Many modern-day commercial feeds add sources of lysine and guarantee those levels in their products. | Photo: Erica Larson/The Horse

A: Lysine is one of nine essential amino acids, which are the building blocks that make up protein. I think of them as the letters that make up words where the words are the protein. We give a lot of attention to protein but, in reality, horses do not have a dietary protein requirement; instead, they have a dietary amino acid requirement and, in particular, they have an essential amino acid requirement. This is because essential amino acids cannot be made by the horse the way non-essential amino acids can. Therefore, they must be present in the diet.

The amounts of the various amino acids in the diet and their relative proportions to each other is very important for protein synthesis. Mechanisms within cells take the amino acids that are absorbed from the diet and reconnect them to make the new proteins that are needed by the body. Muscle tissue is the most commonly thought of protein, but there are many other proteins—for example, hormones, enzymes, important components of the immune system, and the keratin that makes hair and hoof tissue.

Each protein has a specific structure and requires amino acids to be available to connect in the right order, like links in a chain. If one of the amino acids needed is present in a smaller quantity than is needed, protein synthesis will be negatively impacted. This is a complicated concept, so here are a couple of analogies that can help you visualize