Animal Nutrition: Carbohydrates

Carbohydrates

• Carbohydrates have the formula (CH₂O)n.
• They consist of carbon, hydrogen and oxygen.
• They provide energy and compose 70-80% of an animal's diet.
• Carbohydrates have to be provided daily in an animal’s diet.

Carbohydrate Types

• Simple carbohydrates include Monosaccharides (single sugars) and Disaccharides (double sugars).
• Complex carbohydrates include Polysaccharides (many sugars).

Monosaccharides: Simple Carbohydrates

• Monosaccharides are hexoses (six-carbon sugars) or pentoses (five-carbon sugars).
• Glucose, fructose, galactose, and mannose all have the formula C6H12O6

Glucose

• Glucose is also known as dextrose or blood sugar.
• It is the body’s primary fuel.
• It is found in all disaccharides and polysaccharides.
• Only fructose and sucrose are sweeter.
• It occurs in nature combined with other compounds.

Fructose

• Fructose is also known as fruit sugar.
• It is found in fruit, honey, and syrup.
• Fructose converts to glucose in the body.

Galactose

• Galactose is part of lactose.
• It is found in milk.
• Galactose converts to glucose in the body.
• It is found in combination with lipid in nervous tissue.

Disaccharides: Simple Carbohydrates

• The disaccharides are sucrose, maltose and lactose.

Sucrose

• Sucrose is commonly known as table sugar.
• Glucose plus fructose make sucrose.
• Sucrose is refined from sugar beets and sugar cane.

Lactose

• Lactose is milk sugar.
• Glucose plus galactose make lactose.
• Lactose intolerance is caused by a missing digestive enzyme required to split lactose into monosaccharides for absorption.

Maltose

• Maltose is malt sugar.
• Glucose plus glucose make maltose.
• It is found in germinating seeds.
• Maltose is used in fermentation to produce malted beverages.

Polysaccharides: Complex Carbohydrates

• The polysaccharides are glycogen, starches, and fibers.

Glycogen

• Glycogen is a long chain of glucose found in animals.
• It is stored in the liver and muscles.
• Glycogen helps maintain blood glucose and is a source of quick energy, especially during exercise (lasting only 12 hours).

Starch

• Starch is a long chain of glucose found in plants.
• It can be found in cereal grains, legumes, and root vegetables.

Fiber

• Fiber is mostly indigestible carbohydrate, such as lignin.
• It is a component of plant cell walls.
• Fibers are classified according to solubility in water.
• Fiber is abundant in wholegrains, legumes, and fruits.

Types of Fiber

• Insoluble fibers are non-viscous, e.g., cellulose, lignin.
• Soluble fibers are viscous and fermentable, e.g., pectins.

Carbohydrate Digestion (Monogastrics)

• Nitrogen-free extract (NFE) refers to soluble carbohydrates like starches and sugars.
• Enzymatic digestion breaks down NFE into hexoses like glucose in monogastric animals.
• NFE breakdown occurs mostly in the small intestine.

Digestion in the Mouth

• Carbohydrate digestion starts in the mouth.
• Salivary glands secrete α-amylase (ptyalin) which initiates the hydrolysis of starch.
• Salivary α-amylase acts on dietary starch during mastication, randomly breaking α-(1 → 4) bonds and hydrolyzing starch into dextrins.

Digestion in the Stomach

• Carbohydrate digestion halts temporarily in the stomach.
• High acidity inactivates salivary α-amylase.

Digestion in the Intestine

• Further digestion of carbohydrates by pancreatic enzymes occurs in the small intestine.
• The two phases of intestinal digestion are due to pancreatic α-amylase and to intestinal enzymes: sucrase, maltase, lactase, isomaltase.

Role of Pancreatic α-Amylase

• Pancreatic α-amylase degrades dextrins further into a mixture of maltose, isomaltose, and α-limit dextrin.
• α-limit dextrins are small oligosaccharides containing 3 to 5 glucose units.

