Q: What is lysine and why is it included in horse feed?

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 this:

  • First, think of it as though you’re going to write a sentence. The essential amino acids are the vowels and the non-essential amino acids are the consonants. Let’s say that you only have three Es (which happens to be the most used vowel in the English language). As soon as you have used those three Es in words, you can’t build any other words that require the letter E. You might have lots of other letters around, but you can’t finish your sentence without additional Es. Essentially, in this scenario, E would be your limiting amino acid.
  • Another common way to think about this is to visualize a wine barrel with each board that goes up the side representing an amino acid. The boards on this wine barrel are all different lengths, representing the relative amounts of each of the amino acids. If you were to try to fill the barrel with water you could only fill it to the top of the shortest board. That board is your limiting amino acid.

To recap, for protein synthesis to occur all amino acids have to be present in the correct amounts and ratios. Even if only one amino acid is below the amount necessary for protein synthesis to occur, synthesis will be limited. And given the range of structures made up of protein in your horse’s body, this can have major consequences.

Compared to other monogastric species (those with a single-chambered stomach) such as pigs and chickens, our knowledge of amino acid requirements in the horse is currently pretty limited. However, we do know that in traditional equine diets made up of forage and cereal grains lysine appears to be the first limiting amino acid. Lysine is, in fact, the only amino acid for which we have an equine requirement—4.3% of the crude protein requirement (NRC, 2007).

Because a low dietary lysine intake can limit protein synthesis and because lysine can be low in diets fed to horses, many modern-day commercial feeds add sources of lysine and guarantee those levels in their products. Not all feeds show the lysine level; this doesn’t mean that the feed has no lysine in it (as many of the protein sources in that feed will provide some level of lysine), it just means that the manufacturer is not guaranteeing the lysine level. As such, seeing it listed on the tag can help give you some peace of mind that the feed company understands the importance of lysine in the equine diet and that the feed is designed to support adequate lysine in the ration if fed properly.