FIBROUS CARBOHYDRATES

All foods (fodder or concentrates) are made up of different fractions: carbohydrates, proteins, lipids, vitamins, minerals and water. From these different food fractions, carbohydrates represent the most important energy source and the main precursors for the fat and sugar (lactose) found in milk.

Rumen microorganisms enable the host to obtain energy from fibrous carbohydrates (cellulose and hemicellulose):

Fiber in general, tends to be bulky and is retained in the rumen where it ferments slowly. Serving as a support for rumen microorganisms (MOOs)

There are different fiber qualities depending on the type and place of growth of the plant as well as its age.

As the plant matures, lignin content in fiber increases while the degree of ruminal fermentation of both cellulose and hemicellulose is reduced.

Lignification in plants does not only occur due to aging as it also depends on climate. For example, under increased heat conditions lignification also increases. The presence of fiber in long particles is necessary to stimulate rumination.

Rumination increases fiber separation and fermentation, stimulates rumen contractions, and increases the flow of saliva into the rumen.

Saliva contains bicarbonate and phosphates which contribute to maintaining the acidity (pH) of rumen content and pH values close to neutrality.

NON-FIBROUS CARBOHYDRATES

Non-fibrous carbohydrates (starches and sugars) ferment rapidly and in a large percentage within the rumen.

The presence of non-fibrous carbohydrates increases dietary energy density, improves energy supply as well as determining the amount of bacterial protein which is produced in the rumen.

However, non-fibrous carbohydrates do not stimulate rumination nor saliva production and when found in excess they can inhibit fiber fermentation.

Thus, an adequate balance between fibrous and non-fibrous carbohydrates is very important in ruminant feeding in order to obtain profitable production levels.

Meanwhile, acetate and butyrate directly generate energy in the form of ATP in the rumen (Van Huotert, 1993).

In high-producing animals, the main organs involved in carbohydrate metabolism are: the rumen, liver, and mammary gland.

Rumen Fermentation

The rumen digestion of starch generates a high production of VFA. Highlighting the production of propionate (C3) whose molar proportion increases in regards to the rumen fermentation of fibrous forage. Generating a higher molar proportion of acetate (C2).

Propionate found in the rumen is absorbed through the walls (more than 80%) reaching the liver. It is then transformed into glucose through the metabolic process known as gluconeogenesis.

Oxaloacetate(C4)

During rumen fermentation, microorganism populations, mainly bacteria, ferment carbohydrates to produce energy, gases (methane – CH₄ and carbon dioxide – CO₂), heat and organic acids.

The pathways involved in the formation of final fermentation products are very complex. Involving different reactions, that lead from pyruvate or phosphoenolpyruvate (glycolysis products), to CO₂, CH₄ and acetic, propionic, butyric and valeric acids (VFA).

Acetate and CO₂ are produced by numerous species of rumen microorganisms from carbohydrates, while other species produce them from propionic, butyric and valeric acids.

However, many of these individual species also have one or more end products such as succinate, lactate, formate, hydrogen, and ethanol, which are not final products of fermentation.

These compounds, except for ethanol, are subsequently metabolized by other species which produce the typical end products.

VFAs are end products of microbial fermentation which are absorbed through the keratinized epithelium of the rumen.

These acids are short chain carbon compounds, which are produced during the fermentative degradation of food and can be converted into glucose, amino acids or fatty acids by rumen bacteria or by the animal’s cells.

Formic, acetic, propionic, butyric, isobutyric, 2-methyl butyric, valeric, isovaleric, caproic and caprylic acids are considered VFAs.

Acetic, propionic and butyric acids are the ones produced in greater quantities. These are produced by intra- and extracellular enzymes of bacteria and protozoa.

Most of the acetate and all of the propionate are transported to the liver. On the other hand, most of the butyrate is converted to a ketone known as [beta]-hydroxybutyrate in the rumen wall. (Figure 1)

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Propionate 20 Butyrate 10

Acetic, propionic and butyric acids make up the majority (>95%) of the acids produced in the rumen.

The fermentation of amino acids in the rumen also produces acids, which are called iso-acids.

The energy and iso-acids which are produced during fermentation are used by bacteria to grow (i.e. mainly to synthesize protein).

Ethanol, succinate, and lactate are catabolized to final products: acetic, propionic and butyric by the different microorganism species (Santini, 1994).

Acetic acid is transported in the bloodstream from the rumen and incorporated directly into the cellular proto-plasma. It is then used for obtaining energy via the acetyl-CoA and Krebs cycle, or for fat synthesis (both fat tissue and milk fat).

O nutrients

CO₂ and CH₄ are belched, and the energy which is still present in CH₄ is lost.

The heat produced during fermentation is dissipated, if it is not necessary for maintaining body temperature.

The degradation and fermentation of polysaccharides (complex carbohydrates) in the rumen occurs essentially in three steps:

Attachment of microorganisms to forage particles, causing the dissociation of carbohydrates from the structure of the cell matrix.

Hydrolysis of released polysaccharides to monosaccharides.

