Carbohydrate Types & Breakdown

Structural Carbohydrates (SCHO)

• Examples include cellulose, hemicellulose, pectin, and fiber
• Mammalian enzymes cannot hydrolyze SCHO

Non-Structural Carbohydrates (NSCHO)

• Examples include starch and sugars
• Mammalian enzymes can hydrolyze NSCHO

Carbohydrate Breakdown: Bacterial/Microbial Enzymes (BE)

• NSCHO is converted to monomers or monosaccharides
• Microbial enzymes facilitate the breakdown of monomers
• Fermentation yields VFAs, CO2, and CH4

Carbohydrate Breakdown: Mammalian Enzymes (ME)

• NSCHO becomes monomers or monosaccharides such as glucose, galactose, and fructose
• Mammalian enzymes break down monomers
• Absorption occurs in the small intestine
• Used for respiration

Monosaccharides

• Simple sugars, are composed of a single sugar molecule
• Examples are glucose, galactose, and fructose
• Animals absorb monosaccharides from the small intestine
• Mammals use monosaccharides for energy
• Microbes ferment them into VFAs, CO2, and CH4
• VFAs provide animals with energy

Disaccharides

• Sugars consists of two sugar molecules
• Examples include lactose, sucrose, maltose, and cellobiose
• Lactose is glucose + galactose
• Sucrose is glucose + fructose
• Maltose is glucose + glucose (alpha bond), a product of starch digestion
• Cellobiose is glucose + glucose (beta bond)
• BE can digest lactose, sucrose, maltose, and cellobiose
• ME can digest lactose, sucrose, and maltose, but not cellobiose

Polysaccharides

• Complex carbohydrates composed of multiple sugar molecules
• Starch, cellulose, hemicellulose, and pectin are examples

Starch

• A polymer of glucose linked by alpha-linkages
• A homopolymer, composed of just glucose
• Digestible by both BE and ME
• Functions as a storage polysaccharide

Cellulose

• A polymer of glucose linked by beta-linkages
• A homopolymer that is composed of just glucose
• Digestible by only BE

Hemicellulose

• A heteropolymer
• Digestible by BE
• Consists of glucose, xylose, mannose, galactose, arabinose, and rhamnose

Pectin

• A heteropolymer that contains arabinose and galactose
• It is digestible by BE

Starch Structure: Amylose

• Digestion breaks down (hydrolyzes) maltose structure
• Has an alpha linkage between C1 and C4 (the oxygen)
• Alpha bonds create a coiled shape, exposing them for easier hydrolysis
• The linkages are alpha-1,4
• Length is ~100 glucose molecules

Starch Structure: Amylopectin

• Digestion breaks down (hydrolyzes) maltose structure
• Has an alpha linkage between C1 and C4 (the oxygen)
• Alpha bonds create a coiled shape, exposing them for easier hydrolysis
• The linkages are alpha-1,4 and alpha-1,6-linkages (a-1,6 are branch points)
• Approx 10,000-100,000 glucose molecules in length
• Similar to glycogen

Cereal Grains

• Typically contain 15-20% amylose
• Typically contain 80-85% amylopectin

Glycogen

• It is the stored form of glucose
• Found in muscle and liver tissue
• Glucagon mobilizes glycogen
• Increases blood glucose concentration

Cellobiose Structure

• Composed of two glucose molecules with a β(1, 4) glycosidic bond
• Only broken down (hydrolyzed) by BE
• Mammalian enzymes cannot break it down 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, which hydrolyzes α − 1, 4 bonds

Digestion in Monogastrics (Horse/Pig) - Stomach

• Has an acidic environment, meaning a low pH
• Inactivates salivary amylase

Digestion in Monogastrics (Horse/Pig) - Small Intestine

• Pancreatic alpha amylase is secreted
• Pancreatic alpha amylase cleaves 2 glucose molecules at a time
• Requires at least 5 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

• Examples include glucoamylase, maltase, isomaltase, sucrase, lactase, and phlorizin hydrolase

Enzyme 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) 1 glucose 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

Brush border enzymes

• Intestinal saccharides
• Produced by enterocytes
• Have 2 active sites
• Undergo extensive post-transitional modification

