Carbohydrates and Glycosidic Bonds

• Carbohydrates were once thought to be carbon and water, now known as polyhydroxylated compounds including aldehydes, ketones, and derivatives.
• The presence of multiple hydroxyl groups gives carbohydrates their water solubility.
• Some carbs have amines (NH2) and carboxyl groups (COOH).

Carbohydrate Categories

• Non-fiber carbohydrates can be degraded by an animal's endogenous enzymes.
• Fibers are long-chain carbohydrates not degraded by animal enzymes but fermentable by microbes like bacteria, yeast, and fungi.
• Fibers are found in the cell walls of plants and include cellulose, hemicelluloses, pectin, gums, mucilage, and algal polysaccharides.
• Fiber encompasses lignin, a non-carbohydrate polyphenolic compound that is indigestible and plays a role in plant structure.

Glycosidic Bonds

• Alpha (α) bonds form when the bond originates from the anomeric carbon in the α-configuration.
• Beta (β) bonds form when the bond originates from the anomeric carbon in the β-configuration.
• Animal enzymes can hydrolyze α-glycosidic bonds but generally not β-glycosidic bonds, except when the first sugar is galactose.
• Microbial enzymes from bacteria, fungi, and yeast can efficiently degrade β-linkages.

Starch

• Starch consists of long chains of glucose units in two forms.

Amylose

• Amylose is a linear, unbranched chain of 200–300 glucose units.
• Amylases can only degrade starch from the ends of the chains; amylose degrades slowly.

Amylopectin

• Amylopectin is a highly branched starch with short chains of 24-30 glucose units attached at regular intervals.
• Amylopectin is more easily degraded by amylases due to its branching, leading to faster breakdown than amylose.

Starch Granules

• In grains, starch is stored in starch granules, embedded in a protein matrix, consisting of alternating layers of crystalline and amorphous starch.
• A-type crystals are degraded faster, B-type more slowly, and C-type are intermediate.
• The structure of starch granules influences the rate of starch degradation.
• Smaller granules are degraded faster.
• Granules with pores are degraded more rapidly.
• Starch with higher amylopectin content degrades faster.
• Denser granules are degraded more efficiently.
• A-type crystals are more easily hydrolyzed.

Starch Digestibility

• Starch digestibility can be improved through cooking via gelatinization, where water and heat cause granules to swell and lose their crystalline structure, becoming solubilized.
• Retrogradation occurs as starch cools, with amylose and amylopectin recrystallizing, making the starch less digestible.
• Resistant starch becomes resistant to amylase activity and is degraded more slowly, influenced by starch type, cooking conditions, storage time, and moisture content.

Starch and Animal Health

• Starch digestibility and degradability regulate the glycemic, insulinemic response, and starch fermentation in the digestive tract.
• The impact of starch digestibility varies between species.
• In monogastrics, starch digestibility affects glycemic and insulinemic responses; highly digestible starch leads to rapid glucose and insulin increases.
• It is essential to regulate starch intake in species like pets and horses to avoid metabolic disorders.
• In ruminants, starch digestibility is linked to fermentation dynamics in the rumen; rapid fermentation can cause rumen acidosis.
• In this condition the fermentation of digestible starch produces short-chain fatty acids (SCFAs) faster than the rumen can buffer them.
• Managing starch digestion requires balancing digestible, resistant, and total starch.
• Equids and lagomorphs rely on enzymatic digestion in the small intestine and microbial fermentation in the large intestine.

Fibers

• As grass matures, hemicellulose, cellulose, and lignin increase, decreasing digestibility.
• These are classified as dietary fibers, where lignin is not a carbohydrate.
• Lignin impacts fiber digestibility by binding to hemicellulose and cellulose.
• Animal enzymes and microbial populations cannot break down lignin, but some ruminal fungi can partially separate lignin from cellulose.
• Fiber-rich forages become difficult to digest as fiber content rises; animal enzymes cannot hydrolyze fiber, and microbial digestion is time-consuming.
• Higher fiber content in grass lowers overall digestibility and energy availability.

Fiber Analysis Methods

• The Weende method is the official method of determining fiber content on feed labels; it involves treating the sample with acid and base but underestimates total fiber content.
• The Van Soest method is a three-step procedure offering a detailed classification of fiber fractions.
• Neutral Detergent Fiber (NDF) uses a neutral detergent and amylase to extract hemicellulose, cellulose, and lignin; it is associated with forage bulkiness and affects rumination time, feed intake, and digestibility.
• Acid Detergent Fiber (ADF) uses an acid detergent to extract cellulose and lignin, removing hemicellulose; it correlates with diet digestibility.
• Acid Detergent Lignin (ADL) uses sulfuric acid to isolate lignin, the indigestible fiber fraction.
• The Prosky method differentiates between soluble and insoluble fibers (NDF); soluble fibers modulate digestion and gut health, while insoluble fibers contribute to intestinal motility and bulk.
• Ruminants digest fibers through microbial fermentation and rumination; microbiota produces enzymes that break β-glycosidic bonds.
• Rumination involves regurgitating and rechewing ruminal content to reduce particle size and increase surface area.
• During fiber fermentation, microbiota produces short-chain fatty acids (SCFAs), primarily acetic, propionic, and butyric acids, serving as a major energy source.
• Carbon dioxide and hydrogen are released as byproducts, converted to methane by methanogenic archaea.
• Acetic and butyric acids are converted into fatty acids for milk production; propionic acid is metabolized into glucose.
• Acetic and butyric acids are produced by fibrolytic bacteria, while propionic acid derives from amylolytic bacteria
• The production of acids also results in methane release.

Fiber Classification

Solubility

• Some fibers dissolve in water.
• Soluble fibers are easier to degrade.
• Insoluble fibers are preferred for ruminants and herbivorous monogastrics, while soluble fibers are favored in non-herbivorous monogastrics.

Viscosity

• Viscosity is the ability to form a gel through water retention.
• Viscous fibers normalize digestion, slowing transit and softening stools.

Water retention

• Some fibers swell in the presence of water.
• In animals like horses, fibers with high water retention can lead to choke unless hydrated first.

Ionic exchange capacity

• Some fibers adsorb minerals, decreasing digestibility and bioavailability.

Filter effect

• Some high-viscosity fibers filter between nutrients, enzymes, and gut transporters.

Fermentescibility

• Some fibers are more quickly fermented.
• In ruminants and herbivorous monogastrics, highly fermentable fibers limit acidosis risk, while non-herbivorous monogastrics benefit from them for butyric acid production.