Intracellular fermentation of monosaccharides, from which VFAs are obtained. Pyruvate and phosphoenolpyruvate are the main products obtained from intracellular fermentation of hexoses and pentoses. Microorganisms transform these into di erent end products of rumen fermentation through di erent metabolic pathways (Leng 1973).

Ketones are the main energy source for most body tissues.

Ketones are mainly derived from butyrate production in the rumen, but they can also come from the mobilization of adipose tissues during the initial stages of lactation

The liver is the main organ where the formation of ketone bodies occurs, while kidneys participate to a lesser extent

The process takes place in the mitochondrial matrix and occurs in different steps.

The rst reaction is the condensation of two acetyl-CoA molecules to form acetoacetyl-CoA. This ocurrs through the action of the enzyme β-ketothiolase. The acetoacetyl-CoA molecule is condensed with another acetyl-CoA molecule to form β-hydroxy-β-methylglutaryl-CoA (HMG-CoA), by the action of the enzyme HMG-CoA synthase. Finally, HMG-CoA lyase hydrolyzes HMG-CoA into an acetoacetate molecule and an acetyl-CoA molecule.

A part of the acetoacetate is reduced to β-hydroxybutyrate within the mitochondria, consuming an ATP equivalent (NADH molecule); This reaction is catalyzed by the β-hydroxybutyrate dehydrogenase which is is closely associated to the mitochondrial inner membrane.

Acetone is formed by a slow spontaneous, non-enzymatic decarboxylation of acetoacetic acid. Thus, acetone levels are much lower than those of the other two types of ketone bodies.

Since it cannot be transformed back into acetyl-CoA, it is excreted through urine or exhaled. Acetone exhalation is responsible for the characteristic fruity smell in the breath of cattle(apple-like smell).

The nutrients absorbed in the rumen or small intestine, are mainly directed to the liver and other organs for catabolism or anabolism, as needed.

Catabolism And Anabolism 1 2

The metabolism of monosaccharides revolves around the supply and destination of hexose, this monomer being the main source of energy for cells, the catabolic pathway is glycolysis, carried out in the cell cytoplasm. This process consists of eight reactions:

Glucose enters the cytoplasm to be phosphorylated (addition of a phosphate group), from ATP. This reaction is catalyzed by hexokinase. The resulting glucose 6-phosphate (aldohexose) is abundant in all cells, since the vast majority that enters the cytoplasm ends up being phosphorylated, in order to prevent it from crossing back the cytoplasmic membrane and diffusing to the extracellular environment.

Aldohexose is isomerized and transformed into fructose 6-phosphate (ketohexose). Reaction catalyzed by glucose-6-phosphate isomerase.

3 4

Fructose-6-phosphate is phosphorylated from ATP at carbons 1 and 6 to give rise to fructose-1,6 bisphosphate Phosphofructokinase-catalyzed reaction.

Fructose-1,6-bisphosphate is divided into glyceraldehyde-3-phosphate and dihydroxyacetone phosphate. -catalyzed reaction.

Triose phosphate isomerase catalyzes the conversion of dihydroxyacetone phosphate to obtain more glyceraldehyde-3-phosphate

Glyceraldehyde-3-phosphate is oxidized and phosphorylated, at carbons 1 and 6 forming 1,3-bisphosphoglycerate by glyceraldehydephosphate dehydrogenase, then transfers its phosphate group, to synthesize ATP and is transformed into 3-phosphoglycerate.

7 8 Fructose 6-phosphate Phosphoenolpyruvate Pyruvate CO2 Acetil-CoA Lactate

3-phosphoglycerate isomerizes C3 to C2 and is transformed into 2-phosphoglycerate by phosphoglycerate mutase. Subsequently, the enolase promotes the formation of a double bond, eliminating a molecule of H₂O and forming phosphoenolpyruvate.

Pyruvate enters the mitochondrial matrix, therefore, each hexose (glucose) generates:

= Hemicellulose CARBOHYDRATES Acetate Butyrate Propionate VFA Methanogenesis

Figure 2. Metabolism of Carbohydrates

4 Oxalacetate Succinate

79 nutriNews International March 2023 | Ruminal metabolism of volatile fatty acids (Part 1) nutrients

+ ANABOLISM

Gluconeogenesis

All propionate is converted to glucose in the liver. In addition, the liver uses amino acids for glucose synthesis. This process is known as gluconeogenesis.

It is very important, because in general there is not enough glucose absorbed in the digestive tract and all the sugars found in milk (lactose approximately 1575g in a cow that produces 35 kg of milk a day) must be produced by the liver in

Gluconeogenesis also synthesizes GLUCOSE from lactate and glycerol, in the cell cytoplasm and mitochondria of hepatocytes.

In this metabolic pathway, six ATPs are consumed for each hexose produced and the CH3-CH2-COOH propionate is the only glucogenic VFA.

The importance of gluconeogenesis in ruminants is mainly due to the fact that their bodies absorb small amounts of glucose in the digestive tract and that their ability to store glycogen in the liver is more limited than that of monogastrics.

Ruminal metabolism of volatile fatty acids (Part 1)

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