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.
energyCarbon dioxideWaterChlorophyll
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
1-4 link
GG
G G
G
G GG
GG
GGG 1-6 link
GG
G
GG G Major storage Major storage carbohydrate in carbohydrate in animalsanimals
Long straight Long straight glucose chains (glucose chains (α 1-1-4)4)
Branched every 4-8 Branched every 4-8 glc residues (glc residues (α 1-6) 1-6)
More branched than More branched than starchstarch
Less osmotic Less osmotic pressurepressure
Easily mobilizedEasily mobilized
Digestion Pre-stomach – Salivary amylase : 1-4 endoglycosidase
GGG G
G
G G G 1-4 linkG
GGG 1-6 link
GG
G
GG G G G G
GG
G
G G
Gmaltose
G
GG
isomaltose
amylase
maltotriose
G
G
G
G
Limit dextrins
Stomach Not much carbohydrate digestion Acid and pepsin to unfold proteins Ruminants have forestomachs with
extensivemicrobial populations to breakdown andanaerobically ferment feed
Small Intestine Pancreatic enzymes-amylase
G G GG G
GG G G
G G GGGG G
amylose
amylopectin
G G G G G
amylase
+
GG G
G G
maltotriosemaltose
Limit dextrins
G
Oligosaccharide digestion..cont
GG G
G GG
G
GG GG
G
Glucoamylase (maltase) or
-dextrinase
G GG
GG
-dextrinase
G GG
G
G G
Gmaltase
sucrase
Limit dextrins G
Small intestinePortal for transport of virtually all nutrients
Water and electrolyte balance
Enzymes associated with intestinal surface membranesi. Sucraseii. dextrinaseiii.Glucoamylase (maltase)iv.Lactasev. peptidases
Carbohydrate absorption
Hexose transporter
apical basolateral
Carbohydrate
Comparative Ruminant vs. Non-Ruminant Animal
Digestion and Absorption
Non-ruminant RuminantCHO in feed
digestiveenzymes
Glucose insmall intestine
Absorption intoblood circulation
microbialfermentation
Volatile fatty acidsin rumen
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 DisaccharidesAmylase
Digestion in Small Intestine Digestion mediated by enzymes
synthesized by cells lining the small intestine (brush border)
Disaccharides MonosaccharidesBrush Border Enzymes
* Exception is β-1,4 bonds in cellulose
Digestion in Small Intestine
Maltose Glucose + GlucoseMaltase
Lactose Lactase Glucose + Galactose
* Poultry do not have lactase
Sucrose Glucose + FructoseSucrase
* Ruminants do not have sucrase
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 thebeta 1,4- 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 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
Carbohydrates Monosaccharides
Small Intestine
Active Transpor
t
Liver
Portal Vein
Distributed to tissue through circulation
Nutrient Absorption - Carbohydrate
Active transport for glucose and galactose 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
Rumen fermentation is highly efficient considering the feedstuffs ingested
Most carbohydrates fermented by microbes
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) 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
ADP
ATP
NADP+
NADPH
Sugars
Ca
tabo
lism
Biosyn
thesis
GrowthMaintenanceReplication
Microbial Metabolism
VFACO2
CH4
Heat
in rumen:
Bacterial Digestion of Carbohydrates
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
Rumen:
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
Summary of Carbohydrate 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
Glucose
Glucose-6-P
Pyruvate
Hexokinase
PentosePhosphateShunt
glycolysis
Carbohydrates Metabolism in Monogastrics
Serve as primary source of energy in the cell Central to all metabolic processes
Glc-1- phosphate
glycogen
Cytosol - anaerobic
Pyruvatecytosol
Aceytl CoAmitochondria (aerobic)
Krebscycle
Reducingequivalents
OxidativePhosphorylation(ATP)
AMINOACIDS
FATTY ACIDS
Control of enzyme activity
Rate limiting step
Glucose utilization
Stage 1 – postparandial All tissues utilize glucose
Stage 2 – postabsorptive KEY – Maintain blood glucoseGlycogenolysisGlucogneogenesisLactatePyruvateGlycerolAAPropionateSpare glucose by metabolizing fat
Stage 3- Early starvationGluconeogenesis
Stave 4 – Intermediate starvationgluconeogenesisKetone bodies
Stage 5 – Starvation
Carbohydrate Metabolism/ Utilization- Tissue Specificity 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 200-350Fermentation and Organic Acids 1-2
Starch 10-40Soluble Available Fiber 40-60 Pectins B glucans
Insoluble Available Fiber 2-10 Cellulose Hemicellulose
Unavailable Fiber (lignin) 0
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a
Carbohydrate Metabolism in Ruminants
Ingested carbohydrates are exposed to extensive pregastric fermentation
Rumen fermentation is highly efficient considering the feedstuffs ingested
Most carbohydrates fermented by microbes
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 CO
O–CH2 C
O
O–CH3 CH2 C
O
O–CH2CH3
Propionic acid (3c) Butyric acid (4c)
VFA Formation
2 acetate + CO2 acetate + CO22 + CH4 + heat
2 propionate + water2 propionate + water
1 butyrate + CO1 butyrate + CO22 + CH4
1 Glucose
VFAs absorbed passively from rumen to portal bloodProvide 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
Energy Fatty acid synthesis
Propionate 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 _____________________________________________
Rumen
_____________________________________________Blood_____________________________________________Tissue
Bacterial Protein
Amino Acids
Protein
Starch VFA
Propionate Acetate Butyrate
Glucose
Lactose
Fatty Acids
Fatty Acids
Fat
Carbohydrate Digestion and Absorption Ruminant vs. Monogastrics
Digestive Feature Ruminant Nonruminant Salivary amylase Zero High – primates Moderate – pig Low - carnivores
Pregastric fermentation High+ Zero in MOST cases
Gastric Very low Very low
Pancreatic amylase High in SI
Moderate
Glucose absorption Zero to High from SI low
Post SI Low Low to High