Carbohydrate Digestion (PowerPoint) by MikeJenny

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									Carbohydrate Digestion
       Digestion and Absorption
Non-ruminant                             Ruminant
                    CHO in feed
digestive                                   microbial
enzymes                                   fermentation

 Glucose in                       Volatile fatty acids
small intestine                        in rumen

                   Absorption into
                  blood circulation
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
Non-Ruminant Carbohydrate
   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
  Carbohydrate Digestion
      Pancreas
        Pancreatic amylase
         Hydrolyzes alpha 1-4 linkages
         Produces monosaccharides, disaccharides,
          and polysaccharides
         Major importance in hydrolyzing starch and
          glycogen to maltose
Polysaccharides               Disaccharides
   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
   Digestion in Small Intestine
Sucrose                              Glucose + Fructose
       * Ruminants do not have sucrase

Maltose                              Glucose + Glucose

Lactose       Lactase            Glucose + Galactose
     * Poultry do not have lactase
    Digestion of Disaccharides

 Newborns  have a
 full complement of

                           Miller et al. (eds.), 1991
     Digestion in Large Intestine
   Carnivores and omnivores
     Limited anaerobic fermentation
     Bacteria produce small quantities of cellulase
     Volatile fatty acid (VFA) produced by microbial
    digestion of fibers
        Propionate
        Butyrate
        Acetate
Digestion in Large Intestine
   Post-gastric fermenters (horse and
     Can utilize large quantities of cellulose
     Cecum and colon contain bacteria which
      produce cellulase
     Cellulase is capable of hydrolyzing the
      glucose-4-beta-glucoside linkage
        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
Carbohydrate Absorption in
   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
 Small Intestine

Carbohydrates           Monosaccharides

                   Portal Vein           Active

                           Distributed to
     Liver                 tissue through
Nutrient Absorption - Carbohydrate
   Active transport for glucose and
       Sodium-glucose transporter 1 (SGLT1)
       Dependent on Na/K ATPase pump
   Facilitated transport for fructose
         Summary of Carbohydrate
         Digestion and Absorption in
   Consist of starches, glycogen, sucrose, lactose,
    glucose, fructose
   Polysaccharides broken down to
   Monosaccharides taken up by active transport or
    facilitated diffusion and carried to liver
   Glucose is transported to cells requiring energy
      Insulin influences rate of transport
    Carbohydrate Digestion
     in Ruminants
   Ingested carbohydrates are exposed to
    extensive pregastric fermentation
     Most carbohydrates fermented by microbes
    before they can be exposed to typical gastric
    and small intestinal enzymes

   Rumen fermentation is highly efficient
    considering the feedstuffs ingested

   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
Microbial Populations
   Cellulolytic bacteria (fiber digesters)
     Produce cellulase - cleaves β1→4 linkages
     Prefer pH 6-7
     Utilize N in form of NH3
     Require S for synthesis of sulfur-containing amino
      acids (cysteine and methionine)
     Produce acetate, propionate, little butyrate, CO2
     Predominate in animals fed roughage diets
      Microbial Populations
   Amylolytic bacteria (starch, sugar digesters)
        Digest starch
        Prefer pH 5-6
        Utilize N as NH3 or peptides
        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
                           Microbial Metabolism


in rumen:
 VFA                            NADPH
 CO2                                      Growth
 CH4                                      Maintenance
 Heat                                     Replication
Bacterial Digestion of
   Microbes attach to (colonize) fiber
    components and secrete enzymes
       Cellulose, hemicellulose digested by cellulases and
       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
   Small Intestine
     Secretion of digestive enzymes
     Digestive secretions from pancreas and liver
     Further digestion of carbohydrates
     Absorption of H2O, minerals, amino acids, glucose,
    fatty acids

   Cecum and Large Intestine
    Bacterial population ferments the unabsorbed
      products of digestion
     Absorption of H2O, VFA and formation of feces
    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
     B glucans
  Insoluble Available Fiber              2-10
  Unavailable Fiber (lignin)               0
       Volatile Fatty Acids
                           Microbial Fermentation
Carbohydrates                                       VFA’s

    Short-chain fatty acids produced by microbes
       - Rumen, cecum, colon
  3 basic types:
                     O                         O                            O
          CH 3   C             CH3   CH2   C         CH3    CH2   CH2   C
                     O–                        O–                           O–

        Acetic acid (2c)    Propionic acid (3c)          Butyric acid (4c)
     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
Rumen Fermentation
   Gases (carbon dioxide and methane)
    are primary byproducts of rumen
   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
   Acetate
       Energy
       Fatty acid synthesis
   Propionate
       Energy
       Gluconeogenic – glucose synthesis
   Butyrate
       Energy
       Rumen epithelial cells convert to ketone

        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
   Overview
       Acetate and butyrate are the major energy
        sources (through oxidation)
       Propionate is reserved for gluconeogenesis
       Acetate is the major substrate for
          Hence the importance of adequate effective
           fiber in dairy diets to maintain milkfat levels
          Propionate is also lipogenic (though glucose)
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
Overview of Carbohydrates and Ruminants

 Diet      Protein    Carbohydrate         Fat

              Bacterial                      Fatty Acids
                          Starch      VFA

                               Propionate   Acetate

 Blood    Amino Acids                 Fatty Acids
 Tissue     Protein      Lactose         Fat
Carbohydrate Digestion
and Absorption
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     Low to     High
in SI                  moderate
Glucose absorption     Zero to    High
from SI                low
Post SI                Low        Low to High

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