Carbohydrate Digestion and Metabolism

Overview of 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

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.

Stomach Not much carbohydrate digestion Acid and pepsin to unfold proteins Ruminants have forestomachs with extensive microbial populations to breakdown and anaerobically ferment feed

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

Comparative Ruminant vs. Non-Ruminant Animal Carbohydrate Comparative Ruminant vs. Non-Ruminant Animal

Digestion and Absorption Non-ruminant Ruminant CHO in feed microbial fermentation digestive enzymes Glucose in small intestine Volatile fatty acids in rumen Absorption into blood circulation

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

Digestion of Disaccharides Miller et al. (eds.), 1991 Newborns have a full complement of brush-border enzymes

Digestion in Large Intestine Carnivores and omnivores Limited anaerobic fermentation Bacteria produce small quantities of cellulase SOME volatile fatty acids (VFA) produced by microbial digestion of fibers Propionate Butyrate Acetate

Digestion in Large Intestine Post-gastric fermenters (horse and rabbit) Can utilize large quantities of cellulose Cecum and colon contain high numbers of bacteria which produce cellulase Cellulase is capable of hydrolyzing the beta 1,4- linkage

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

Carbohydrate Digestion in Ruminants Ingested carbohydrates are exposed to extensive pregastric fermentation Rumen fermentation is highly efficient considering the feedstuffs ingested Most carbohydrates fermented by microbes

Reticulorumen Almost all carbohydrate is fermented in the rumen Some ‘bypass’ starch may escape to the small intestine No salivary amylase, but have plenty of pancreatic amylase to digest starch

Microbial Populations Cellulolytic bacteria (fiber digesters) Produce cellulase - cleaves β1→4 linkages Primary substrates are cellulose and hemicellulose Prefer pH 6-7 Produce acetate, propionate, little butyrate, CO2 Predominate in animals fed roughage diets

Microbial Populations Amylolytic bacteria (starch, sugar digesters) Digest starches and sugars Prefer pH 5-6 Produce propionate, butyrate and sometimes lactate Predominate in animals fed grain diets Rapid change to grain diet causes lactic acidosis (rapidly decreases pH) Streptococcus bovis

Microbial Metabolism ADP ATP NADP+ NADPH VFA CO2 CH4 Heat Sugars Biosynthesis Catabolism in rumen: VFA CO2 CH4 Heat Growth Maintenance Replication

Bacterial Digestion of Carbohydrates Rumen: Microbes attach to (colonize) fiber components and secrete enzymes Cellulose, hemicellulose digested by cellulases and hemicellulases Complex polysaccharides are digested to yield sugars that are fermented to produce VFA Starches and simple sugars are more rapidly fermented to VFA Protozoa engulf starch particles prior to digesting them

Ruminant Carbohydrate Digestion Small Intestine Cecum and Large Intestine Secretion of digestive enzymes Digestive secretions from pancreas and liver Further digestion of carbohydrates Absorption of H2O, minerals, amino acids, glucose, fatty acids Bacterial population ferments the unabsorbed products of digestion Absorption of H2O, VFA and formation of feces

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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Carbohydrate Metabolism in Ruminants Ingested carbohydrates are exposed to extensive pregastric fermentation Rumen fermentation is highly efficient considering the feedstuffs ingested Most carbohydrates fermented by microbes

Volatile Fatty Acids Carbohydrates VFA’s Glucose 3 basic types: Microbial Fermentation Carbohydrates VFA’s Glucose Short-chain fatty acids produced by microbes 3 basic types: - Rumen, cecum, colon Acetic acid (2c) Propionic acid (3c) Butyric acid (4c)

VFA Formation 1 Glucose 2 acetate + CO2 + CH4 + heat 2 propionate + water 1 butyrate + CO2 + CH4 VFAs absorbed passively from rumen to portal blood Provide 70-80% of ruminant’s energy needs

Rumen Fermentation Gases (carbon dioxide and methane) are primary byproducts of rumen fermentation Usually these gases are eructated or belched out - if not, bloat occurs Bloat results in a severe distension of the rumen typically on the left side of the ruminant and can result in death

Uses of VFA Acetate Propionate Butyrate Energy Fatty acid synthesis Propionate Gluconeogenic – glucose synthesis Butyrate Rumen epithelial cells convert to ketone (beta hydroxybytyrate) Proportions produced depends on diet

Absorption of VFAs all simple passive diffusion VFA metabolism in the rumen wall Cells use most of the butyrate for their own energy needs Acetate and propionate are ‘exported’ to blood

VFA Production – Molar Ratios Forage:Grain Acetate Propionate Butyrate 100:0 71.4 16.0 7.9 75:25 68.2 18.1 8.0 50:50 65.3 18.4 10.4 40:60 59.8 25.9 10.2 20:80 53.6 30.6 10.7

Rumen VFA Profiles

Metabolism of VFA Overview Acetate and butyrate are the major energy sources (through oxidation) Propionate is reserved for gluconeogenesis Acetate is the major substrate for lipogenesis Propionate is also lipogenic (though glucose)

Glucose Requirements There is less fluctuation in blood glucose in ruminants and blood glucose is lower at 40-60 mg/dl Reduced fluctuation due to: Eat more constantly than monogastrics Continuous VFA production Continuous digesta flow Continuous gluconeogenesis

Overview of Carbohydrates and Ruminants Diet Protein Carbohydrate Fat _____________________________________________ Rumen Blood Tissue Bacterial Protein Fatty Acids Starch VFA Propionate Acetate Butyrate Amino Acids Fatty Acids Glucose Lactose Fat Protein

Carbohydrate Digestion and Absorption Ruminant vs. Monogastrics Digestive Feature Ruminant Non ruminant Salivary amylase Zero High – primates Moderate pig Low - carnivores Pregastric fermentation High+ Zero in MOST cases Gastric Very low Pancreatic amylase in SI Glucose absorption Zero to from SI low Post SI Low to High