Carbohydrate Digestion and Metabolism Overview of Carbohydrate Digestion
Carbohydrate Digestion and Metabolism
Overview of Carbohydrate Digestion and Metabolism
Carbohydrates • Carbohydrates are composed of carbon and water and have a composition of (CH 2 O)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 CO 2 + 6 H 20 + energy (sun)C 6 H 12 O 6 + 6 O 2
Simple Sugars -
Disaccharides
Complex carbohydrates n n Oligosaccharides Polysaccharides n n n Starch Glycogen Dietary fiber (Dr. Firkins)
Starch n n Major storage carbohydrate in higher plants Amylose – long straight glucose chains (α 1 -4) Amylopectin – branched every 24 -30 glc residues (α 1 -6) Provides 80% of dietary calories in humans worldwide
Glycogen n n n G G G Major storage G G G carbohydrate in animals G G Long straight glucose G a 1 -6 link G GG chains (α ( 1 -4) a 1 -4 link G G G Branched every 4 -8 glc G residues (α 1 -6) ( G More branched than starch Less osmotic pressure Easily mobilized
Digestion n Pre-stomach – Salivary amylase : a 1 -4 endoglycosidase G G G a Limit dextrins GG G G G amylase G GGG G a 1 -6 link G GG maltotriose GG a 1 -4 link G GG G G maltose G G isomaltose
Stomach n n n 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 n maltotriose maltose G G G G amylose G G a amylase G G GG GG G G amylopectin + G GG G a Limit dextrins
Oligosaccharide digestion. . cont a Limit dextrins G GG G G sucrase G G G maltase Glucoamylase (maltase) or G G G a-dextrinase G G GG G G G
Small intestine Portal for transport of virtually all nutrients Water and electrolyte balance Enzymes associated with intestinal surface membranes i. Sucrase ii. a dextrinase iii. Glucoamylase (maltase) iv. Lactase v. peptidases
Carbohydrate absorption Hexose transporter apical basolateral
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 n Monosaccharides n n n Do not need hydrolysis before absorption Very little (if any) in most feeds Di- and poly-saccharides n n n Relatively large molecules Must be hydrolyzed prior to absorption Hydrolyzed to monosaccharides Only monosaccharides can be absorbed
Non-Ruminant Carbohydrate Digestion n Mouth Ø 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 n Pancreas Ø Pancreatic amylase § Hydrolyzes alpha 1 -4 linkages § Produces monosaccharides, disaccharides, and polysaccharides § Major importance in hydrolyzing starch and glycogen to maltose Polysaccharides Amylase Disaccharides
Digestion in Small Intestine n Digestion mediated by enzymes synthesized by cells lining the small intestine (brush border) Disaccharides Brush Border Enzymes Monosaccharides * Exception is β-1, 4 bonds in cellulose
Digestion in Small Intestine Sucrose Sucrase Glucose + Fructose * Ruminants do not have sucrase Maltose Lactose Maltase Lactase Glucose + Galactose * Poultry do not have lactase
Digestion of Disaccharides n Newborns have a full complement of brush-border enzymes Miller et al. (eds. ), 1991
Digestion in Large Intestine n 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 n 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) Small Intestine Pancreatic Amylase Brush Border Enzymes Large Intestine None Dextrin→Maltose Glucose Fructose Galactose + + + Glucose Bacterial Microflora Ferment Cellulose
Carbohydrate Absorption in Monogastrics n n 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
Small Intestine Carbohydrates Monosaccharides Portal Vein Liver Active Transport Distributed to tissue through circulation
Nutrient Absorption - Carbohydrate n Active transport for glucose and galactose n n n Sodium-glucose transporter 1 (SGLT 1) Dependent on Na/K ATPase pump Facilitated transport for fructose
Carbohydrate Digestion in Ruminants n Ingested carbohydrates are exposed to extensive pregastric fermentation § Most carbohydrates fermented by microbes n Rumen fermentation is highly efficient considering the feedstuffs ingested
