Lysine, methionine and threonine are known as rate-limiting amino acids that 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 most 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, methionine and threonine 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 can not 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.
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 constrain 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. But 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 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.
In the same study, horses undergoing light exercise that were given supplemental lysine and threonine had lower levels of 3-methylhistidine, indicating less muscle damage. 
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 might 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 high 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 require added methionine in their diet.
Several other nutrients are crucial to supporting strong hooves including zinc, copper, and biotin. A properly balanced diet with adequate vitamins, minerals and amino acids is the best approach for supporting hoof health in horses.
6) Coat Quality
The protein keratin is also abundant in hair follicles. Adequate levels of methionine in the diet can support a healthy coat by increasing availability of the amino acid cysteine.
The sulfur bonds between cysteine residues in keratin provide the rigid structure to this protein. Horses with coarse, brittle hair might particularly benefit from added methionine in the diet.
7) Immune Function
Amino acids are required to make the many antibodies and signaling molecules that are needed to fight infections by viruses and bacteria. In other animals, lysine has been shown to help lower the impact of herpes virus infections. Lysine competes with the amino acid arginine for absorption and metabolism. Arginine is needed by herpes viruses to function properly and replicate.
Equine herpes virus (EHV-1 and EHV-4) causes a respiratory disease called rhinopneumonitis in horses. It can also cause abortions in pregnant mares and impair neurological health. Although it has not been directly studied in horses, supplemental lysine could help diminish the severity of symptoms and minimize outbreaks, as reported in other animals. 
Wondering whether your horse needs more lysine, methionine or threonine in their diet? You can submit your horse’s diet for evaluation and our equine nutritionists can help you determine if amino acid supplementation is right for your horse.
Signs of Amino Acid Deficiency in Horses
Horses that are not meeting their protein and amino acid requirements can show a wide range of symptoms.
The severity of symptoms will depend on the length of time they have been deficient, how severe the deficiency is and whether other nutrients are imbalanced.
When protein and amino acid intake is not sufficient to support protein synthesis in the body horses might show the following signs including:
- Slow hoof growth
- Cracked, brittle hooves
- Poor coat quality
- Loss of muscle mass
- Flagging stamina
- Low milk production in lactating mares
Amino Acid Requirements in Horses
There are several factors that will determine an individual horse’s current protein and amino acid requirements including:
- Workload and exercise intensity
- Physiological status including pregnancy & lactation
- Health status
According to the National Research Council’s Nutrient Requirements for Horses (2007), mature horses at maintenance should have 8-10% protein in their ration. This is equivalent to a crude protein requirement of approximately 630 grams per day for a 500 kg horse.
The minimum amount of lysine required is 18 grams per day for a 500 kg horse at maintenance. However, optimal levels of intake are estimated at 27 grams per day for a 500 kg horse.
In general, the lysine requirement for horses is 4.3% of the dietary crude protein requirement. At different life stages, the protein and lysine requirements will change.
Physiological amino acid requirements for horses have only truly been researched for lysine. Requirements for other amino acids are based on their typical levels in horse muscle tissue. Based on this data, methionine and threonine should represent 1.2% and 2.7% of crude protein intake, respectively.
Threonine requirements can be affected by fibre intake in the diet. Higher levels of fibre can cause more damage to the mucin proteins that line the digestive tract. This will increase the threonine requirement because more will be needed to re-build the protective mucous barrier. 
The following are NRC (2007) requirements for horses at different life stages and physiological status.
Mature Horses at Maintenance
Daily requirements for a 500 kg mature horse at maintenance with no workload:
- Crude Protein: 630g
- Lysine: 27g
- Methionine: 7.5g
- Threonine: 17g
Moderate and light exercise will not substantially change the horse’s protein requirement. However, horses engaged in heavy or very heavy work can require more protein and amino acids to support tissue recovery and for making energy.
Horses in Heavy Work
Daily requirements for a 500 kg horse engaged in heavy or very heavy work:
- Crude Protein: 860 – 1000g
- Lysine: 29 â€“ 35g
- Methionine: 8 â€“ 10g
- Threonine: 19 â€“ 20g
Growing horses have higher requirements for protein and amino acids to support rapid tissue growth. Their overall diet should contain between 14Â – 16% crude protein.
Daily requirements for weanlings and yearlings increase as they age, within the following ranges:
- Crude Protein: 720 â€“ 950g
- Lysine: 29 â€“ 50g
- Methionine: 8 â€“ 11g
- Threonine: 18 â€“ 29g
Pregnant and Lactating Horses
Pregnant (9-11 months of gestation) and lactating horses require higher intake of good quality proteins that meet their essential amino acids requirements. Their overall diet should contain between 10 – 14% crude protein. This is especially important during the final 3 months of gestation and first 4 months of lactation when protein demands are highest.
Daily requirements for pregnant and lactating mares:
- Crude Protein: 800 â€“ 850g
- Lysine: 28 â€“ 50g
- Methionine: 8 â€“ 16g
- Threonine: 18 â€“ 31g
Older horses (over 20 years of age) might require higher levels of protein to help maintain body condition, similar to the levels necessary for growing animals. Nutrient absorption is reduced in senior horses which may mean that more amino acids need to be supplied to ensure adequate uptake by the body.
In older horses, increasing the protein intake should be done carefully with advice from an equine nutritionist and veterinarian. Senior horses should be supplemented with additional amino acids slowly and only if the horse has adequate liver and kidney function to support a higher protein intake.
Excess amino acids will not be stored in the horse but will be broken down into urea to be excreted in urine and manure. This can cause horses to drink more water and urinate more frequently.