Post-Transitional Modification of Enzymes

• Activation done through hydrolysis of a single peptide
• Protection from proteolytic enzymes with N-terminus
• Modification can attach a pyroglutamate to decreases proteolytic enzyme access
• Glycosylation covers the enzymes with sugars and decreases proteolytic enzyme access
• Embedding the enzyme in the glycocalyx

Enzyme/Nutrient Consumption Relationship

• Secretion of an enzyme increases with increased consumption of the nutrient that it digests

Digestion & Insulin

• Increased CHO intake/digestion increases insulin secretion
• Insulin secretion increases with increased enzyme production
• Produced in response to high blood glucose concentration

Diabetes Type 1

• Beta cells no longer produce insulin
• Solution: insulin shots

Diabetes Type 2

• Muscle and adipose tissue no longer respond to insulin
• Caused by genetics or environment
• Increased risk with increased weight and specific food consumption patterns
• Decreasing the risk requires weight loss and exercise
• Needs increased expression of GLUT4 by muscle cells

Glucose, GLUT4, and Adipocytes - Low Blood Glucose

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

Glucose, GLUT4, and Adipocytes - High Blood Glucose

• Many glucose molecules and insulin in the blood vessel
• Adipocytes lined on the edge of an adipocyte
• Glucose being transported from the blood vessel to the adipocyte

Glucose, GLUT4, and Adipocytes - High Blood Glucose (Type 1 Diabetes)

• Many glucose molecules in the blood vessel, but no insulin
• GLUT4 remains in the adipocyte, unable to transfer glucose out of the blood vessel

Glucose, GLUT4, and Adipocytes - High Blood Glucose (Type 2 Diabetes)

• Many glucose molecules and insulin in the blood vessel
• GLUT4 remains in the adipocyte, not transporting glucose out of the blood vessel

Glucose Pathways

• Polysaccharides (starch) become disaccharides, then monosaccharides (glucose, galactose, fructose)
• Monosaccharides enter enterocytes
• Pass through enterocytes into the blood
• Goes to the liver and becomes glucose
• Glucose has 3 pathways: to other tissues (GLUT 1-3), to muscle and adipose (GLUT4), and to the pancreas for insulin production

Insulin Effects

• Increases protein synthesis, glycogen synthesis (glucose storage in muscle and liver), and fatty acid synthesis
• Decreases glucagenesis, where the body is making glucose

Glucagon

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

Microbial Metabolism & Hydrogen Sinks

• Minerals deposit hydrogen to allow microbial metabolism to continue

Hydrogen Sinks

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

Acidosis in Ruminants

• Accumulation of lactate in the rumen, resulting in a pH below 5.5
• Generally occurs in newly arrived feedlot cattle
• Too much corn can cause this
• A pH below 5.5 is bad because the acidic environment kills protozoa, fungi, and many microbes
• Can also cause erosions in the rumen
• Corn (starch) becomes glucose, then lactate, and then propinate (requires M. elsdenii)

Streptococcus Bovis & M. Elsdenii

• Too much corn can cause a buildup of Streptococcus bovis in the transitions
• Can cause build up of lactate, which causes acidosis.
• M. elsdenii further decreases pH when not enough are killed off by the low pH
• Low pKa decreases ruminal pH, kills M. elsdenii, and results in a further pH decrease

Low pH Effects

• Ruminal ulcers allow bacteria into the bloodstream (Fusobacterium necrophrum), which leads to the liver and liver abscesses

Acidosis Prevention

• Transition cattle from low to high grain diet over ~28 days
• This increases the population of M. elsdenii to convert lactate to propinate
• Decrease SCHO and increase NSCHO overtime
• Feed hay to increases rumination, saliva production, and pH

Ionophores

• Antibiotics that select for a more favorable microbial population.
• Decreases acetate and increases propinate (decreases CH4)
• Regulate intake
• Improves protein utilization
• Not good for horses

Bacterial Enzymes for Starch Hydrolysis

• Amylolytic bacteria produces amylase
• Alpha-endo enzyme cleaves starch randomly, creating oligosaccharides
• Beta exo enzyme creates maltose
• Glucoamylase produces glucose and cleaves 1,4 and 1,6 bonds
• Pullulamase debranches and cleaves 1,6 bonds

Starch Breakdown

• Starch is hydrolyzed by BE into oligosaccharides
• BE breaks down to maltose, isomaltose, and maltotriose
• BE breaks down to glucose
• Glucose is fermented into 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 and cow is almost identical
• Increasing processing (grinding, cracking, etc.) decreases the starch amount in the small intestine.
• It is available for digestion and fermentation in the Large Intestine

