Protein Nutrition, Part I: Protein Digestion in the Horse
This entry was posted on November 7, 2010 by Omega Fields.
Written By Dr. Kris Hiney
This month we begin a new series focused on protein requirements for our horses, and the various feeds which provide protein. Protein is commonly the first concern of many horsemen when selecting their feeds, and the most frequently discussed. In the equine industry, there tends to be a common fallacy that if a product costs more, it clearly is a more desirable product to feed. As feedstuffs which contain more protein are often more costly, this tends to make higher protein feeds more attractive to the consumer. However, that higher protein content may not be necessary to feed to your horse! Before we can delve too deeply into the proper amounts of protein to feed to the various classes of horses, we should back up and break down what protein does for the animal, and examine how the horse digests this very important nutrient.
Proteins are actually complex molecules that are comprised of a series of amino acids joined together by peptide bonds. All proteins are built from a specific series of the 20 most common amino acids. While many more amino acids are found in nature, we typically limit our discussion of amino acid nutrition to these basic 20 amino acids. Of these amino acids, some are characterized as dietary essential amino acids or non-essential amino acids. The dietary essential amino acids include lysine, methionine, phenylalanine, the branched chain amino acids (valine, leucine and isoleucine), tryptophan, threonine, histidine and arginine. The distinction between these two classes is that mammalian systems lack either the ability to synthesize some amino acids (they lack the appropriate synthetic enzymes) or they cannot produce enough of that amino acid to support normal bodily function. The non-essential amino acids can be produced in the animal’s diet from other amino acids. In contrast, bacteria can produce all amino acids. This is what gives the ruminant animals such as cattle, sheep and goats, such a unique advantage. They can rely very heavily upon bacterial synthesis of amino acids to meet their requirements.
Figure 1. The levels of protein structure from primary (just the linear chain of amino acids) up to the completely folded, functional proteins.
All animals, in addition to plants, bacteria and fungi produce a wide array of proteins necessary for normal function. These include structural proteins found in muscle, bone, cartilage etc., enzymes, hormones, antibodies, and cell signaling molecules. The specific make up of a protein is coded for by the organisms DNA. The DNA contains billions of individual genes that contain the information on how to build each specific protein found in the organism. Different tissues will turn on or express different genes, thus making liver produce the necessary proteins for its function, and the heart a separate set of proteins. However, every cell will contain the genetic information to produce every protein. That is why you can use a skin cell to clone an entirely new animal. All the necessary information to make a new horse is contained in that one cell. While this information may not seem germane to feeding horses, it is important to understand why the animal must receive particular amino acids in the diet. Without the right amount of essential amino acids, the animal is unable to synthesize protein correctly.
When we feed protein, we need to be concerned with more than just the amount of protein contained in the feed. Feed tags will provide us with the percent of crude protein present in the feed. This number is really based on the percent of nitrogen found in the feed. That number is then multiplied by 6.25 to achieve the amount of crude protein. That is based on the fact that the average protein contains 16% nitrogen. However, there may be non-protein nitrogen present in the feed which may or may not be useful to the animal. While all protein contains nitrogen, not all nitrogen is protein. For example, in ruminant animals, non-protein nitrogen is often fed in the form of urea, due to the unique makeup of the animals digestive system which allows it to utilize these feed sources. But what the crude protein percentage does not provide us with is information about how useful is that protein to the animal. As horseman, we need to be concerned not only with the quantity of the protein we feed, but more importantly, with the quality.
