Lysine, methionine and threonine are known as rate-limiting amino acids. They are required in the horse’s diet because they cannot be made in the body. Of the 21 amino acids that exist, these three are most commonly deficient in the horse’s diet.
Amino acids are the molecular building blocks of proteins. Proteins have many functions in the body; they form structural components of cells, act as hormones and act as enzymes that carry out metabolic processes.
Horses that have inadequate protein or amino acid intake might experience:
- Poor hoof and coat quality
- Persistent fatigue
- Slow recovery from illness
- Loss of muscle mass
When an essential amino acid is deficient to the point of limiting protein synthesis, it is called a “limiting” amino acid. Lysine, threonine, and methionine are typically considered the first, second, and third limiting amino acids in equine diets.
Cereal grains and grasses that make up much of the horse’s diet are naturally low in these three amino acids. Legumes such as alfalfa and soybeans are typically higher in protein and provide more lysine, threonine, and methionine.
Horses do not store excess amino acids in their body and they must be supplied regularly by the diet to avoid deficiency. Some horses may benefit from supplementation to ensure they obtain adequate amounts of protein.
Mad Barn’s Three Amigos is an essential amino acid blend providing lysine, threonine and methionine in a 5:3:2 ratio.
Benefits of Amino Acids for Horses
Horses of all ages and workloads need sufficient amino acids in their diet to support protein synthesis in all cells of their body.
There are 21 amino acids that are used to make proteins. Of these, 10 are considered essential because they cannot be made in the body and must be obtained through the diet. If one or more of these is not present at adequate levels, protein synthesis will be limited by their availability.
Lysine is considered the first rate limiting amino acid in horses because it is the one that is most likely to be low in the diet and restrict protein synthesis. Threonine and methionine are typically considered the second and third most limiting amino acids, respectively.
Making sure your horse’s feeding program provides adequate essential amino acids will support optimal protein synthesis, performance and well-being. Additionally, it is especially important to ensure they are getting a balanced profile of the rate limiting amino acids.
Below are 7 research-backed reasons to ensure your horse is getting adequate amounts of lysine, methionine and threonine:
1) Improved Top Line and Muscle Function
Mature, exercising horses given supplemental lysine and threonine have improved muscle mass regardless of age. In one study, they had lower body condition scores without any change in body weight suggesting they were gaining muscle mass while lowering body fat. 
2) Antioxidant Protection
The amino acid methionine can be converted into another amino acid called cysteine. Cysteine is important for making the antioxidant glutathione which is found in all cells of the body.
Having adequate antioxidant protection can help tissues recover quickly from exercise, speed up recovery from illness and support healthy aging.
3) Gut Health
The amino acid threonine is crucial for synthesis of mucin proteins in the gut. These proteins form a protective barrier between the acidic environment inside the gut and the tissues of the stomach and intestines. 
Horses with digestive issues may benefit from supplemental threonine to support a healthy gut lining, lower the risk for ulcers and improve nutrient absorption.
4) Joint Health
Threonine is converted into other amino acids, glycine and serine, that are important for healthy connective tissue. Glycine and serine are required to make the proteins collagen and elastin which are abundant in connective tissues like ligaments and tendons.
Threonine and lysine are also directly integrated into the structure of collagen, particularly inter-chain bonds that allow for link between collagen chains (inter-fibrils of collagen). Lysine also inhibits the enzyme matrix metalloproteinases (MMPs) which increases collagen’s resistance to degradation.
5) Hoof Health
Keratin is a protein that is abundant in the hoof. It contains high levels of the amino acid cysteine. Although cysteine itself is not an essential amino acid, much of it is derived from converting methionine to cysteine.
If methionine is limited, there will be less cysteine available to make keratin in hoof tissues. Horses with cracked or crumbling hooves might benefit from added methionine in their diet.
Several other nutrients are crucial to supporting strong hooves including zinc, copper, and