Animal Nutrition Topic 5

TOPIC 5 OBJECTIVES

Carbohydrates are a vital component of the diets of omnivores and herbivores. By the end of this week, you will be able to:

1. Summarize the differences between simple and complex carbohydrates.

2. Recognize important dietary carbohydrates.

3. Relate the differences in their chemical structure despite having the same chemical formulas (isomers).

4. Distinguish between complex carbohydrates of plant and animal origin.

5. Analyze structural differences that contribute to complex carbohydrate digestibility.

6. Compare and contrast carbohydrate absorption efficiency and circulating glucose concentrations.

CARBOHYDRATES & WHAT THEY ARE

• Carbohydrates are formed via Photosynthesis, a process that converts solar energy into chemical energy using only carbon dioxide (CO2), water (H2O), and solar radiation.

Main Structural Components of Carbohydrates

• Composition: Carbohydrates primarily consist of Carbon (C), Hydrogen (H), and Oxygen (O).

• In plants, carbohydrates compose a significant portion of their structural framework:

  • Lignin and Cellulose are key structural components.

  • More than 70% of Dry Matter (DM) in forages is carbohydrates.

  • More than 80% of DM in grains is carbohydrates.

• In contrast, in animals, carbohydrates make up a much smaller proportion of total weight:

  • Glucose and glycogen (CHO) represent approximately 1% of total weight.

Classifying Carbohydrates

Classification Breakdown:

• Simple Carbohydrates

  • Monosaccharides:

  • Glucose

  • Fructose

  • Galactose

  • Disaccharides:

  • Maltose

  • Sucrose

  • Lactose

• Complex Carbohydrates

  • Oligosaccharides:

  • Mannan-OS

  • Fructo-OS

  • Raffinose

  • Polysaccharides:

  • Starch

    • Amylose

    • Amylopectin

  • Fiber

    • Pectin

    • Hemicellulose

    • Cellulose

  • Glycogen

Simple Carbohydrates

• Monosaccharides (1 saccharide):

  • Examples include Glucose, Fructose, and Galactose.

• Disaccharides (2 saccharides):

  • Include Maltose, Sucrose, and Lactose.

• Simple carbohydrates are classified according to the number of carbon atoms per molecule, are rapidly digested and absorbed, and serve as the principal energy source in animal metabolism.

Sugar Molecules (Saccharides)

• Glucose molecule formula: C6H12O6

• Saccharides are sub-divided into the following groups based on the number of carbons:

  • Trioses: 3 carbons

  • Tetroses: 4 carbons

  • Pentoses: 5 carbons

  • Hexoses: 6 carbons

• In their most stable forms, monosaccharides predominantly exist as ring structures (either pyranose or furanose) that link together to form all other carbohydrate molecules.

Monosaccharide Structures

• Glucose Structure:

  • CH2OH HO OH OH OH

• Fructose Structure:

  • CH2OH OH OH OH O

• Galactose Structure:

  • HOCH2 OH HO OH

• Notably, glucose and galactose have similar structures, with the primary difference being the orientation of the hydroxyl (OH) group at the fourth carbon.

• Fructose, while having the same chemical formula as glucose (C6H12O6), has a distinct ring structure.

• The ratio of Carbon, Hydrogen, and Oxygen in monosaccharides is 1:2:1.

• Over 70 monosaccharides have been identified; however, glucose, fructose, and galactose are the most commonly referenced dietary monosaccharides.

Organic Chemistry Concepts Related to Carbohydrates

1. Functional Groups

• Functional groups are defined as specific clusters of atoms within a molecule that determine its chemical behavior.

2. Chirality

• Chirality: A chiral carbon is defined as a carbon atom with four different groups attached. This leads to the creation of stereoisomers, which are molecules that contain the same atoms but differ in their 3D arrangement (analogous to left and right hands—mirror images that cannot be superimposed).

Monosaccharide Isomers

• Examples of monosaccharide isomers:

  • D-glucose

  • D-fructose

  • L-fructose

• D-fructose is an example of a ketone, while D-glucose is an aldehyde.

• Chiral Carbon Details:

  • The structure includes four different groups attached to one of the chiral carbons, classifying it as an isomer.

  • Chemical structure isomers are categorized as D or L. Enzymes in the body specifically recognize the D isomer of carbohydrates.

Confirmation of Glucose

• The different forms of glucose, including:

  • Alpha-glucose

  • D-Glucose

  • L-Glucose

  • Beta-glucose

• Glucose can exist in both linear (chain) and cyclic forms.

Disaccharides

• Disaccharides are formed by the linkage of two monosaccharide units:

  • Maltose: Formed by the linkage of two glucose units. It is considered the rarest of disaccharides in nature and is found in beer.

  • Sucrose: Formed by the linkage of glucose and fructose; commonly known as table sugar.

  • Lactose: Formed by the linkage of galactose and glucose, known as milk sugar and is predominantly found in milk.

• Each disaccharide contains one glucose molecule.

• Unique Characteristics of Lactose:

  1. More slowly digested, providing a steady release of glucose and minimizing glucose spikes in the bloodstream.

  2. Reduces digestive upset.

  3. Promotes the growth of beneficial bacteria in the hindgut.

  4. Enhances mineral absorption, particularly calcium and phosphorus.

• The physiological impacts of lactose digestion are attributed to a lower activity of lactase compared to other carbohydrate-digesting enzymes, leading to longer retention times of lactose in the digestive tract.

