Equine Carbohydrate Disorders Part 2: Understanding the Terminology

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Written By Dr. Kris Hiney
Last month we began a new series looking at carbohydrates commonly fed to horses, and the different forms they come in. We discussed the differences between simple sugars and polysaccharides and how the bonds which join these sugars have direct implications on how they are digested in the equine. We also discussed a few carbohydrates in particular in which many horse owners have particular interest, including fructans, which may carry health risks, and fructooligosaccharides, which can be used as digestive aids. In the following article I will attempt to define the wide array of terminology that one finds in equine nutrition, such as crude fiber, neutral detergent fibers, and non-structural carbohydrates, among others.
Horses’ diets primarily consist of plants which are largely made up of carbohydrates. Certainly plants also contain protein, lipids, vitamins and minerals, as well as water, but their primary composition is carbohydrates. If we use the carbohydrate definitions from last month, we know that the cell wall of plants is composed of cellulose, hemicellulose, lignin (which is indigestible by horses), beta glucans, gums and pectins. Inside the cell wall we find the more digestible cell contents which contain the simple sugars, starches, oligosaccharides and fructans. Often horse owners need to know what percentage of these compounds appear in a particular feedstuff. We use a variety of analytical methods to partition these carbohydrates into groups.
Crude Fiber
The most commonly used descriptor of carbohydrates on feed tags is usually crude fiber. Crude fiber content of a feed is determined using proximate analysis or the Weende system. Crude fiber is the residue remaining after subtracting water, lipids, proteins and the nitrogen-free extract from the feed. Nitrogen-free extract is said to represent mainly sugars and starches.   It uses a chemical method of solubilizing the feed using acids and bases. One of the problems in using crude fiber as a descriptor is that any lignin, cellulose or hemicellulose which is solubilized in the process is lost and appears in the nitrogen-free extract value for the feed. Thus, crude fiber values may actually underestimate the fiber value of the feed, and over estimate the nitrogen-free extract portion.
NDF and ADF
The Van Soest system of analysis improves on simple values of crude fiber, by separating out different fiber fractions into neutral detergent fiber and acid detergent fiber. These are the most common values which are reported if you have a forage analyzed. Unfortunately this method is most applicable to the value of feedstuffs for ruminants, rather than horses. Regardless, it still does offer the horse owner some important information. The neutral detergent fiber of a feed contains hemicelluloses, cellulose and lignin, although some soluble hemi-cellulose may escape into solution.    It does not provide information about fructans, pectins, gums, or beta glucans which are quite readily fermented by the equine digestive tract. Acid detergent fiber represents the lignin and cellulose content of the feed, as the hemicelluloses have been removed. The amount of hemi-cellulose in a feed, which is easily fermented by horses as well, is represented by the difference in value of the NDF and ADF value for the feed.
Total Dietary Fiber
Total dietary fiber is more frequently used in human nutrition, but may actually offer better information for species which are hind gut fermentors, as is the horse. Total dietary fiber combines many plant values together, and essentially represents the portion of the plant that resists enzymatic digestion which takes place in the small intestine. This includes the traditional fiber components of hemi-cellulose, cellulose and lignin, but also includes pectins, oligosaccharides, mucilages, gums, etc. which are often referred to as soluble fibers in human nutrition. Thus this method recovers more of the true fiber portion of the feed which is susceptible to fermentation in the horse and contributes to their energy supply. Unfortunately this method is not commonly used in commercial feed testing laboratories which serve the animal industry.
Non-Fiber Carbohydrates (NFC)
Now the definitions are going to start getting a little trickier, and the horse owner may encounter many different acronyms. Non-fiber carbohydrates, or NFC, represent the carbohydrates which are not in the cell wall and are not recovered when doing an NDF feed analysis.    The NFC is what remains after subtracting the NDF, protein, fat and ash. It represents the sugars, starches, fructans, galactans, pectins, beta glucans and organic acids. NFC is determined via a mathematical procedure and not a chemical analysis. You may also see values reported as NSC, which stands for non-structural carbohydrates. These values are actually determined chemically and differ from NFC in that NFC values may also include pectins and gums which will not appear when analyzed for NSC.
