Carbohydrate Digestion Glucose Requirements There is less

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Carbohydrate Digestion
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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
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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
Monosaccharides
Disaccharides
Exception is ß-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

• 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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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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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

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

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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

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Microbial Populations

• Cellulolytic bacteria (fiber digesters)

• Produce cellulase - cleaves ß1?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
Sugars
ADP ATP NADP NADPH
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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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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a
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Volatile Fatty Acids
Microbial Fermentation

• Carbohydrates VFAs

Glucose

• Short-chain fatty acids produced by microbes
• 3 basic types

- Rumen, cecum, colon
Butyric acid (4c)
Acetic acid (2c)
Propionic acid (3c)
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VFA Formation
2 acetate CO2 CH4 heat
2 propionate water
1 Glucose
1 butyrate CO2 CH4
VFAs absorbed passively from rumen to portal
blood Provide 70-80 of ruminants 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
• Energy
• Fatty acid synthesis
• Propionate
• Energy
• Gluconeogenic glucose synthesis
• Butyrate
• Energy
• Rumen epithelial cells convert to ketone (beta
  hydroxybytyrate)
• Proportions produced depends on diet

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Absorption of VFAs

• No evidence for active transport (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
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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 40-60
  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
Fatty Acids
Bacterial Protein
Starch VFA
Propionate Acetate
Butyrate
Fatty Acids
Amino Acids
Glucose
Lactose
Fat
Protein
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Carbohydrate Digestion and Absorption