Carbohydrate Digestion and Metabolism

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

120 grams of glucose / day = 480 calories

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

Disaccharides

Complex carbohydrates Oligosaccharides Polysaccharides Starch Glycogen Dietary fiber (Dr. Firkins)

Starch Major storage carbohydrate in higher plants Amylose – long straight glucose chains (α1-4) Amylopectin – branched every 24-30 glc residues (α 1-6) Provides 80% of dietary calories in humans worldwide

Glycogen Major storage carbohydrate in animals a 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

Digestion Pre-stomach – Salivary amylase : a 1-4 endoglycosidase a 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.

Small Intestine + Pancreatic enzymes a-amylase a amylase maltotriose maltose + 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

Oligosaccharide digestion..cont a 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

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

Carbohydrate absorption Hexose 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

Digestion of Carbohydrates Monosaccharides 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

Non-Ruminant Carbohydrate Digestion Mouth 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

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

Digestion in Small Intestine Digestion 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

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

Overview Monogastric Carbohydrate Digestion Location 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

Carbohydrate Absorption in Monogastrics With 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

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

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

Summary of Carbohydrate in Monogastrics Polysaccharides 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

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

Carbohydrates Metabolism in Monogastrics Serve 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

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

Control of enzyme activity Rate limiting step

Glucose utilization

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

Carbohydrate Metabolism/ Utilization- Tissue Specificity Muscle – 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

Carbohydrate Digestion Rate Composition and Digestion of Carbohydrate Fractions ___________________________________________________________ _____________________________________________________ Composition Rumen Digestion (%/h) Sugars 200-350 Fermentation and Organic Acids 1-2 Starch 10-40 Soluble Available Fiber 40-60 Pectins B glucans Insoluble Available Fiber 2-10 Cellulose Hemicellulose Unavailable Fiber (lignin) 0 ___________________________________________________________ ___________________________________________________________ ___________________________________________________________

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