1. Carbohydrate Digestion

2. 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

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
Disaccharides
Monosaccharides
* 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. Digestion of Disaccharides
Miller et al. (eds.), 1991
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
Volatile fatty acid (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 bacteria which
produce cellulase
Cellulase is capable of hydrolyzing the
glucose-4-beta-glucoside linkage

11. 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

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. Carbohydrates Monosaccharides
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

16. Summary of Carbohydrate Digestion and Absorption in Monogastrics
Consist of starches, glycogen, sucrose, lactose, glucose, fructose
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 transport

17. 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

18. 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

19. 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

20. 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

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

22. 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

23. 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

24. Carbohydrate Digestion Rate
Composition and Digestion of Carbohydrate Fractions
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_____________________________________________________
Composition Rumen Digestion (%/h)
Sugars
Fermentation and Organic Acids
Starch
Soluble Available Fiber
Pectins
B glucans
Insoluble Available Fiber
Cellulose
Hemicellulose
Unavailable Fiber (lignin)
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25. a

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

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

28. 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

29. Uses of VFA Acetate Propionate Butyrate
Energy
Fatty acid synthesis
Propionate
Gluconeogenic – glucose synthesis
Butyrate
Rumen epithelial cells convert to ketone
Proportions produced depends on diet

30. Absorption of VFAs No evidence for active transport
VFA metabolism in the rumen wall
Cells use most of the butyrate for their own energy needs
Acetate and propionate are ‘exported’ to blood

31. 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

32. Rumen VFA Profiles

33. 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
Hence the importance of adequate effective fiber in dairy diets to maintain milkfat levels
Propionate is also lipogenic (though glucose)

34. Glucose Requirements
There is less fluctuation in blood glucose in ruminants and blood glucose is lower at mg/dl
Reduced fluctuation due to:
Eat more constantly than monogastrics
Continuous VFA production
Continuous digesta flow
Continuous gluconeogenesis

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

36. 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
Pancreatic amylase
Low to
in SI
moderate
Glucose absorption
Zero to
from SI
low
Post SI
Low to High