Post-Ruminal Digestion of Starch (Escape Starch)

• Includes pancreatic α amylase (ruminants produce limited amount)
• Brush border enzymes (maltase, isomaltase, glucamylase)
• Absorption via SGLT1

Important Enzymes for Digesting Starch

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

Starch Digestion in Horses

• Digestion is possible in both the small and large intestine
• Not very good at digesting it in the small intestine
• Starch creates VFAS and gives energy
• Starch creates mcp (mpl and vits.) which give protein

Starch Digestion in the Small Intestine Pros/Cons

• PRO: glucose that is absorbed in the small intestine provides more energy than fermented glucose
• CON: capacity to digest starch in the small intestine is limited Glucose, when fermented, is less energetically beneficial to the animal. 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 and other products which can be absorbed post-ruminally to address the animal’s requirements
• Starch digestion in small intestine is limited, and undigested starch goes to the Li (produces VFAs) without the benefit of mcp.

Ruminal Responses to Starch

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

Starch Increases VFA and MCP Production

• mcp inc. bc microbes have more energy, which increases microbe reproduction

Starch Decreases the Acetate:Propinate Ratio

• Starch fermenters produce more propinate.
• Propinate is glucagenic and a requirement for cows and more glucose favors marbling

Starch Decreases CH4

• Starch increases propinate, therefore decreases CH4
• Less CH4 lost due to CH4 being a greenhouse gas

Starch Decreases Ruminal pH

• Increases VFA production, lowers pH
• Decreases rumination
• Decreases pH which kills fiber digesting bacteria
• Decreasing ammonia concentration in the rumen also prevents fiber bacteria from growing.

Lactose and Sucrose

• Lactose is in milk and bypasses the rumen using the esophageal groove
• Microbes ferment lactose instantly
• There is no sucrose in the small intestine, meaning most ruminants don't produce sucrase

Fiber Content

• Crude fiber can be misleading and underestimates the amount of fiber

Detergent Fiber System

• More accurately identifies the carbohydrate fraction of feedstuffs, based on 3 analyses
• Neutral detergent fiber (NDF, which includes cellulose, hemicellulose, and lignin)
• Acid detergent fiber
• Acid detergent lignin

Fiber-Fermenting Bacteria

• Include Fibrobacter succinogens, Ruminococcus albus, and Ruminococcus flavefaciens
• Grow slow
• Dislike low pH
• Favor acetate production (increases acetate, increasing CH4)

Fiber-Fermenting Bacteria Locations

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

Cellulose Breakdown

• Cellulose is broken down by BE to oligosaccharides
• BE breaks down to cellobiose
• BE breaks down to glucose
• Fermented to VFAs, CO2, and CH4

Natural Detergent Fiber (NDF)

• Predicts intake (the animal is filled with fiber)
• Made of hemicellulose, cellulose, and lignin

Acid Detergent Fiber (ADF)

• Predicts digestibility (as ADF increases, digestibility decreases)
• Digestibility is decreased because there is more lignin, which is hard to digest
• Products are cellulose and lignin

Acid Detergent Lignin (ADL)

• Products are lignin

Sample Equations

• Hemicellulose: NDF – ADF = hemicellulose
• Cellulose: ADF – ADL = cellulose
• Lignin: ADL = lignin

Cellulose Complex (6 Enzymes)

• exoglucanase
• endoglucanase
• cellobiohydrolas
• cellobiosiclasee
• cellobiase
• cellodextrinase

Lignins Impact on Digestibility

• Harder to digest because Lignins are present
• 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
• A pH of 6.5 is required for Cellulose complex enzymes

SCHO Characteristics (Structural)

• Examples are grasses, stems, and leaves
• Not digestible by ME
• Digestible by BE
• Supports Microbes
• Generally less digestible, with fewer VFAs, requiring more rumination
• Grow slow and produce more acetate and CH4
• pH Like is near neutral

NSCHO Characteristics (Non-Structural)

• Examples are grains and seeds
• Digestible by ME
• Not digestible by BE
• Supports microbes
• More digestible, with more VFAs, requiring less rumination
• Grow fast and produce less acetate and CH4
• pH Like is tolerant of pH