Transcript Carbohydrate Digestion

Carbohydrate
Digestion and
Metabolism
Overview of Carbohydrate
Digestion and Metabolism
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.
energy
Carbon dioxide
Water
Chlorophyll
GLUCOSE
6 CO2 + 6 H20 + energy (sun)C6H12O6 + 6 O2
Simple Sugars -
Disaccharides
Complex carbohydrates


Oligosaccharides
Polysaccharides



Starch
Glycogen
Dietary fiber (Dr. Firkins)
Starch




Major storage carbohydrate
in higher plants
Amylose – long straight
glucose chains (α1-4)
Amylopectin – branched
every 24-30 glc residues (α
1-6)
Provides 80% of dietary
calories in humans
worldwide
Glycogen
G
G
G
 Major storage
G
G
G
G
G
carbohydrate in
G
G
animals
G
a 1-6 link
G GG
 Long straight glucose
a 1-4 link G G
chains (α 1-4)
G
G
 Branched every 4-8 glc
G



residues (α 1-6)
More branched than
starch
Less osmotic pressure
Easily mobilized
Digestion

Pre-stomach – Salivary amylase : a 1-4
endoglycosidase
G
G
G G GG a Limit dextrins
GG
G
G
G
G
G
G
G
amylase
G
G
G
GGG
G
a
1-6
link
G
GG
maltotriose
GG
a 1-4 link
G
GG
G
G
maltose
G
G
isomaltose
Stomach



Not much carbohydrate digestion
Acid and pepsin to unfold proteins
Ruminants have forestomachs with
extensive
microbial populations to breakdown and
anaerobically ferment feed
Small Intestine
Pancreatic enzymes
a-amylase

maltotriose maltose
G G G
G G G G G
amylose
G G
a amylase
G G G G G
GG GG G G
amylopectin
+
G G G
GG G
a Limit dextrins
Oligosaccharide digestion..cont
a Limit dextrins
G G G
GG G
G
sucrase
G G
G
maltase
Glucoamylase (maltase)
or
G G G
a-dextrinase
a-dextrinase
G G
GG G
G
G
GG G
G
G
Small intestine
Portal for transport of virtually
all nutrients
Water and electrolyte balance
Enzymes associated with
intestinal surface membranes
i. Sucrase
ii. a dextrinase
iii.Glucoamylase (maltase)
iv.Lactase
v. peptidases
Carbohydrate absorption
Hexose transporter
apical
basolateral
Carbohydrate
Comparative Ruminant vs.
Non-Ruminant Animal
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



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
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
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
Amylase
Disaccharides
Digestion in Small Intestine

Digestion mediated by enzymes
synthesized by cells lining the small
intestine (brush border)
Disaccharides
Brush Border Enzymes
Monosaccharides
* Exception is β-1,4 bonds in cellulose
Digestion in Small Intestine
Sucrose
Sucrase
Glucose + Fructose
* Ruminants do not have sucrase
Maltose
Lactose
Maltase
Lactase
Glucose + Glucose
Glucose + Galactose
* Poultry do not have lactase
Digestion of Disaccharides
 Newborns
have a
full complement of
brush-border
enzymes
Miller et al. (eds.), 1991
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
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
Overview Monogastric
Carbohydrate Digestion
Location
Enzymes
Form of Dietary CHO
Mouth
Salivary Amylase
Starch Maltose Sucrose Lactose
Stomach
(amylase from saliva)
Small Intestine
Pancreatic Amylase
Brush Border Enzymes
Large Intestine
None
Dextrin→Maltose
Maltose
Glucose Fructose Galactose
+
+
+
Glucose Glucose Glucose
Bacterial Microflora Ferment Cellulose
Carbohydrate Absorption in
Monogastrics

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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
Small Intestine
Carbohydrates
Monosaccharides
Portal Vein
Liver
Active
Transport
Distributed to
tissue through
circulation
Nutrient Absorption - Carbohydrate

Active transport for glucose and
galactose
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
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Sodium-glucose transporter 1 (SGLT1)
Dependent on Na/K ATPase pump
Facilitated transport for fructose
Carbohydrate Digestion
in Ruminants

