carbohydarte digestion

1. Carbohydrates, Classification,
Digestion, Absorption and
Utilization in Ruminants

2. Carbohydrates
 Carbohydrates are composed of carbon and water
and have a composition of (CH2O)n.
 The major nutritional role of carbohydrates is to
provide energy and digestible carbohydrates
provide 4 kilocalories per gram.

3. Carbohydrates Classification
Monosaccharides Oligosaccharides (2-10) Polysaccharides (many)
Functional
Group
No. of C
atoms
Di-saccharides Tri-saccharides
Tetra-
saccharides
Homopoly-
saccharides
Heteropoly-
saccharides
Aldoses
e.g Glucose
Trioses Maltose Raffinose Stachyose Starch Hyaluronic Acid
Tetroses Lactose Rhamnose Dextrin Heparin
Pentoses Sucrose Glycogen Chondroitin Sulfate
Ketoses
e.g Fructose
Heptoses Cellulose Dermatan Sulfate
Inulin Keratan Sulfate

4. Overview of Carbohydrate
Digestion and Metabolism

5. Stomach
Ruminants have fore-stomach with extensive
microbial populations to breakdown and
anaerobically ferment feed

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

7. 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

8. Carbohydrate Digestion in Ruminants
 Ingested carbohydrates are exposed to
extensive pre-gastric fermentation
 Rumen fermentation is highly efficient
considering the feedstuffs ingested
 Most carbohydrates fermented by microbes

9. Reticulorumen
 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

10. 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

11. 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

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

13. Bacterial Digestion of Carbohydrates
in 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 produceVFA
 Starches and simple sugars are more rapidly fermented
toVFA
 Protozoa engulf starch particles prior to digesting
them

14. Ruminant Carbohydrate Digestion in
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

15.  Bacterial population ferments the unabsorbed
products of digestion
 Absorption of H2O,VFA and formation of feces
Ruminant Carbohydrate Digestion in
Cecum Large Intestine

16. Volatile Fatty Acids
Carbohydrates VFA’s
Glucose
Microbial Fermentation
 Short-chain fatty acids produced by microbes
 3 basic types:
- Rumen, cecum, colon
Acetic acid (2c)
CH3 C
O
O–
CH2 C
O
O–
CH3
CH2 C
O
O–
CH2
CH3
Propionic acid (3c) Butyric acid (4c)

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

18. 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

19. Uses ofVFA
 Acetate
• MajorVFA present in blood & absorbed as such
• Energy (10 ATP/mole)
• Fatty acid synthesis (SCFA for milk fat)
• Never converted to Glucose
 Propionate
• Energy (17 ATP/mole)
• Carried to liver through blood for glucose synthesis
 Butyrate
• Energy (25 ATP/mole)
• Rumen epithelial cells convert to ketone (beta-hydroxy butyrate)
• Ketone bodies also from adipose tissues in early lactation
 Proportions produced depends on diet

20. 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

21. RumenVFA Profiles

22. Metabolism ofVFA
 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)

23. 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
• ContinuousVFA production
• Continuous digesta flow
• Continuous gluconeogenesis

24. Overview of Carbohydrates in Ruminants
Diet Protein Carbohydrate Fat
_____________________________________________
Rumen
_____________________________________________
Blood
_____________________________________________
Tissue
Bacterial
Protein
Amino Acids
Protein
Starch VFA
Propionate Acetate
Butyrate
Glucose
Lactose
Fatty Acids
Fatty Acids
Fat

25. Carbohydrate Digestion and Absorption
Ruminant vs. Monogastrics
Digestive Feature Ruminant Nonruminant
Salivary amylase Zero High – primates
Low - carnivores
Pre-gastric fermentation High+ Zero in MOST cases
Gastric fermentation Very low Very low
Pancreatic amylase High
in SI
Moderate
Glucose absorption Zero to High
from SI low
Post SI Low Low to High