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Carbohydrate Digestion and Metabolism
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Overview of Carbohydrate Digestion and Metabolism
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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
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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)
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Disaccharides
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Complex carbohydrates
Oligosaccharides Polysaccharides Starch Glycogen Dietary fiber (Dr. Firkins)
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Starch Major storage carbohydrate in higher plants
Amylose – long straight glucose chains (α1-4) Amylopectin – branched every glc residues (α 1-6) Provides 80% of dietary calories in humans worldwide
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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
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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.
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Stomach Not much carbohydrate digestion
Acid and pepsin to unfold proteins Ruminants have forestomachs with extensive microbial populations to breakdown and anaerobically ferment feed
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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
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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
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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
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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
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Comparative Ruminant vs. Non-Ruminant Animal
Carbohydrate Comparative Ruminant vs. Non-Ruminant Animal
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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
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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
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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
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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
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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
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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
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Digestion of Disaccharides
Miller et al. (eds.), 1991 Newborns have a full complement of brush-border enzymes
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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
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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
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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
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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
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Carbohydrates Monosaccharides
Small Intestine Carbohydrates Monosaccharides Portal Vein Active Transport Liver Distributed to tissue through circulation
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Nutrient Absorption - Carbohydrate
Active transport for glucose and galactose Sodium-glucose transporter 1 (SGLT1) Dependent on Na/K ATPase pump Facilitated transport for fructose
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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
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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
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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
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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
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Microbial Metabolism ADP ATP NADP+ NADPH VFA CO2 CH4 Heat Sugars
Biosynthesis Catabolism in rumen: VFA CO2 CH4 Heat Growth Maintenance Replication
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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
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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
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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
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Lactate transported back to liver for glucose production “Cori Cycle”
Lactate transported back to liver for glucose production “Cori Cycle”. Costs energy
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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
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cytosol Pyruvate mitochondria (aerobic) Aceytl CoA FATTY ACIDS Krebs cycle Reducing equivalents AMINO ACIDS Oxidative Phosphorylation (ATP)
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Control of enzyme activity
Rate limiting step
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Glucose utilization
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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
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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
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Carbohydrate Digestion Rate
Composition 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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a
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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
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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)
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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
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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
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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
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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
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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
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Rumen VFA Profiles
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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)
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Glucose Requirements There is less fluctuation in blood glucose in ruminants and blood glucose is lower at mg/dl Reduced fluctuation due to: Eat more constantly than monogastrics Continuous VFA production Continuous digesta flow Continuous gluconeogenesis
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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
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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
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