Carbohydrate Digestion & Classification

Structural Carbohydrates (SCHO)

• SCHO examples include cellulose, hemicellulose, pectin, and fiber
• SCHO aren't hydrolyzed by mammalian enzymes

Non-Structural Carbohydrates (NSCHO)

• NSCHO examples include starch and sugars
• NSCHO are hydrolyzed by mammalian enzymes

Breakdown of Carbohydrates via Bacterial/Microbial Enzymes (BE)

• NSCHO are broken down into monomers or monosaccharides by BE
• Microbial enzymes break down monomers
• This process results in fermentation into VFAs (volatile fatty acids), CO2, and CH4

Breakdown of Carbohydrates via Mammalian Enzymes (ME)

• NSCHO break down into monomers or monosaccharides, such as glucose, galactose, and fructose using ME
• Mammalian enzymes break down monomers
• Nutrients are absorbed in the small intestine
• Used for respiration

Monosaccharides

• Simple sugars made of one sugar molecule
• Examples: glucose, galactose, and fructose
• Animals absorb monosaccharides from the small intestine
• Mammals use monosaccharides for energy
• Microbes ferment monosaccharides into VFAs, CO2, and CH4
• VFAs can be used by animals as energy

Disaccharides

• Sugars composed of two sugar molecules
• Examples: lactose, sucrose, maltose, and cellobiose
• Lactose contains glucose + galactose
• Sucrose contains glucose + fructose
• Maltose contains glucose + glucose with an alpha bond
• Cellobiose contains glucose + glucose with a beta bond
• BE can digest lactose, sucrose, maltose, and cellobiose
• ME can digest lactose, sucrose, and maltose, but not cellobiose

Polysaccharides

• Complex carbs made of many sugar molecules, such as starch, cellulose, hemicellulose, and pectin

Starch

• Starch is a polymer of glucose connected by alpha-linkages
• Starch id a homopolymer composed of just glucose
• Digestible by BE and ME
• Starch is a storage polysaccharide

Cellulose

• Cellulose is a glucose polymer connected by beta-linkages
• Cellulose is a homopolymer composed of just glucose
• Digestible by only BE

Hemicellulose

• Hemicellulose is a heteropolymer
• Digestible by BE
• Made of glucose, xylose, mannose, galactose, arabinose, and rhamnose

Pectin

• Pectin is a heteropolymer made of arabinose and galactose
• Digestible by BE

Starch Structure

• Digestion breaks down (hydrolyzes) the maltose structure
• Alpha linkage occurs between carbon 1 (C1) and carbon 4 (C4), involving oxygen
• Cereal grains contain 15-20% amylose and 80-85% amylopectin
• Alpha bonds cause a coiled shape, exposing bonds for easier hydrolysis

Amylose

• Contains alpha-1,4-linkages
• Length is ~100 glucose molecules

Amylopectin

• Contains both alpha-1,4 and alpha-1,6-linkages
• Alpha-1,6-linkages are the branch points
• Length is 10,000-100,000 glucose molecules
• Similar to glycogen

Glycogen

• Stored form of glucose in muscle and liver tissue
• Glucagon mobilizes glycogen to increase blood glucose concentration

Cellobiose

• Two glucose molecules linked with a β(1, 4) glycosidic bond
• Only broken down/hydrolyzed by BE
• Mammalian enzymes can't break down cellobiose due to the β − 1, 4 bond

Lignin

• Not a carbohydrate
• Approximately 0% digestible
• Decreases the digestibility of cellulose and hemicellulose
• Part of the cell wall

Digestion in Monogastrics (Horse/Pig)

• Mouth decreases particle size
• Salivary amylase is secreted in the mouth, which hydrolyzes α − 1, 4 bonds

Stomach

• Acidic environment (low pH)
• Inactivates salivary amylase

Small Intestine

• Pancreatic alpha amylase is secreted
• Pancreatic alpha amylase cleaves two glucose molecules at a time
• Requires at least five glucose molecules with α − 1, 4 bonds
• Amylose breaks down to maltose and maltotriose via pancreatic α amylase
• Amylopectin breaks down to maltose, maltotriose, and isomaltose (α − 1, 6 bonds) via pancreatic α amylase

Brush Border Enzymes

• Include glucoamylase, maltase, isomaltase, sucrase, lactase, and phlorizin hydrolase

Enzymes and Function

• Maltase breaks down 40% of maltose into 2 glucose molecules
• Isomaltase breaks down 30% of isomaltose into 2 glucose molecules
• Maltase/Isomaltase breaks down 30% of maltotriose into 3 glucose molecules
• Glucoamylase cleaves (hydrolyzes) one glucose molecule from starch
• Sucrase hydrolyzes sucrose into fructose and glucose
• Isomaltase hydrolyzes isomaltose
• Lactase cleaves lactose into glucose and galactose
• Phlorizin hydrolase cleaves sugars bound to lipids

