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Carbohydrate Digestion

Carbohydrate Digestion. 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

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Carbohydrate Digestion

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  1. Carbohydrate Digestion

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

  3. 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

  4. 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

  5. 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

  6. Digestion in Small Intestine • Digestion mediated by enzymes synthesized by cells lining the small intestine (brush border) Brush Border Enzymes Monosaccharides Disaccharides * Exception is β-1,4 bonds in cellulose

  7. 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

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

  9. 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

  10. 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

  11. Overview Monogastric Carbohydrate Digestion Location EnzymesForm 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

  12. 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

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

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

  15. Summary of Carbohydrate Digestion and Absorption 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

  16. 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 before they can be exposed to typical gastric and small intestinal enzymes

  17. Reticulorumen • Almost all carbohydrate is fermented in the rumen • Some ‘bypass’ starch may escape to the small intestine • Do not have salivary amylase, but have plenty of pancreatic amylase to digest starch

  18. 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

  19. 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

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

  21. 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

  22. 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

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

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

  27. 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

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

  29. VFA Production – Molar Ratios

  30. Rumen VFA Profiles

  31. 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)

  32. 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

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

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

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