WSC
Confused? Let’s add some more letters. Water soluble carbohydrates are also used in equine nutrition to determine the most easily digested carbohydrate portion. The monosaccharides, disaccharides, oligosaccharides and some polysaccharides appear in this portion of feed separation. Compared to the NSC value for feeds, it would equal the WSC portion added to the starch content of the feed. Starch content of feed can also be analyzed separately as well.
Finally, some equine specialists have suggested that carbohydrates need to be redefined in terms of horse nutrition. As they are hind gut fermentors, they handle carbohydrates much differently than do ruminants. Certainly, they also get much more use of cell wall components than do simple monogastrics. Many of the particular disorders found related to carbohydrate digestion in the horse may require us to look differently at feeds than is needed in other species. One of the suggested systems created by Rhonda Hoffman (currently of Middle Tennessee State University) is to separate equine carbohydrates into hydrolysable carbohydrates (CHO-H) subject to enzymatic digestion, and fermentable carbohydrates (CHO-F) which undergo fermentation in the hind gut. The fermentable carbohydrate fraction can further be defined as either rapidly fermentable carbohydrates (CHO-FR) or slowly fermentable (CHO-FS). Slowly fermentable carbohydrates would include those seen in NDF values, with the indigestible lignin portion removed from the value. The rapidly fermentable carbohydrates, whose presence can increase the energy content of a feed, include the oligosaccharides, fructans, beta glucans and pectins. Feeds higher in rapidly fermentable carbohydrates can offer more calories to the horse. Alternatively, some horse owners may need to be aware of the fructan content of feeds or forages in horses more prone to developing laminitis.
So what values do you need to know and why should you care?
In general, crude fiber values are listed on most feed tags. Typically the lower the crude fiber value, the higher the energy density of the feed. This is not true of feeds which are designed to have higher fat values, and may include fibers to provide a healthier type of feed for the horse. Owners who have horses with insulin resistance or metabolic syndrome, as well as PSSM horses, should try and choose feeds which are lower in the soluble carbohydrates such as sugars and starches. However, these values are not always provided on feed tags. Owners may also select away from forages which may be higher in sugars and starches as well for these particular types of horses with demonstrated metabolic disorders.
Next month, we delve more deeply into particular carbohydrate disorders seen in horses.

Equine Carbohydrate Disorders, Part 1: Definitions and Relationship to Equine Diseases

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Written By Dr. Kris Hiney
Equine disorders related to carbohydrate consumption have received much attention by owners and researchers alike, as of late. This has resulted in almost a mistrust or fear of feeding horses carbohydrates.  But in reality, almost all of the horse’s calories come from carbohydrates – there is no way to avoid them in the horse’s diet. What one must do is understand all of the forms in which CHO (carbohydrates) are found, identify horses at risk for CHO disorders and select the appropriate feeds to keep them healthy.
To begin, carbodydrates are simply molecules composed of carbon, hydrogen and water. Monosaccharides are single units of sugars which vary slightly in their structure.  Common monosccahrides in the horse’s diet consist of glucose, galactose, fructose, mannose, arbinose and xylose. While these monosaccharides are not normally found in their single form in plants, they are joined together to make  longer polysaccharides. However, monosaccharides are produced through  enzymatic digestion by the horse.  Disaccahrides, then, are just two sugar units linked together. Common disaccharides include lactose (found in mare’s milk and is formed by glucose and galactose linked together) and maltose (two glucose units linked together).
Figure 1. Glucose and galactose. The two structures only differ by the location of the hydroxyl group on the left side of the structure.