Characteristics of Brush Border Enzymes

• They are intestinal saccharides
• Produced by enterocytes, have two active sites
• Undergo extensive post-transitional modification

Post-Transitional Modification of Enzymes

• Activation occurs via hydrolysis of a single peptide
• Protection from proteolytic enzymes occurs via the N-terminus
• The N-terminus modifies by attaching a pyroglutamate, decreasing proteolytic enzyme access
• Glycosylation covering the enzymes with sugars, decreases proteolytic enzyme access
• Embedding the enzyme in the glycocalyx

Enzyme/Nutrient Consumption Relationship

• Enzyme secretion increases with increased consumption of the nutrient it digests

Digestion & Insulin

• Increased CHO intake/digestion increases insulin secretion with increased enzyme production
• Insulin is produced due to high blood glucose concentration.

Diabetes Type 1

• Beta cells no longer produce insulin
• Solution: Insulin shot

Diabetes Type 2

• Muscle and adipose no longer responds to insulin.
• Can be caused by genetics or environment and increased risk with increased weight/pattern of food consumption
• Decrease by greater weight loss and exercise, which increases GLUT4 expression by muscle cells

Glucose, GLUT4, and Adipocytes Under Low Blood Glucose Conditions

• Fewer glucose molecules are present in the blood vessel
• GLUT4 is in the adipocyte

Glucose, GLUT4, and Adipocytes Under High Blood Glucose Conditions

• Many glucose molecules are in the blood vessel along with insulin
• Adipocytes line on the edge of the adipocyte
• Glucose is transported from the blood vessel to the adipocyte

High Blood Glucose - Type 1 Diabetics

• Many glucose molecules exist in the blood vessel, but there is no insulin
• GLUT4 remains in the adipocyte and cannot transfer glucose out of blood vessel

High Blood Glucose - Type 2 Diabetes

• Many glucose molecules exist in the blood vessel with insulin
• GLUT4 remains in the adipocyte and doesn't transport glucose out of blood vessel

Glucose Pathways

• Polysaccharides (starch) disaccharides monosaccharides such as glucose, galactose, fructose
• Monosaccharides enter enterocytes
• Monosaccharides pass through enterocytes into the blood
• In the liver, they become glucose
• Then glucose can go three ways: to other tissues via GLUT 1-3, to muscle and adipose via GLUT4, or to the pancreas for insulin production

Insulin Effects

• Increases protein synthesis, glycogen synthesis (store glucose in muscle and liver), and fatty acid synthesis
• Decrease Glucagenesis, where the body is making glucose

Glucagon

• Produced by alpha cells (opposite of insulin)
• Increases glycogen breakdown
• Increases glucagenesis

Microbial Metabolism & Hydrogen Sinks

• Minerals deposit hydrogen to allow microbial metabolism to continue

Hydrogen Sinks Characteristics

• CH4 contains energy
• Propionate contains energy, is glycogenic, and has a greater occurrence in the liver

Acidosis

• Accumulation of lactate in rumen, resulting in pH below 5.5
• Generally occurs in newly arrived feedlot cattle
• Too much corn can cause acidosis
• A pH below 5.5 is bad because an acidic environment will kill off the Protozoa, fungi, and a lot of microbes
• Can cause erosions in the rumen
• Corn Chart (Acidosis): corn(starch) --> glucose --> lactate --> propinate (want it to become propinate) via M. elsdenii

Streptococcus Bovis & M. Elsdenii

• Too much corn lead to buildup of Streptococcus bovis in the transitions from corn to lactate which causes acidosis
• M. elsdenii further decreases pH if not enough gets killed off by the low pH (bad)
• Low pKa (fairly acidic) decrease in ruminal pH kills M. elsdenii results in a further decrease in pH

Low pH Effects

• Ruminal ulcers allow bacteria into the bloodstream (Fusobacterium necrophrum) bacteria into liver liver abscesses

Acidosis Prevention

• Transition cattle from low grain to high grain diet. (approx. 28 day process)
• This allows the population of M. elsdenii to increase so they can convert lactate to propinate
• Decrease SCHO and increase NSCHO overtime
• Feed hay: increases rumination, saliva production, and pH

Ionophores

• Antibiotic that selects for a more favorable microbial population
• Decreases acetate and increases propinate to decrease CH4
• Regulates intake
• Improves protein utilization
• Note: this is detrimental for horses

Bacterial Enzymes for Starch Hydrolysis

Enzyme Function

• Amylolytic bacteria produces amylase
• Alpha-endo enzyme cleaves starch randomly, creating oligosaccharides of various lengths
• Beta exo enzyme creates maltose
• Glucoamylase produces glucose by cleaving 1,4 and 1,6 bonds
• Pullulamase performs debranching and cleaves 1,6 bonds

Starch Breakdown Chart

• Starch is hydrolyzed by BE to form oligosaccharides, which BE then breaks down to maltose, isomaltose, and maltotriose
• BE then breaks these down to Glucose is fermented to VFAs

