1. Carbohydrate Digestion and Metabolism

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.
energy
Carbon dioxide
Water
Chlorophyll
GLUCOSE
6 CO2 + 6 H20 + energy (sun)
C6H12O6 + 6 O2

3. 120 grams of glucose / day = 480 calories

4. Simple Sugars - Glucose – basic unit of polysaccharides
Galactose – found as part of lactose in milk
Fructose – found in fruits, vegetables and honey (consumption on the rise….High fructose corn syrup)

5. Disaccharides

6. Complex carbohydrates
Oligosaccharides
Polysaccharides
Starch
Glycogen
Dietary fiber (Dr. Firkins)

7. Starch Major storage carbohydrate in higher plants
Amylose – long straight glucose chains (α1-4)
Amylopectin – branched every glc residues (α 1-6)
Provides 80% of dietary calories in humans worldwide

8. Glycogen Major storage carbohydrate in animals
a 1-4 link G
a 1-6 link
Major storage carbohydrate in animals
Long straight glucose chains (α 1-4)
Branched every 4-8 glc residues (α 1-6)
More branched than starch
Less osmotic pressure
Easily mobilized

9. Digestion Pre-stomach – Salivary amylase : a 1-4 endoglycosidase
a 1-4 link
a 1-6 link
maltose
isomaltose
amylase
maltotriose
a Limit dextrins
Key- We must breakdown these very large oligosaccharides into monosaccharides in order to absorb them.
Alpha amylase. – Cannot attack a1-4 linkase close to 1-6 branch points.

10. Small Intestine + Pancreatic enzymes a-amylase a amylase maltotriose
maltose + G G G G G G G G G G
a amylase
amylose G G G G G G G G G G G G G G G G G
amylopectin
a Limit dextrins

11. Oligosaccharide digestion..cont
a Limit dextrins G G G G
sucrase G G G G G
maltase G G
Glucoamylase (maltase) or
a-dextrinase G G G
Maltase – specifically removes a single glucose from the nonreducing end of a linear a1-4 glucose chain…breaking down maltose into glucose. (exosaccharidases)
Alpha dextinase – cleaves 1,6-alpha glucosidic linkages
a-dextrinase G G G G G G G G G G G

12. Small intestine Portal for transport of virtually all nutrients
Water and electrolyte balance
Enzymes associated with
intestinal surface membranes
Sucrase
a dextrinase
Glucoamylase (maltase)
Lactase
peptidases

13. Carbohydrate absorption
Hexose transporter
Monosaccharides are still too large for passive diffusion across brush border membrane. We use facilitated diffusion to absorb these molecules.
Glucose and galactose use a sodium-glucose symport (SGLUT1) while fructose uses the glut5
We must transport Na out of the cell to maintain proper electrochemical gradient (sodium potassium pump)
Water will also follow sodium into enterocyte. This is critical to maintain proper water balance.
apical
basolateral

14. 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

15. 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

16. 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

17. 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

18. 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

19. 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

20. 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

21. Carbohydrates Monosaccharides
Small Intestine
Carbohydrates Monosaccharides
Portal Vein
Active Transport
Liver
Distributed to tissue through circulation

22. Nutrient Absorption - Carbohydrate
Active transport for glucose and galactose
Sodium-glucose transporter 1 (SGLT1)
Dependent on Na/K ATPase pump
Facilitated transport for fructose

24. 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

25. Lactate transported back to liver for glucose production “Cori Cycle”
Lactate transported back to liver for glucose production “Cori Cycle”. Costs energy

26. 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
Glc-1- phosphate
glycolysis
glycogen
Pyruvate

27. cytosol
Pyruvate
mitochondria
(aerobic)
Aceytl CoA
FATTY ACIDS
Krebs
cycle
Reducing
equivalents
AMINO
ACIDS
Oxidative
Phosphorylation
(ATP)

28. Control of enzyme activity
Rate limiting step

29. Glucose utilization

30. 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

31. 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

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

33. a