By Caroline Loos, PhD, University of Kentucky
Protein is the second most abundant substance in the body (after water). It is part of almost every tissue and cell, and plays a critical role in all metabolic pathways. For example, protein is needed to make anything from blood cells, organs, muscle, hair, and hoof to enzymes, hormones, antibodies, milk, and much more. Therefore, protein is critical not only for growth and performance, but also for digestion and a healthy immune system. Without this essential nutrient, horses could not survive.
What is protein?
Protein is a chain of amino acids linked together, which is then folded up into a complex structure. So, what makes a muscle protein different from milk protein? The ratio and sequence of how amino acids are arranged within the chain as well as how the chain is folded, will ultimately determine the function of the protein. Because the horse needs to make thousands of different types of protein, each with a unique amino acid sequence, is it more essential for the diet to provide a complete and diverse amino acid profile than a supplement providing only a few specific amino acids.
How much protein do horses really need?
Although general daily requirements for protein have been determined, truthfully, we still know very little about the “ideal” amount of protein for different types of horses. This is mainly because horses do not have a requirement for protein, but for its elementary building blocks, the amino acids. Unfortunately, there are 21 amino acids, and determining what level of each is needed in every type of horse (young, old, exercising, broodmares, etc.), under a variety of circumstances, would take a lot of time (and money… something not easy to come by as a researcher).
Luckily, we know enough about the ‘quality’ of most common protein ingredients in horse diets (e.g. soybean meal, alfalfa, seed meals, peas, whey, etc.), which allows us to estimate pretty closely how much of these types of ingredients should be fed to ensure adequate amino acid intake.
But, let’s pause for a moment to talk about protein ‘quality’, a word commonly tossed around in the industry, but perhaps not always completely understood.
What is protein quality?
Protein quality stands for 2 main characteristics: 1. The foregut digestibility of the protein source and 2. the amino acid profile. To be useful to the horse, protein needs to be digested in the small intestine, where it is absorbed as amino acids that can be used to make new body proteins. Protein digested in the hindgut is mainly absorbed as ammonia and does not contribute to the amino acid pool. The digestibility differs greatly depending on the source of the protein. For example, protein from hay is only ~20-50% digestible as amino acids compared to 90% for soybean meal or whey. The amino acid profile means the amount and ratio of different amino acids within the protein, in particular of the essential amino acids which the horse cannot make in the body. Ideally, you want the amino acid profile of the dietary protein to supply exactly those amino acids needed to make specific proteins in the body. For example, muscle-support products will be formulated to supply an amino acid profile that matches that of the horse’s muscle. Like digestibility, the amino acid profile of ingredients varies a lot, sources like whey or soy are known to have superior amino acid profiles compared to forage protein. In general, the higher the quality of the protein, the less you have to feed of it, and the more efficiently the horse will be able to use the dietary amino acids to make new body proteins.
It is very easy to overfeed protein, especially when feeding protein-rich forages (e.g. pasture) in combination with protein-rich feeds or supplements. However, be aware that excessive protein feeding can have negative health consequences in certain situations, and contributes to undesired eutrophication of the environment. In a recent research study1, I showed in fact that a protein-rich meal caused hyperinsulinemia in horses with equine metabolic syndrome, indicating that in addition to sugars and starch, we should also pay attention to protein levels when feeding sensitive horses.
What about protein for muscle development?
Most people worry about protein with regards to muscle and particularly, topline development. While we have estimates on general daily protein requirements for most horses, many sport horses are top level athletes, yet we still know very little about exactly how much protein is needed for maximal muscle development.
That is where my research work at the University of Kentucky has been focused the last few years2. In collaboration with Cavalor®, an equine feed and supplements company, I looked at the effects of different doses of a high-quality protein supplement (Cavalor VitAmino®) on the activation of pathways involved in muscle protein synthesis. As expected, we saw a dose-dependent increase in muscle protein synthesis pathway activation with protein intake (Fig. 1).
Interestingly, we noticed that once maximal activation was achieved, feeding additional protein had no further effect. This indicates that horses, like human athletes, have an upper limit for muscle protein synthesis, and that it probably takes less protein than we think for maximal stimulation, as long as it is of high quality. This is the first time this has been shown in horses, and further supports the argument that overfeeding protein is not beneficial for the horse and should be avoided.
The second misconception when it comes to muscle development is the assumption that a particular feed or supplement will automatically lead to a beautiful topline. Regardless of how much protein you feed, regular exercise, with focus on working the specific muscles that form the topline (i.e. core and back muscles), is the only way to stimulate muscle development. Exercise is key to initiating the build-up and strengthening of the muscle fibers. The micro-damage and mild inflammation of the muscle after an intense workout will trigger an adaptive response in the muscle tissue, stimulating it to grow stronger and become more efficient in an attempt to protect itself from more damage during the next exercise bout. Only repetitive stimulation of this adaptive response will ultimately lead to an increase in muscle mass and overall fitness. So, while protein provides the necessary building blocks, frequent exercise is needed to effectively convert those amino acids into muscle tissue.
There is still much to learn about optimal protein levels for different types of horses. However, the general consensus of recent scientific studies indicates that we can meet requirements for most horses with less protein than we think, especially when feeding high quality protein. More importantly, feeding more than needed does not seem to have any additional benefit on muscle protein synthesis and could have negative health effects in some horses. To feed protein safely and effectively make sure to: 1. estimate the requirements of your horse; 2. determine the protein content (and quality!) of all dietary components (concentrates + forage) and 3. match the feed to your horse’s needs.
Want to learn more about the P’s & Q’s of Protein Feeding in Equine Diets? Dr. Caroline Loos was a featured presenter at the 2020 Adequan®/USDF Virtual Convention, where she led a discussion on the importance of equine nutrition. USDF members can log into the USDF Education Library to watch her presentation on-demand: https://www.usdf.org/education/university/kb/courses/indepth.asp
1Loos, et al., A high protein meal affects plasma insulin concentrations and amino acid metabolism in horses with equine metabolic syndrome. The Veterinary Journal 20 (2019) DOI: 10.1016/j.tvjl.2019.105341
2Loos et al., Pathways regulating equine skeletal muscle protein synthesis respond in a dose-dependent manner to graded levels of protein intake. The Journal of Animal Science (2020): DOI: 10.1093/jas/skaa268