Protein Nutrition, Part II Protein Quality - It's More Than Just Digestibility

Written By Dr. Kris Hiney
In the last article, we introduced the idea of examining the protein in our horse’s diet beyond just the mere percent crude protein on the feed tag or a forage report. We discussed the concept of protein digestibility and the importance of the site of digestion. As there is only limited evidence that uptake of amino acids takes place in the hind gut of the horse, we prefer to feed proteins which are digested and absorbed in the small intestine of the horse. Typically concentrates offer more pre-cecal digestibility of protein than do forages. Therefore site and extent of digestion are key components of protein quality. However, there is another equally important factor in protein quality, and that is the amino acid profile.
Last month the amino acids that horses could synthesize on their own were listed, as well as the amino acids that are essential to be supplied by the diet. Horses must receive the proper balance of amino acids in order to synthesize the many complete proteins in their body. When they do not receive enough of a particular amino acid, protein synthesis is limited. That amino acid is then referred to as the limiting amino acid. Typically, lysine is the most common limiting amino acid in the horse’s diet, or the one in shortest supply. It does not mean it is the most abundant amino acid found in the horse’s body, rather the one that is most commonly deficient in feeds. Now remember, every protein in the body is coded for by the DNA that provides a blue print to build that protein. The DNA provides the proper sequence of amino acids that must be linked together to form the protein. Let’s say that to build one molecule of actin (a protein found in muscle cells responsible for their contractile activity) there are 25 lysines, 30 threonines and 46 alanines. (The actual polypeptide chain of actin is over 300 amino acids long.)    If the diet contained only enough for 20 lysines to be added to the peptide chain, protein synthesis would halt, even if you had 35 threonines. If you added those lysines back in through the diet, protein synthesis could continue. Now, this is an arbitrary example, instead try to think of protein synthesis occurring throughout the body, constantly adding amino acids that have come from the diet, synthesized by the horse, or that have been recycled by tearing down old proteins.   Obviously the more protein the horse is synthesizing (think young growing horses) the more critical the amino acid profile of the diet.

DNA molecule which contains the information to build every protein in your horses body.
So what is the amino acid profile? Simply put, it is the percentage of each amino acid that appears in the diet, or even in a horse’s tissue. For example, in equine muscle tissue, lysine is set at a relative value of 100%, while other amino acids such as arginine, leucine, phenylalanine and threonine appear at 76, 107,60 and 61% respectively of the amount of lysine present in muscle(Bryden, 1991). In other animal species such as swine and poultry, nutritionists try to match the amino acid profile of the diet to the amino acid profile of the actual animal. In this manner, the least amount of amino acids are wasted. Instead, they are incorporated into the animals’ body to allow for growth, reproduction etc. One of the goals of many animal nutritionists is to achieve something called zero nitrogen balance in the animal. That means the amount of N going into the animal matches the amount of N going out of the animal. Or we are replacing the amount of N that is being lost by the animal through normal tissue turnoever. If we feed protein beyond what the animal requires, the animal will still digest and absorb those amino acids. However, if they exceed the horse’s requirements to synthesize protein, the horse will instead catabolize those amino acids for fuel, and excrete the nitrogen in the urine as urea. If you have ever walked into a poorly ventilated barn with horses that were fed high protein diets, you probably have smelled the ammonia that comes with excessive protein feeding. Furthermore, feeding excess N just adds to the N being added back to the ground through runoff from facilities.
Protein quality and it’s amino acid profile can alter how much protein the horse actually requires. While we often just discuss protein requirements generically as a percentage, in fact horses can be fed a lower total amount of protein if it is of higher quality. For example, in young horses, the lysine requirement is 4.3% of their crude protein requirement.    The higher amount of lysine in the feed, the less of that feed will need to be fed. Let’s take a 4 month old foal which requires 669 grams of CP and 28.8 g of lysine. We then feed our horse either a 16% crude protein feed of high or low quality. We feed him at 2.5 % of his body weight and he weighs 370 lbs. That provides an intake of 9.25 lbs per day. If his feed provides 16% protein, he gets 660 grams of protein. We have almost completely met his protein requirements. But what if one of our feed sources contained only 2% lysine? That means that the foal would be deficient by 15.8 grams (the feed would provide 13 g of lysine) and thus his growth rate would be limited. Therefore the foal would have to eat much more of that diet (more than he can consume) in order to consume the correct amount of lysine. Our foal on the high quality diet would receive 28 g of lysine, meeting his requirements, and allow his body to grow normally. In older horses whose protein requirements are easier to meet, we can actually lower the total amount of protein in the diet provided it is of a high quality. In fact, lowering the total protein in the diet while supplementing key amino acids has been proven effective in both growing horses (Graham et al., 1994, Stanier et al, 2001) and in exercising horses (Graham-Thiers et al, 1999, 2001)
The future of protein nutrition in the horse may very well focus on identifying the correct amino acids needed in the horse’s diet, and moving to a lowering of the absolute % CP in the diet, therefore minimizing waste, and decreasing the amount of N added back to the environment. While nutritionists still have much to learn, the goal when feeding protein is to feed just the right amount the horse needs, and not to overfeed needlessly.
Next month, we put the theories into practice and discuss protein requirements for various classes of horses.

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