Carbohydrate Digestion and Metabolism. Overview of Carbohydrate Digestion and Metabolism

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