Starch Fermentation in the Rumen

• About 25% of starch is not fermented in the rumen
• Starch digestion in the small intestine of the pig vs. cow is almost identical
• Increasing processing, such as grinding or cracking, decreases the amount of starch

Undigested Starch

• Available for digestion and fermentation in the Large Intestine

Post-Ruminal Digestion of Starch (Escape Starch)

• Including pancreatic α amylase (ruminants produce a limited amount)
• Brush border enzymes like maltase, isomaltase, and glucamylase
• Absorption occurs via SGLT

Important Enzymes for Digesting Starch

• Pancreatic α amylase
• Maltase
• Isomaltase
• Glucamylase (last 3 are brush border enzymes)

Starch Digestion in Horses

• Horses can digest in the small and large intestine
• Digestion is not favored in the small intestine
• Digestion is favored in the large intestine
• Starch creates VFAs, which give energy
• Starch creates mcp (& mpl and vits.) which gives protein

Starch Digestion in the Small Intestine Pros/Cons

• Glucose absorbed in the small intestine gives more energy than glucose that has undergone fermentation
• Limited capacity to digest the small intestine
• If glucose gets fermented, it's less energetically beneficial, since there is less energy available from VFAs than glucose

Starch Digestion Ranking

• Cow: rumen > small intestine > large intestine
• Horse: small intestine

Benefits of Microbes Fermenting Starch

• Microbes produce mcp (& other products) which can be absorbed post-ruminally to address the animal`s requirements
• The digestion of starch in the small intestine is limited, and undigested starch goes to the Large intestine, which produces VFAs, without the benefit of mcp

Ruminal Responses to Starch

• Increases VFA and mcp production
• Decreases acetate:propinate ratio, CH4 production, ruminal pH, and fiber (SCHO) digestion

Starch production

• Starch increases VFA and MCP production
• mcp inc, because microbes have more energy, which means more microbe reproduction
• Starch fermenters produce more propinate
• More propinate produces glucagenic, which is a requirement for cows, and more glucose favors marbling
• Starch increases propinate so there is less CH4, which leads to less energy lost

Starch Decreases Ruminal pH

• Increases the VFA production, which decreases the pH
• Starch also decreases rumination
• Low pH resulting from starch kills fiber digesting bacteria
• Decreasing the concentration of ammonia in the rumen also prevents the fiber bacteria from growing

Lactose and Sucrose

• Lactose is milk and bypasses the rumen via the esophageal groove, where microbes ferment it instantly
• Ruminants don't produce sucrase since there is no sucrose in the small intestine

Fiber Content Approximation

• Crude Fiber can be misleading because it underestimates how much fiber exists

Detergent Fiber System

• More accurately determines the carbohydrate fraction of feedstuffs based on 3 analyses:
  • Neutral detergent fiber (NDF, cellulose, hemicellulose, ligin)
  • Acid detergent fiber
  • Acid detergent lignin

Fiber-Fermenting Bacteria

• Fibrobacter succinogens
• Ruminococcus albus
• Ruminococcus flavefaciens

Characteristics of Fiber-Fermenting Bacteria

• Growth occurs slowly
• Bacteria dislikes low pH
• Favors acetate production (Inc acetate, inc. CH4)

Locations of Fiber-Fermenting Bacteria

• Cows: reticulorumen and large intestine
• Horses and Pigs: large intestine
• All 3 Animals: highest amount of bacteria in Large intestine (horse > cow > pig)

Cellulose Breakdown Chart

• Cellulose broken down by BE to oligosaccharides then by BE to cellobiose and lastly, BE converts to glucose and is fermented to VFAs, CO2, and CH4

Natural Detergent Fiber (NDF)

• Predicts intake because the animal is filled with fiber
• Composed of hemicellulose, cellulose, and lignin

Acid Detergent Fiber (ADF)

• Predicts digestibility because increased ADF decreases digestibility
• Digestibility decreases because there is more lignin, which is associated with food particles that are very difficult to digest
• Composed of cellulose and lignin

Acid Detergent Lignin (ADL)

• Composed of lignin

Sample Equations

 Hemicellulose = NDF – ADF
 Cellulose = ADF – ADL 
 Lignin = ADL

Cellulose Complex

• The cellulose complex involves six enzymes: exoglucanase, endoglucanase, cellobiohydrolase, cellobiosiclase, cellobiase, and cellodextrinase

Lignin's Impact on Digestibility

• Harder for enzymes to digest/attack cellulose when lignin is present

Enzymes Products and Function

• Exoglucanase produces glucose from cellulose
• Endoglucanase produces disaccharides of various lengths
• Cellobiohydrolase produces cellobiose (two glucoses, disaccharides)
• Cellobiosiclase produces cellobiose
• Cellobiase produces 2 glucose molecules from cellobiose
• Cellodextrinase Cleaves cellulose randomly
• The pH of primal activity for the cellulose complex enzymes is 6.5

SCHO vs. NSCHO Characteristics