Whatever the age, workload or physiological status of your horse, it is important to evaluate the horse’s diet as a whole before making changes. This is necessary to ensure your feeding program is well-balanced for protein, energy, vitamins and minerals.
Submit your horse’s diet online and Mad Barn’s equine nutritionist will provide a complementary evaluation to determine how best to balance your horse’s diet.
Roles and Sources of Lysine, Methionine & Threonine
All amino acids are required for protein synthesis, but some proteins require more of specific amino acids. The sequence of amino acids in a protein is dictated by the DNA that encodes the gene for that protein.
Amino acids can also be broken down into other molecules that the cell needs to perform a variety of functions.
Lysine is found throughout the body and is especially important for the following reasons:
- Increases calcium absorption in the gut and minimizes calcium loss in urine  
- Supports collagen and elastin formation which are key proteins in skin and connective tissue
- Found in high levels in muscle proteins such as myosin which is involved in muscle contraction
- Makes carnitine which is a vitamin-like nutrient that is required for cells to get energy from fat
- Supports the immune system by helping fight viral infections
Grains and grasses are typically low in lysine. Legumes like soybeans, and soybean meal, are high in lysine. Canola meal can also provide good levels of lysine. Lysine within proteins is found in the form of L-lysine, which can also be top-dressed on its own as an equine supplement.
Every protein starts with the amino acid methionine. The first part of each gene tells the cell to start assembling the protein beginning with methionine.
Methionine is a typically a sulfur-containing amino acid but can also exist in low quantities as selenomethionine where selenium is found in place of sulfur.
Selenomethionine can be converted to selenocysteine which is an important part of the antioxidant enzyme glutathione peroxidase.
The methionine cycle which breaks down and re-makes this amino acid also produces other compounds that have important functions: 
- s-adenosylmethionine (sAM):Â a methyl donor to a wide variety of processes which affects gene expression and protein function
- Adenosine: the backbone of adenosine triphosphate which is the main energy currency of the cell
- Taurine: an amino acid that is not used for protein synthesis but is important for supporting cells of the nervous system
- Phosphatidylcholine: a phospholipid that is a major part of cell membranes
Methionine is found at high levels in alfalfa, flax, rice bran, sunflower seeds and beet pulp where it exists as either D-methionine or L-methionine. It can also be found in animal proteins like whey or casein. Concentrated DL-methionine can be added to the horse’s diet and provides both the D and L isomers of methionine. The levels to feed will depend on their current diet and methionine requirement.
Threonine is primarily used to make mucin proteins in the gut. Up to 60% of threonine in the diet stays in the gut to form this protective mucous barrier.
Threonine has additional important roles that could affect horse health:
- Enzyme function: Threonine is often the site that is modified to make an enzyme work more or less efficiently. Signals from outside a cell, such as hormones, change how a cell functions by changing how well the enzymes work, often by phosphorylating threonine in proteins
- Reduce fat mass: Threonine might â€œturn on” genes that burn fat and â€œturn off” those that make fat to induce net loss in fat mass 
- Energy source: Threonine can be converted to intermediates of the Kreb’s cycle (or TCA cycle) which is the most efficient process that cells have to make energy from fat and sugar. It can also be converted into glucose through gluconeogenesis in the liver.
- Creatine: Threonine can be converted to glycine, an amino acid that is required to make creatine which is a high energy compound naturally found in muscle
This amino acid is found in most plant and animal proteins. Threonine can also be supplemented as a single ingredient based on the individual horse’s needs.
Dangers of Excess Amino Acid Intake
Ensuring your horse is meeting their amino acid requirements based on their individual needs is critical to optimize their health. However, adding too much lysine, methionine and threonine might have negative effects in some horses.
Excess amino acids will be broken down and excreted in urine and feces. This is an energy demanding process and can be taxing on the liver and kidney. Breakdown (or catabolism) of amino acids produces ammonia which is converted to urea in the liver. Urea travels in the blood to the kidney where it is removed and excreted in urine and it can also move into the intestine and excreted in manure.
To get rid of extra amino acids in urine, the horse might need to urinate more frequently which will stimulate thirst and increase water intake.
For healthy horses at maintenance, excess intake of amino acids is likely not a problem but they should always have free access to water to support excretion of amino acid waste products.
Side Effects of Too Much Protein for Horses
For horses with poor kidney function, or heavily exercised horses, excess amino acids could cause some issues:
Extra demand on water intake: For exercising horses that need a lot of water for sweating, using up water to eliminate amino acids can cause poor performance and electrolyte imbalances.
Tying-up: Issues with water and electrolyte balance in heavily exercised horses can cause a build up of lactic acid in muscles leading to poor exercise endurance and increased risk for tying up.
Respiratory irritation: In the stable environment, high ammonia and urea in the urine can evaporate and affect horses and humans. Inhaling these compounds can irritate the lungs, making horses and handlers more prone to respiratory infections.
Changes in behaviour: Build up of ammonia or urea in the blood can affect the nervous system, making horses irritable or restless
Additional strain on the kidney: Older horses might already have weakened kidneys which limit their capacity to excrete excess urea. Similarly, horses with equine metabolic syndrome often have strained kidney function from dealing with a high sugar load and might benefit from lower dietary protein.
Given these potential concerns for some horses, it is especially important to consult with an equine nutritionist before making changes to the amino acid and protein nutrition of your horse. Submit your horse’s diet online for a complementary diet evaluation by our equine nutritionists and we can help you make adjustments to your feeding program.
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