10 Questions: Protein, Amino Acids, and Exercising Horses

Though muscle is recognized and appreciated for its importance in the physical prowess of horses, the role of muscle in maintaining normal protein metabolism throughout the body is likely undervalued. The horse’s body is composed of thousands of proteins that are in a constant state of turnover, breaking down and synthesizing simultaneously.

Because muscle is the main reservoir of amino acids in the body, and amino acids are the building blocks of proteins, basic knowledge of protein and amino acids is essential for all horse owners interested in nutrition. Using a simple Q&A format, here’s your chance to learn more about protein in equine nutrition.

What’s the difference in protein needs between a horse at maintenance and a horse involved in exercise?

The difference can be substantial depending on the amount of exercise. Horses asked to perform intense exercise require much more daily protein compared to those asked to do little or no exercise. A 1,100-lb (500-kg) horse, for example, needs 630 g of crude protein daily at maintenance but requires 1,004 g when in intense work. The additional protein required by exercising horses is usually met when feed is increased to meet energy demands.

Why do exercised horses need more protein?

Exercise induces an increased need for protein due to maintenance of new muscle mass, repair of existing muscle mass, and replacement of nitrogen lost in sweat.

Is feeding elevated protein to exercising horses advantageous?

Yes, researchers have found that feeding protein promotes muscle protein synthesis after exercise, but some specific amino acids are thought to be particularly beneficial.

Which amino acids are most advantageous?

Leucine, isoleucine, and valine, collectively referred to as branched-chain amino acids (BCAA), increase muscle protein synthesis after exercise by providing building blocks and influencing metabolic pathways, acting as a fuel source, and stimulating cell signaling. Leucine is especially important in activating muscle protein synthesis.

What about lysine and threonine?

Lysine and threonine have long been considered the most limiting amino acids in typical horse diets, and they are frequently supplemented as crystalline forms in feeds to improve body protein synthesis. When supplemented together to mature horses, lysine and threonine improved muscle mass scores and lowered body condition scores with no decrease in body weight, suggesting improved lean tissue accretion.¹

What is a crystalline amino acid?

Crystalline amino acids are synthetic, though they are exactly like naturally occurring amino acids and are digested completely.

What is a limiting amino acid?

Amino acids must be present within the body in certain ratios to support protein synthesis. A limiting amino acid is an essential (or indispensable) amino acid that is provided the most below its requirement, thus obstructing (or limiting) protein synthesis. If an amino acid is not used for protein synthesis because another amino acid is limiting, it is catabolized to carbon dioxide and urea for excretion.²

Why is pinpointing specific amino acids important? Can we feed a high-protein ration instead?

While practical, feeding a high-protein diet results in an elevated nitrogen concentration of the urine and feces. Excretion of excess nitrogen raises ecological concerns. When horses are stalled, increased nitrogen secretion can irritate eyes and respiratory tissues. Furthermore, protein is an expensive ingredient in rations.

Instead of focusing on the amount of protein offered, the quality of protein should be considered. Quality refers to the ratios of specific amino acids in a protein source. Soybean meal, for example, is often used as a protein source in horse feeds because of the desirable amino acid composition. Whey protein contains a full complement of amino acids, too, but it is expensive and not useful for all types of horses.

Does a high-protein ration have any negative influence on performance?

Exercising horses fed 160% of their crude protein requirement showed increased water intake, nitrogen excretion, and urine volume compared to exercising horses that received the recommended crude protein requirement. Carrying additional water weight may hamper performance. Moreover, a lower blood pH resulted from feeding a high-protein diet to exercising horses. Intense exercise reduces pH through lactic acid production. Additional decreases in blood pH created by a high-protein diet could lead to body-wide acidosis.

What is N-acetyl cysteine?

An amino acid analog, N-acetyl cysteine acts as a precursor for the potent antioxidant glutathione, which is vital for many biological processes, including recovery of skeletal muscle after injury. Specifically, N-acetyl cysteine is an especially important dietary component for horses with a muscle disease known as myofibrillar myopathy (MFM). Horses diagnosed with this disorder should be supplemented a high-quality source of N-acetyl cysteine.³

¹Graham-Thiers, P.M., and D.S. Kronfeld. 2005. Amino acid supplementation improves muscle mass in aged and young horses. Journal of Animal Science 83:2783-2788.

²Mok, C.H., and K.L. Urschel. 2020. Amino acid requirements in horses. Asian-Australasian Journal of Animal Sciences 33(5):679-695.

³Valberg, S.J., S. Perumbakkam, E.C. McKenzie, and C.J. Finno. 2018. Proteome and transcriptome profiling of equine myofibrillar myopathy identifies diminished peroxidation 6 and altered cysteine metabolic pathways. Physiological Genomics 50:1036-1050.