate Absorption in MonogastricsWith exception of newborn animal (first 24 hours), no di-, tri-, or polysaccharides are absorbed Monosaccharides absorbed primarily in duodenum and jejunum Little absorption in stomach and large intestine

21 Carbohydrates MonosaccharidesSmall Intestine Carbohydrates Monosaccharides Portal Vein Active Transport Liver Distributed to tissue through circulation

22 Nutrient Absorption - CarbohydrateActive transport for glucose and galactose Sodium-glucose transporter 1 (SGLT1) Dependent on Na/K ATPase pump Facilitated transport for fructose

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24 Summary of Carbohydrate in MonogastricsPolysaccharides broken down to monosaccharides Monosaccharides taken up by active transport or facilitated diffusion and carried to liver Glucose is transported to cells requiring energy Insulin influences rate of cellular uptake

25 Lactate transported back to liver for glucose production “Cori Cycle”Lactate transported back to liver for glucose production “Cori Cycle”. Costs energy

26 Carbohydrates Metabolism in MonogastricsServe as primary source of energy in the cell Central to all metabolic processes Glucose Cytosol - anaerobic Hexokinase Pentose Phosphate Shunt Glucose-6-P Glc-1- phosphate glycolysis glycogen Pyruvate

27 cytosol Pyruvate mitochondria (aerobic) Aceytl CoA FATTY ACIDS Krebs cycle Reducing equivalents AMINO ACIDS Oxidative Phosphorylation (ATP)

28 Control of enzyme activityRate limiting step

29 Glucose utilization

30 Stage 1 – postparandial All tissues utilize glucose Stage 2 – postabsorptive KEY – Maintain blood glucose Glycogenolysis Glucogneogenesis Lactate Pyruvate Glycerol AA Propionate Spare glucose by metabolizing fat Stage 3- Early starvation Gluconeogenesis Stave 4 – Intermediate starvation gluconeogenesis Ketone bodies Stage 5 – Starvation

31 Carbohydrate Metabolism/ Utilization- Tissue SpecificityMuscle – cardiac and skeletal Oxidize glucose/produce and store glycogen (fed) Breakdown glycogen (fasted state) Shift to other fuels in fasting state (fatty acids) Adipose and liver Glucose  acetyl CoA Glucose to glycerol for triglyceride synthesis Liver releases glucose for other tissues Nervous system Always use glucose except during extreme fasts Reproductive tract/mammary Glucose required by fetus Lactose  major milk carbohydrate Red blood cells No mitochondria Oxidize glucose to lactate Lactate returned to liver for Gluconeogenesis

32 Carbohydrate Digestion RateComposition and Digestion of Carbohydrate Fractions ___________________________________________________________ _____________________________________________________ Composition Rumen Digestion (%/h) Sugars Fermentation and Organic Acids Starch Soluble Available Fiber Pectins B glucans Insoluble Available Fiber Cellulose Hemicellulose Unavailable Fiber (lignin) ___________________________________________________________ ___________________________________________________________ ___________________________________________________________

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