Oligosaccharides are longer chains of a variety of monosaccharides linked together, typically between three and ten sugar units.  The primary oligosaccharides in the horse’s diet are stachyose, raffinose and fructo-oligosaccharides (FOS).  FOS have received attention in animal nutrition as a way to supply pre-biotics to the animal. Pre-biotics are often oligosaccharides which are resistant to digestion in the foregut of the horse but are digested by bacteria in the hindgut. These supply a source of nutrition which supports the growth of beneficial bacteria and perhaps reduces the population of disease causing – or “pathogenic” – bacteria. In fact they are looked at as an alternative to feeding antibiotics in livestock. FOS are believed to alter the pH of the colon to a more favorable environment for the most productive bacteria. Mannose specific oligosaccharides are also thought to reduce the adherence of pathogenic bacteria to the epithelium of the gut wall. In yearling horses, feeding FOS reduced fecal pH and increased the production of volatile fatty acids from the hind gut. FOS supplementation also decreased the incidence of diarrhea when fed to foals. It has also been shown to have a protective effect on the development of foal diarrhea when fed to their dams. However, it is not known if that was an indirect effect passed through the milk, or if the foals simply ingested some of their dams’ feed containing the supplement. While feeding oligosaccharides does not appear to have an immune boosting effect that has been suggested in other species, it does appear to have beneficial effects on gut health in the equine. Horses receiving FOS and challenged with a large barley meal had less lactobacilli in their colon compared to controls. Thus FOS may help prevent GI disturbances due to diet changes or CHO overload.
Fructooligosaccharides also belong to the category of carbohydrates labeled as fructans.  Fructans are polysaccharides which have multiple fructose units. Inulin is also classified  as a  fructan. Many horse owners have heard of fructans as a risk factor for pasture associated laminitis. A sudden increase in fructans in the diet can alter the microbial population in the hindgut which may then subsequently lead to the development of laminitis. Fructan concentrations in grasses vary with both season and time of day.  Fructans and other starch concentrations are highest in the spring, lowest in the summer and intermediate in the fall. During the day, the process of photosynthesis results in the highest concentrations of fructans in the afternoon with sometimes half or less in the morning or evening hours.
Other CHO include longer chains of sugar units and are known as polysaccharides. Most commonly we think of starches and fibers as the common polysaccharides in the equine diet. Starch occurs in either linear form known as amylose or branched form, amylopectin.  It is composed of only glucose linked by bonds that can be enzymatically digested by the horse. In contrast, cellulose is also a straight chain of glucose but is linked by a different type of bond , a beta bond, which must be broken by microbes. Fermentation of this fiber fraction results in formation of volatile fatty acids which are metabolized by the horse to produce energy. Pectin and hemicelluloses are also common polysaccharides found in the equine diet.
Figure 2. Amylose is a chain of glucose units linked by alpha bond.
Figure 3. Cellulose is a similar chain of glucose units, but linked by beta bonds instead, making it indigestible by mammals.
Those CHO linked with alpha bonds can be digested in the foregut, allowing the monosaccharides to be absorbed intact. In contrast, cellulose, hemicelluloses, pectin, raffinose and stachyose, which contain beta bonds, will all need to undergo microbial fermentation to provide energy to the horse.   Hemicellulose, compared to cellulose, is a mixture of arabinose, xylose, glucose , mannose and galactose. Pectin is made up of beta linked galacturonic acid, arabinose and galactose. Pectin and hemi-cellolose are more rapidly fermented than cellulose and increase the digestibility of the feed if present in a greater proportion.
Now that we know what different types of carbohydrates exist in the horse’s diet, let’s look more closely at some differences that occur in forages. Typically, forages should always make up the bulk of the horse’s diet. They are made up of structural CHO which make up the cell wall as well as some indigestible lignin.  The plant cell wall is made of cellulose, hemicelluloses and pectin. Forages also have non-structural CHO or NSC in the cell content, though certainly not as much as concentrates. The NSC is a mixture of monosaccharides (glucose, fructose, etc.) and disaccharides as well as starch and fructans.