The quality of the protein can be thought of in two different ways. One indicator of protein quality is how digestible is that protein to the animal. A feedstuff which has a high percentage of protein that is unavailable to the animal, or is unable to be absorbed, is essentially useless. So let’s take a moment and explore protein digestion in the equine. Protein digestion begins in the stomach of the horse, where it is exposed to both inorganic acids (hydrochloric acid) and proteolytic enzymes – pepsin. These two digestive secretions begin the breakdown of protein by initiating the unfolding of the protein structure. While proteins are coded for by DNA in a linear form, the particular amino acids present in the protein cause the peptide chain to fold and wrap around itself to form its unique functional shape. In fact, one misplaced amino acid can render an entire protein useless. This is often the basis of many genetic diseases, a mutation which causes the wrong amino acid to be added to the peptide chain. Pepsin begins the disruption of the polypeptide chain by cleaving after specific amino acids, primarily tryptophan, tyrosine and phenylalanine. While gastric digestion does not completely break protein down to a point it can be absorbed by the animal, it shortens the polypeptides and allows more access to the enzymes which will be present in the small intestine
As the dietary protein enters the small intestine, it will be digested much further by a series of proteolytic enzymes which arise from the pancreas. These include chymotrypsin, trypsin, elastase, carboxypeptidase etc. These enzymes essentially complete the breakdown of protein into small enough pieces that can be absorbed including single amino acids, dipeptides and tripeptides. Once these are absorbed into the enterocyte, the dipeptides and tripeptides are further hydrolyzed into single amino acids. These then enter the blood stream of the animal where they can be delivered to various tissues to be used for protein synthesis. Non-protein nitrogen can also be absorbed out of the small intestine of the horse. Feeding urea causes an elevation of blood urea nitrogen,as well as urinary nitrogen, as the animal flushes the nitrogen out of the animal’s system. There is no evidence to support the usage of feeding nonprotein nitrogen to the horse, as it is unable to be incorporated into bodily protein.
Sidebar: Foal management. The exception to protein digestion occurs right after birth. The foal is born with a naïve immune system and must receive antibodies from the dam. These come from ingestion of colostrum or the first milk a mare produces. The colostrum contains a trypsin inhibitor which prevents the antibodies which are present in the milk from being digested.The foal also helps out the process by having an “open gut”. The proteins initially presented to the gut can be absorbed intact through the process of pinocytosis, or engulfing of the entire protein by the gut cell membrane. However, there is a finite capacity of the gut to perform this action. The most efficient uptake of intact proteins takes place within the first 12 hrs, with relatively little to no absorption past 24 hrs. Additionally, there is a limit to how much protein can be absorbed. If a foal was first provided milk replacer prior to the administration of colostrum, they may not be able to absorb the important antibodies if fed later with the appropriate colostrum. So always make sure your foal receives high quality colostrum in the first 24 hours of life. Don’t skip straight to the milk replacer in an orphaned foal!
Any protein which escapes digestion in the small intestine is then delivered to the hind gut of the horse, where microbial fermentation can take place. The microbes are quite adept at further degrading feedstuffs, and producing compounds that are of value to themselves, as well as to the horse. The bacteria in the hindgut of the horse are quite capable of synthesizing all of the amino acids, however there is little evidence this is useful to the horse. In contrast to the ruminant animal, whose site of fermentation and microbial protein synthesis is prior to the site of enzymatic digestion and protein absorption in the hindgut, the horse is unable to absorb these proteins. Unless the horse practices copraphagy, or the ingestion of feces, this protein is believed to be lost to the animal. In horses infused with lysine either in the stomach or in the cecum, only gastric infusion resulted in an elevation of plasma lysine, indicating the inability to absorb lysine out of the hindgut. However, there has been some recent evidence that some absorption of amino acids may occur in the hindgut. Messenger RNA coding for specific amino acid transporters have been isolated from the hindgut of the horse. This indicates a potential ability of the horse to absorb at least some specific amino acids from the hindgut. However, the transporter proteins themselves have yet to be isolated, or the disappearance of specific amino acids from the hind gut to be proven.
Finally, let’s compare different feedstuffs in the extent and site of their digestion in the equine. Protein digestibility differs according to both type and amount. It has been repeatedly documented across a wide number of studies that increasing the amount of protein in the diet, also increases the percentage of that protein which will be digested. Perhaps this is due to an adaptation of the digestive system to increase the synthesis of proteolytic enzymes when presented with larger amounts of protein. Total tract digestibility of protein in forages ranges from 73-83% for alfalfa, 57-64% for coastal Bermuda grass hay, and 67-74% for other grass hays. Total tract digestibility for grains or concentrates is much higher, ranging between 80-90%. If however, the site of digestion is take into account, prececal digestion of forages ranges between 25-30% while concentrates are digested much more extensively in the small intestine, up to 70-75%. So what does this mean for the horse owner? Ideally when selecting protein sources for horses, both the digestibility as well as the site of digestion must be considered. Ideally,protein should be digested in the small intestine rather than the hindgut, to optimize amino acid absorption. However, there may be some evidence that this species which does rely so heavily on hind gut fermentation may be capable of some absorption out of the hind gut. The extent of that is yet to be proven.
For next month, we will look at the second piece of protein quality, the amino acid profile of the diet.