Ingested carbohydrates are exposed to
extensive pregastric fermentation
 Most carbohydrates fermented by microbes

Rumen fermentation is highly efficient
considering the feedstuffs ingested
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
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
Microbial Populations

Amylolytic bacteria (starch, sugar digesters)
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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
Microbial Metabolism
Sugars
ADP
Catabolism
in rumen:
VFA
CO2
CH4
Heat
ATP
NADP+
NADPH
Growth
Maintenance
Replication
Bacterial Digestion of
Carbohydrates
Rumen:

Microbes attach to (colonize) fiber
components and secrete enzymes
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


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
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
Summary of Carbohydrate in
Monogastrics
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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
Carbohydrates Metabolism in Monogastrics


Serve as primary source of energy in the cell
Central to all metabolic processes
Glucose
Cytosol - anaerobic
Hexokinase
Pentose
Phosphate
Shunt
Glucose-6-P
glycolysis
Pyruvate
Glc-1- phosphate
glycogen
cytosol
Pyruvate
mitochondria
(aerobic)
Aceytl CoA
FATTY ACIDS
Krebs
cycle
Reducing
equivalents
AMINO
ACIDS
Oxidative
Phosphorylation
(ATP)
Control of enzyme activity
Rate limiting step
Glucose utilization
Stage 1 – postparandial
All tissues utilize glucose
Stage 2 – postabsorptive
KEY – Maintain blood glucose
Glycogenolysis
Glucogneogenesis
Lactate
Pyruvate
Glycerol
AA
Propionate
Spare glucose by metabolizing fat
Stage 3- Early starvation
Gluconeogenesis
Stave 4 – Intermediate starvation
gluconeogenesis
Ketone bodies
Stage 5 – Starvation
Carbohydrate Metabolism/
Utilization- Tissue Specificity
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Muscle – cardiac and skeletal
 Oxidize glucose/produce and store glycogen (fed)
 Breakdown glycogen (fasted state)
 Shift to other fuels in fasting state (fatty acids)
Adipose and liver
 Glucose  acetyl CoA
 Glucose to glycerol for triglyceride synthesis
 Liver releases glucose for other tissues
Nervous system
 Always use glucose except during extreme fasts
Reproductive tract/mammary
 Glucose required by fetus
 Lactose  major milk carbohydrate
Red blood cells
 No mitochondria
 Oxidize glucose to lactate
 Lactate returned to liver for Gluconeogenesis
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
a
Carbohydrate Metabolism
in Ruminants

Ingested carbohydrates are exposed to
extensive pregastric fermentation
 Most carbohydrates fermented by microbes

Rumen fermentation is highly efficient
considering the feedstuffs ingested
Volatile Fatty Acids
Carbohydrates
Microbial Fermentation
VFA’s
Glucose

Short-chain fatty acids produced by microbes
- Rumen, cecum, colon
 3 basic types:
CH3
C
O–
Acetic acid (2c)
O
O
O
CH3
CH2
CH3
C
O–
Propionic acid (3c)
CH2
CH2
C
O–
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
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
Uses of VFA

Acetate

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
Propionate

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
Energy
Fatty acid synthesis
Energy
Gluconeogenic – glucose synthesis
Butyrate


Energy
Rumen epithelial cells convert to ketone (beta
hydroxybytyrate)
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
lipogenesis

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
Protein
Rumen
Fatty Acids
Starch
VFA
Propionate
Acetate
Butyrate
_____________________________________________
Blood
Fatty Acids
Amino Acids
Glucose
_____________________________________________
Tissue
Lactose
Fat
Protein
Carbohydrate Digestion and Absorption
Ruminant vs. Monogastrics
Digestive Feature
Ruminant
Salivary amylase
Zero
Non ruminant
High – primates
Moderate – pig
Low - carnivores
Pregastric fermentation High+
Zero in MOST cases
Gastric
Very low
Very low
Pancreatic amylase
in SI
Moderate
High
Glucose absorption
from SI
Zero to
low
High
Low
Low to High
Post SI