This is an abbreviation of unit 6 from the Advanced Equine Nutrition course. The complete text, a video, and quiz questions are available with The Horse’s Advocate membership. You can also purchase them separately. Note: Passing the Basic Equine Nutrition Course is required before starting the Advanced Course.

Unit 6 – Protein: An Essential Nutrient

A Review Of Protein And Amino Acids

Protein is the most crucial nutrient in any animal’s diet. It forms structures and allows for functions in every part of the body. Let’s look at these as a review:

• Structural proteins include all connective tissue, including muscles, tendons, ligaments, skin, hair, hooves, all organs, tooth dentin, and more.
• Functional proteins include enzymes, neurotransmitters, immune system components, carriers, signaling, repair, and genetic copying proteins.
• Proteins are combined with fats (lipoproteins) to help carry fat through the blood. Proteins are combined with sugars (glycoproteins) that make mucus, hormones, and the subclass, proteoglycans, such as glycosaminoglycan (Adequan®).

I would argue that using “protein” as a nutrient is misleading. It needs to be clarified to understand how to feed horses. Describing proteins as “high-quality,” “good-quality,” or “poor-quality” offers detailed clarity. These terms represent the amino acid profiles of all proteins. The more essential amino acids in the protein, the better quality.

Twenty amino acids make all proteins. Two more amino acids drive the code translation from DNA to RNA. The organelle called the ribosome within the cells puts the amino acids in the correct order. The ribosome outputs small chains of amino acids called peptides. It produces groups of peptides without branching called polypeptides. Finally, it creates long chains called proteins.

To complete the discussion of amino acids, several other newly discovered amino acids are not used for protein synthesis. Instead, they are temporary intermediates in the synthesis of proteins. In other words, amino acids are transformed by adding elements such as selenium, hydrogen, and oxygen. However, they are not incorporated into the final protein.

Of the 20 amino acids known to make the billions of proteins that are within every cell, there are three divisions:

• 9 Essential amino acids – (“PriVaTe TIM HaLL“) Phenylalanine, Valine, Threonine, Tryptophan, Isoleucine, Methionine, Histidine, Leucine, Lysine
• 11 Non-essential amino acids – Alanine, Arginine, Asparagine, Aspartate (Aspartic Acid), Cysteine, Cystine, Glutamate (Glutamic Acid), Glycine, Proline, Serine, and Tyrosine.
• 7 Conditional amino acids—The body usually synthesizes these. However, they are not made under certain conditions. Therefore, they would need to be taken in with food. An example would be liver disease, heavy work, or rapidly growing newborns. These include Arginine, Cysteine, Glutamine (similar to Glutamate), Glycine, Proline, Serine, and Tyrosine.

Key Points:

1. Proteins are made in cells’ ribosomes, orchestrated by the code in RNA. The amino acids that make proteins are grouped into three categories. They are essential, non-essential, and conditional. Essential amino acids must be eaten. Non-essential amino acids are produced by the body. Conditional amino acids are made from essential amino acids under certain conditions.
2. Research on horses to answer questions about protein is minimal relative to food-producing animals and humans. The lack of money earned from horses receiving better protein foods contributes to this. Therefore, few discuss its importance for horse health.
3. Feeding synthetic amino acids bypasses the complex systems of animal amino acid synthesis. No research shows it is equivalent to consuming naturally made amino acids in forage. There is also no guarantee of quality or quantity.
4. If a complete profile of amino acids is fed sufficiently, the horse will make all the proteins needed without deficiencies.
5. Feeding enough forage to meet the daily amino acid needs will cause excess sugar intake, leading to obesity. However, daily feeding of sufficient high-quality protein will satiate horses and decrease their total calorie intake. This effect is called the Protein Leverage Hypothesis.
6. Feeding high-quality vegetable protein to horses within the range of 0.5 to 1.0 g/pound body weight causes no problems.
7. Most amino acids are used, but some are converted into glucose (glucogenic) or ketones (ketogenic) for fuel. This conversion commonly occurs during a catabolic crisis but not from excess in the diet.
8. A catabolic crisis happens when more amino acids become fuel. This results in fewer amino acids for anabolic processes (building with protein). Starvation and excessive glucose intake both paradoxically cause protein catabolism and the loss of amino acids.
9. The free amino acid pool is created when protein structures are regularly broken down. The reason for continuous protein replacement is unknown, but it occurs in every tissue of all animals. Most free amino acids are recycled into other proteins, while small amounts are converted into fuel. Any loss is replaced with dietary protein.
10. Some amino acids, like leucine, act as signaling molecules. They are required to be present for an action to occur.
11. Horses should be fed a high-quality protein-first diet to avoid chronic protein deficiencies.