If we compare common forages, cool season grasses are made up of primarily cellulose, then hemi-celluose and the fairly small amounts of pectin. Cool season grasses include Kentucky Bluegrass, orchard grass, fescues and ryegrass.  Legumes, which are typically high in digestible energy are relatively higher in pectin. Legumes would include alfalfa, clover, lespedeza and peanuts. Warm season grasses grow and mature more rapidly and have much more cell wall/kg DM and thus much more fiber. Warm season grasses include Bermuda grass, switchgrasses, and bluestem. Therefore warm season grasses at a later stage of maturity may be ideal for horses with carbohydrate sensitivities. In general, there is a higher proportion of cell content in a younger, or more immature plant. This makes grasses or hays harvested at an earlier stage more digestible.
Interestingly, the storage form of CHO in legumes and warm season grasses is primarily starch, while cool season grasses prefer to store energy in the form of fructans with much less starch. There is also a limit to how much starch the chloroplasts of warm season grasses and legumes can contain, yet there is no limit to fructan accumulation. Fructan also accumulates more to the base of the plant and more so in the stem than in the leaf. Cool temperatures and droughts (which typically don’t go together) may also increase the fructan production by the plant. Anything that promotes photosynthesis but retards growth ends up increasing NSC (lots of light with cool temperatures).   Therefore, be especially careful to observe growing conditions, especially if the horses are consuming cool season grasses and have carbohydrate sensitivities.

Lipid Nutrition: Part 4, Omega-3 Fatty Acids

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Written By Dr. Kris Hiney
In previous articles we have discussed the many benefits of feeding fats to horses. Typically these fats in feeds are vegetable oils, or even occasionally animal fats. We have not yet discussed specifically the type of fat in the diet. However, researchers in human and animal medicine have much information supporting the idea that specific types of fatty acids can provide numerous health benefits. This month we look at the science behind Omega-3 fatty acids and begin the process of understanding the terminology used.
Omega-3 fatty acids
So what makes Omega-3 (or “n-3”) fatty acids so unique? Quite simply, it’s just the location of the double bonds which occur between the carbons in the fatty acid chain. The location of these bonds are what provide these fatty acids with their naming system.  Omega-3 fatty acids have the last double bond placed three carbons from the methyl end of the carbon chain, which is the opposite end from the attachment to the glycerol backbone in a triglyceride. Compare this to the Omega-6 fatty acids (or “n-6”), which have their last double bond six carbons in from the methyl end. This simple change in location of a double bond can have tremendous impact on the metabolism of these fats in the body.
Essential fatty acids
Previously we mentioned that horses must ingest certain fatty acids in their diet as they do not have the capability of synthesizing them in great enough quantities. These include linoleic acid and linolenic acid. Both of these fatty acids are 18 carbons long but differ in the number and placement of the double bonds.  Linolenic acid has three double bonds with the last one placed three carbons from the methyl end.  Thus, it is an Omega-3 fatty acid.  Linoleic acid has two double bonds, with the last double bond six carbons from the methyl end and is an Omega-6 fatty acid. These two fatty acids represent the essential fatty acids that horses must consume. These fatty acids do occur in forages and concentrates such as corn and oats, just in smaller quantities than we think about in more fat rich feedstuffs. Typically there will be more Omega-3 fatty acids in forages, especially pasture grasses, while grains will contain more Omega-6 fatty acids.
              Linoleic Acid Molecular Diagram               Linolenic Acid Molecular Diagram
The horse, as well as humans, must consume these fatty acids as we both lack the necessary enzymes to build these structures on our own. However, we do possess the enzymes needed to elongate these fatty acids to more complex fatty acid structures.  These elongation enzymes are shared by both linoleic and linolenic acid in their metabolic pathway. Their products in turn can be used to synthesize a whole host of biologically active compounds.  Linolenic acid can be elongated to eicosapentanoic acid or EPA, a twenty carbon fatty acid with five double bonds, and docosahexaenoic acid or DHA, a 22 carbon fatty acid with 6 double bonds, as well as others. Both EPA and DHA are Omega-3 fatty acids, due to their origin from an Omega-3 fatty acid. Linoleic acid is elongated to arachidonic acid, a twenty carbon fatty acid chain with four double bonds which is, of course, an n-6 fatty acid.   These fatty acids can be used to synthesize eicosanoids, which are biologically active lipids.
