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

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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