Proteins are sometimes called the building blocks of life, and when new tissues are made, large amounts of dietary protein are required. The stages in an animal’s life when new tissues are extensively manufactured are:
  • Growth
  • Lactation
  • Pregnancy
Proteins are composed of units called amino acids. Proteins are synthesized from available amino acids that are ingested or synthesized by the horse and are used to build muscle and bone, blood components, enzymes that catalyze biochemical reactions, hormones, peptides, and antibodies. Meeting a protein requirement does not mean just supplying a certain amount of crude protein; it means ensuring levels of specific amino acids as well, such as amino acids that must be provided through dietary sources. Twenty different amino acids are needed for protein synthesis, and several can be made by tissues of the body. However, ten of these amino acids, known as essential amino acids, must be supplied to the horse through dietary sources: arginine, histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine. Lysine, followed by methionine and threonine are the essential amino acids in most demand during protein synthesis in the horse.

10 Essential Amino Acids – Their Functions

The ten essential amino acids that must be supplied by dietary means have a wide range of functions. A trick to remember the 10 essential amino acids is to recall PVT TIM HALL.

Phenylalanine
  • Produces adrenalin & noradrenalin; is an anti-depressant
Valine
  • Regulates protein turnover and energy metabolism along with isoleucine and leucine; vital for muscle coordination
Tryptophan
  • Produces serotonin and is a mood stabilizer; precursor to niacin and may aid in blood clotting
Threonine
  • Enhances growth and food efficiency; produces adrenalin and is a precursor to the thyroid hormone
Isoleucine
  • Helps form hemoglobin and fights nervous system degeneration
Methionine
  • Important for hair coat and growth; helps prevent deposits and adhesions of fat in the liver; essential for selenium bioavailability; second only in importance to lysine as a limiting amino acid
Histidine
  • Maintains plasma, hematocrit and serum albumin; releases histamine from the body; helps treat arthritis; stimulates stomach acid secretion and improves appetite
Arginine
  • Stimulates insulin and growth hormone release, helpful as nutritional aid in cancer therapy; stimulates immune system by boosting T-cell production
Leucine
  • Keeps muscle protein from degrading
Lysine
  • Enhances growth and nitrogen balance; promotes bone growth in foals; stimulates gastric juices

Effects of Amino Acid Deficiency

Lysine is the amino acid that is often in shortest supply in horse feeds and is particularly important for growing horses. Because feed ingredients used for horse rations can be deficient in some amino acids, most feed companies are now adding lysine, methionine, and threonine directly to the formulation of a supplement or concentrate in addition to the crude protein in the product. This is important because if one amino acid needed for protein synthesis is in short supply, that protein cannot be formed. This can lead to:

  • Poor-quality growth and developmental disease in growing horses
  • Poor body or muscle condition and possibly bone breakdown in working horses

If necessary, the horse also can use protein as a source of energy during starvation or intense exercise by removing the amino group (the nitrogen portion of the molecule) and using the carbohydrate skeleton that is left for energy production. Starving horses or protein-deficient horses will catabolize muscle tissue to produce amino acids for other biological processes.

Take Home Message

When thinking about meeting your horse’s protein requirement, remember that it does not mean just supplying a certain amount of crude protein, but ensuring levels of specific amino acids as well. For situations in which protein requirements will be higher, such as growth, pregnancy, and lactation, be sure to re-evaluate your horse’s ration to make sure all requirements are being met.