What are Essential Amino Acids in Protein, and Why Do They Matter?

Nutrition articles frequently refer to protein quality and essential amino acids. When we use the term crude protein, we are essentially talking about a calculation based on measured nitrogen. Protein is about 16% nitrogen by weight, so if we measure the amount of nitrogen and multiply it by 6.25, this gives us a measurement of crude protein. It does not tell us anything about the quality of the protein. If you tested pure nitrogen this way, it would be 625% protein!

Digestible protein is that amount of the protein that is actually digested by the animal. In an over simplified example, if you fed 100 grams of protein and measured 50 grams in the feces, the protein would be 50% digestible.

What is really important to simple stomached animals (horses included) is the content of essential amino acids in the protein. We commonly talk about 10 essential amino acids (EAA), the amino acids that must be in the diet as the animal cannot synthesize them. These are:

  • Phenylalanine
  • Valine
  • Threonine
  • Tryptophan
  • Isoleucine
  • Methionine
  • Histidine
  • Arginine
  • Leucine
  • Lysine

A common memory aid in many nutrition texts books is to use the first letters of these 10 as PVT TIM HALL. (All of you who had a non-ruminant nutrition course still remember this acronym!)

The other 12 amino acids can generally be synthesized in the body and do not need to be in the diet, although there must be a supply of appropriate substrate to produce them. Animal nutrition text books cover this topic in excellent detail.

Limiting Amino Acids
Limiting Amino Acids

When we talk about limiting amino acids, these are the essential amino acids most likely to be restricting the use of the total amount of amino acids present. In most species, lysine is the first limiting amino acid, with methionine and threonine close behind. We commonly talk about amino acids as the building blocks of protein. If you are once you run out of an essential amino acid, you cannot build any more animal protein and the rest of the amino acids are used inefficiently for energy.

If you have a horse on a diet that is calculated to have adequate “crude protein”, but essential amino acids are not present, the horse simply cannot use the protein to build and maintain muscle, hair, hoof and skin and you will see changes in the appearance of the horse, such as loss of muscle mass, rough hair, scaly hoof surface.

3 Replies to “What are Essential Amino Acids in Protein, and Why Do They Matter?”

  1. Great article! Thank you. I am new to owning a horse, but not new to nutrition. If people knew this much about their own nutrition they would be a lot healthier. 🙂 I have been reading the articles and there is an abundance of useful information. Thank you for putting this information on line!

    Regards –

    Sue M

  2. This article is very interesting.
    How would I know if my horse feed has these amino acids? Are they added (and on the label) or they come from the protein source used? Are they in all the different types of horse feed, like senior or pleasure feeds?
    Since this is a Nutrena site, do all of their horse feeds have the correct amount of amino acids for the horse?
    Thanks for the info

    1. Hello Linda, Excellent question. All horse feed has protein, of course. To look at the quality of the protein in the feed, you will want to look to see, at a minimum, the amino acid lysine in the guaranteed analysis. For a 12% protein maintenance horse feed, look for around a 0.5% – 0.6% lysine level. It will be a little higher on performance feeds, up to around 0.8%.

      The additional amino acids, methionine and then threonine, may also be listed – you are more likely to find them on more premium-type feeds. Methionine will typically show up at about 0.25-0.3%, and threonine at 0.45-0.5%. You won’t generally find any beyond those tagged – and that is OK, because of the way ingredients work, if those are guaranteed, the rest will be in place.

      And yes, our Nutrena feeds have the correct amino acid balances! 🙂

      Thank you ~ Gina T.

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