Reticulorumen n 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 n Cellulolytic bacteria (fiber digesters) § § § Produce cellulase - cleaves β 1→ 4 linkages Primary substrates are cellulose and hemicellulose Prefer p. H 6 -7 Produce acetate, propionate, little butyrate, CO 2 Predominate in animals fed roughage diets
Microbial Populations n Amylolytic bacteria (starch, sugar digesters) § § § Digest starches and sugars Prefer p. H 5 -6 Produce propionate, butyrate and sometimes lactate Predominate in animals fed grain diets Rapid change to grain diet causes lactic acidosis (rapidly decreases p. H) § Streptococcus bovis
Microbial Metabolism Sugars ADP VFA CO 2 CH 4 Heat NADP+ Biosynthesis Catabolism in rumen: ATP NADPH 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 n Small Intestine § Secretion of digestive enzymes § Digestive secretions from pancreas and liver § Further digestion of carbohydrates § Absorption of H 2 O, minerals, amino acids, glucose, fatty acids n Cecum and Large Intestine §Bacterial population ferments the unabsorbed products of digestion § Absorption of H 2 O, VFA and formation of feces
Summary of Carbohydrate in Monogastrics n n n 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 n Insulin influences rate of cellular uptake
Carbohydrates Metabolism in Monogastrics n n Serve as primary source of energy in the cell Central to all metabolic processes Glucose Cytosol - anaerobic Hexokinase Pentose Phosphate Shunt Glucose-6 -P glycolysis Pyruvate Glc-1 - phosphate glycogen
cytosol Pyruvate mitochondria (aerobic) Aceytl Co. A 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 n n n Muscle – cardiac and skeletal n Oxidize glucose/produce and store glycogen (fed) n Breakdown glycogen (fasted state) n Shift to other fuels in fasting state (fatty acids) Adipose and liver n Glucose acetyl Co. A n Glucose to glycerol for triglyceride synthesis n Liver releases glucose for other tissues Nervous system n Always use glucose except during extreme fasts Reproductive tract/mammary n Glucose required by fetus n Lactose major milk carbohydrate Red blood cells n No mitochondria n Oxidize glucose to lactate n 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
a
Carbohydrate Metabolism in Ruminants n Ingested carbohydrates are exposed to extensive pregastric fermentation § Most carbohydrates fermented by microbes n Rumen fermentation is highly efficient considering the feedstuffs ingested
Volatile Fatty Acids Carbohydrates Microbial Fermentation VFA’s Glucose § Short-chain fatty acids produced by microbes - Rumen, cecum, colon § 3 basic types: Acetic acid (2 c) Propionic acid (3 c) Butyric acid (4 c)
VFA Formation 2 acetate + CO 2 + CH 4 + heat 1 Glucose 2 propionate + water 1 butyrate + CO 2 + CH 4 VFAs absorbed passively from rumen to portal blood Provide 70 -80% of ruminant’s energy needs
Rumen Fermentation n 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 n Acetate n n n Propionate n n n Energy Fatty acid synthesis Energy Gluconeogenic – glucose synthesis Butyrate n n Energy Rumen epithelial cells convert to ketone (beta hydroxybytyrate) Proportions produced depends on diet
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 n Overview n n n Acetate and butyrate are the major energy sources (through oxidation) Propionate is reserved for gluconeogenesis Acetate is the major substrate for lipogenesis n Propionate is also lipogenic (though glucose)
Glucose Requirements n n 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 _______________________ Bacterial Protein Rumen Fatty Acids Starch VFA Propionate Acetate Butyrate _______________________ Blood Fatty Acids Amino Acids Glucose _______________________ Tissue Lactose Fat Protein
Carbohydrate Digestion and Absorption Ruminant vs. Monogastrics Digestive Feature Salivary amylase Ruminant Zero Non ruminant High – primates Moderate – pig Low - carnivores Pregastric fermentation High+ Zero in MOST cases Gastric Very low Pancreatic amylase in SI Moderate High Glucose absorption from SI Zero to low High Low to High Post SI
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