Glycosidic Bonds

• Glycosidic bonds are covalent bonds that connect one sugar molecule to another.

• They are named based on two criteria:

  1. The configuration of the carbons involved in the linkage (α vs. β).

  2. The carbon position (e.g., 1→4 bond).

Formation of Glycosidic Bonds

• The condensation of two glucose molecules can result in an α1-4 glycosidic bond (where the bond between carbons 1 and 4 is facing downward) or a β1-4 glycosidic bond (illustrated with a zig-zag configuration).

  • Example Structures:

  • α-bond:

    • Structure defined as being digestible by mammals.

  • β-bond:

    • Structure defined as being digestible by microbes.

Complex Carbohydrates

• Oligosaccharides: (composed of 3-10 saccharides) include tapioca, mannan-OS, raffinose, and fructo-OS.

• Polysaccharides: (composed of >10 saccharides) include starch (amylose and amylopectin) and dietary fiber (cellulose, hemicellulose, pectin).

• Complex carbohydrates serve essential functions in diets.

Polysaccharides: Starch

• Consists of two forms, both featuring alpha linkages, primarily broken down by the enzyme amylase.

• Starch exists in coiled structures with branching points, comprising:

  • α1-4 linkages for chain components.

  • α1-6 linkages at branching points.

Polysaccharides: Animal Forms

• Glycogen is analogous to starch in plants:

  • Plants typically contain amylose (25-30%) and amylopectin (70-75%).

  • In animals, glycogen comprises approximately 1% of muscle weight and 5% of liver weight.

Fiber Components in Plant Structures

• Cellulose, Pectin, Hemicellulose:

  • These are carbohydrates forming part of the plant cell walls, collectively referred to as fiber.

  • Pectin: Generally soluble, more straightforward than other forms, acts as a plant "cement."

  • Hemicellulose: More complex than pectin, interspersed with lignin/cellulose, and is more digestible than cellulose.

  • Cellulose: Composed of long chains of glucose, primarily fermented in the foregut.

Lignin

• Lignin is a biopolymer that associates with cellulose and hemicellulose.

• Characteristics:

  • It is considered indigestible and reduces the digestibility of carbohydrates by coating cellulose and associating with hemicellulose.

  • Lignin increases with plant age and maturity, inversely affecting protein and digestibility—older plants typically are higher in lignin.

• Plant Species Comparison:

  • Trees have high lignin content.

  • Vegetables and cereals have intermediate levels of lignin.

  • Grasses have lower levels than legumes.

Mnemonics for Understanding Glycosidic Bonds

• Alpha vs. Beta:

  • Alpha = Anchor (sinks down)

  • Beta = Bubbles or Balloon (floats up)

  • Alpha = A town down; people bow down to the alpha.

Plant Fiber Composition Variability

• The composition of plant carbohydrates varies based on:

  • Species: Legumes contain more protein and lignin compared to grasses.

  • Variety: Low lignin varieties like alfalfa or brown midrib corn.

  • Maturity: Younger plants tend to have higher protein and lower cellulose content.

Nutritional Value in Terms of Digestibility

• Carbohydrates exist in categories within plant content:

  • Nonstructural Carbohydrate (NSC) includes:

  • Monosaccharides

  • Oligosaccharides

  • Starches

  • Structural components include:

  • Cell wall: Pectin, hemicellulose, cellulose.

• Question: Which class is more digestible?

• Conclusion: Nutritional value correlates to digestibility.

Proximate Analysis of Feed Samples

• Dry matter is analyzed by separating:

  • Inorganic material (ash)

  • Organic material (through various tests) such as:

  • Crude protein

  • Ether extract

  • Crude fiber

  • Nitrogen-free extract

  • Moisture.

• Carbohydrates are categorized within various fractions in this analysis.

Van Soest Method for Forage Analysis

• The feed sample is subjected to boiling in different solvents to separate fiber types, resulting in detailed breakdowns:

  • Neutral Detergent Fiber (NDF): Includes hemicellulose, cellulose, lignin.

  • Neutral Detergent Solubles: Includes soluble CHO, starch, CP, fat.

  • Acid Detergent Fiber (ADF): Contains cellulose, lignin.

  • Lignin Synthesis: Through boiling with a concentrated H2SO4 solution, hemicellulose is dissolved.

Detergent Fiber System

• This system allows for a descriptive breakdown of fiber consisting of:

  • NDF: hemicellulose, cellulose, lignin

  • ADF: cellulose, lignin

• Useful for assessing forage quality and digestibility in ruminant diets, with ADF related to crude fiber (CF) found on feed tags.

Comparison of Carbohydrates: Cellulose vs. Starch vs. Glycogen

Complex Carbohydrates in Animals

• In animals, complex carbohydrates play key roles, such as:

  • Glycoproteins: Components of mucous secretions in the intestine and synovial fluid, functioning as cell surface receptors and hormones.

  • Glycosaminoglycans: Essential for joint health and part of mucous secretions in the intestine.

  • Glycolipids: Found in the cell membrane, especially in nerve tissue, significant for cell-to-cell signaling.

Dietary Distribution of Carbohydrates in Animals

• Carbohydrate intake varies across different species and life stages, summarized below:

• Values indicate upper estimates based on common feeding practices, with variations expected depending on individual diets.