Role of Intestinal Enzymes

• A final phase of carbohydrate digestion occurs in the brush-border membrane.
• Maltase acts on Maltose to produce Glucose + Glucose.
• Isomaltase acts on Isomaltose to produce Glucose + Glucose.
• Sucrase acts on Sucrose to produce Glucose + Fructose.
• Lactase acts on Lactose to produce Glucose + Galactose.
• Dextrinase acts on a-Limit dextrin to produce Glucose + Maltose.

End Products of Carbohydrate Digestion

• Glucose, fructose, and galactose are the end products.
• They are readily absorbed through the intestinal mucosal cells into the bloodstream.

Absorption of Carbohydrates

• Carbohydrates are absorbed as monosaccharides from the intestinal lumen.
• Active transport goes against a concentration gradient, i.e., from low to high glucose concentration.
• Facilitative transport goes with a concentration gradient, i.e., from high to low concentration.

Active Transport

• Glucose and galactose are transported across the brush border membrane of mucosal cells via active transport.
• Active transport requires energy, a specific transport protein, and the presence of sodium ions.

Sodium Dependent Glucose Transporter (SGLT-1)

• A sodium-dependent glucose transporter (SGLT-1) binds glucose and Na+ at separate sites.
• It transports them through the plasma membrane of the intestinal cell.

Sodium Transport

• Na+ is transported down its concentration gradient from high to low concentration, while glucose is transported against its concentration gradient.

Active Transport Energy

• Free energy required for active transport is obtained from ATP hydrolysis linked to a sodium pump.
• The sodium pump expels Na+ from the cell in exchange for K+.

Facilitative Transport

• Fructose and mannose are transported across the brush border by a Na+-independent facilitative diffusion process (GLUT-5).
• Glucose and galactose can use this transport if the concentration gradient is favorable.

Carbohydrate Transport to the Liver

• The sodium-independent transporter, GLUT-2, facilitates the transport of sugars out of the mucosal cells.
• This allows sugars to enter the portal circulation for transport to the liver.

Carbohydrate Digestion (Ruminants)

• Ruminant diets contain cellulose, hemicellulose, and lignin.
• Enzymes secreted by rumen microbes break down complex carbohydrates.
• This results in VFAs and gases.

Carbohydrate Digestion in Ruminants

• It occurs through microbial fermentation in the rumen.
• Plant material ferments into simple sugars which microbes utilize and produce waste products.
• The waste products are gases, heat, and VFAs.
• Ratios of VFAs depend on the diet type consumed.

Composition of VFAs and Gases in the Rumen

• VFAs contain 60-70% acetic acid, 15-20% propionic acid, and 10-15% butyric acid.
• Gases in the rumen contain 76% carbon dioxide, 22% methane, and 2% hydrogen.
• Small amounts of oxygen and nitrogen are from ingested air.
• VFAs are absorbed directly from the rumen, reticulum, and abomasum.

Microbial Populations in the Rumen

• Cellulolytic: Ferment plant fibre (cellulose).
• Pectinolytic: Ferment plant fibre (pectins).
• Hemicellulolytic: Ferment plant fibre (hemi-cellulose).
• Amyolytic: Ferment starch.
• Ureolytic: Ferment urea.
• Proteolytic: Ferment proteins.

Cellulolytic Bacteria

• Cellulolytic bacteria produce cellulose to break down cellulose which is the primary substrate.
• They mainly produce acetate, but also some propionate and little butyrate.
• They prefer a pH of 6-7.
• They predominate in animals fed roughage diets.

Amylolytic Bacteria

• Amylolytic bacteria digest starches and sugars.
• They prefer a pH of 5-6.
• They mainly produce propionate, less butyrate, and sometimes lactate.
• They predominate in animals fed grain diets.
• A rapid change to a grain diet can cause lactic acidosis (rapidly decreases pH)
• Streptococcus bovis is an example of this type of bacteria.

Role of the Small Intestine

• The small intestine is responsible for the secretion of digestive enzymes.
• It allows digestive secretions from pancreas and liver.
• It enables further digestion of carbohydrates.
• It facilitates the absorption of H₂O, minerals, amino acids, and glucose.

Role of the Large Intestine

• The large intestine containing the Caecum enables bacterial population to ferment unabsorbed products of digestion.
• It enables absorption of H₂O and VFA and the formation of faeces.