Eicosanoids have hormone-like activity which is typically mediated locally within a tissue. These include prostaglandins, thromboxanes and leukotrienes.   These compounds differ by their structure and perform a host of activities within the body. All of these compounds are necessary for normal bodily function, but an imbalance can contribute to a disease state. Prostaglandins can effect smooth muscle contraction, vasodilation, inflammation, pain, and fever, as well as gastric acid and mucus secretion. Leukotrienes are released during the inflammatory process and can contribute to inflammation and bronchoconstriction. While their role may be to aid in healing the damaged tissue, overproduction of leukotrienes can contribute to asthma or allergic reactions. Finally, thromboxanes cause the aggregation of platelets and constriction of blood vessels. Again, all of these compounds are part of normal bodily function, but their potent effects can contribute to the diseased state.
So how do Omega-3 fatty acids fit into this story of thromboxanes and leukotrienes? When animals ingest greater quantities of Omega-3 fatty acids, these fatty acids can displace arachadonic acid in the cell membrane. Thus, there is less arachadonic acid available to be released and formed into eicosanoids.   Increased linolenic acid also decreases the amount of linoleic acid which is elongated simply due to a competition for the same enzymes. The elongation products of linolenic acid and subsequently EPA may also directly counter act some of the inflammatory products of arachadonic acid metabolism. Thus increased consumption of Omega-3 fatty acids may aid in decreasing inflammation in the diseased state.
Feeding Omega-3 fatty acids may be helpful for horses which may have chronic pain or inflammation. Traditionally horsemen have used NSAIDS, or Non-Steroidal Anti-Inflammatory DrugS, to manage pain and inflammation. When we provide NSAIDS these compounds act by blocking the activity of enzymes which cause the release of inflammatory agents.  However, NSAIDS are not specific and block the activity of both cox-1 and cox-2. These enzymes are essential in the conversion of arachidonic acid to progstaglandins.  Cyclo-oxygenase 1 unfortunately is also intimately involved with the formation of thromboxane in platelets and in gastromucosal integrity. It is the inhibition of cox-1 which leads to the formation of ulcers in horses which have long term NSAID administration. However, many specific cox-2 inhibitors have been produced (Celebrex® and Vioxx®).
Sources of Omega-3s for horses
Compared to humans, it may be harder to increase the consumption of Omega-3 fatty acids in horses, but not impossible. Typically the greatest concentration of Omega-3 fatty acids is found in marine fish. Certainly fish oils have been fed to horses, but there may be limits in the acceptability of fish oil by the horse. Flaxseed, however, is also an excellent source of Omega-3 fatty acids, has a slightly sweet, nutty, whole-grain flavor and aroma, and is readily accepted by horses. Many flax products are now offered to the horse owner.  Increasing consumption of fresh grass will also boost the Omega-3s in your horses’ diet.
Next month we continue to look at Omega-3 fatty acids in the horse’s diet and examine some of the available literature concerning their effects in the horse.

Lipid Nutrition: Part 1, Feeding Fat to Horses

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Written By Dr. Kris Hiney

This month we begin a series looking at the value of incorporating fat into the diets of our horses. We will discuss how fat is digested and handled in the equine, the types of fats fed to horses, and the many beneficial effects that can be realized through the addition of fat to the diet of our horses.
Fat digestion
Feeding fat to horses became more popular in the 1980’s and has continued to see an increase in the share of the feed market. Most feed stores now offer a selection of fat added feeds, or specific fat supplements. While one may not think of horses as a species that routinely consumes fats, horses can handle fats quite well in their digestive system. Lipid digestion occurs primarily in the small intestine, via the production and release of digestive enzymes and bile salts. As the horse does not possess a gall bladder, bile salts are continually released into the intestine. Fats that are added to the diet in the form of oils or fat are very well digested, typically up to 90%. Comparatively, naturally occurring fats in the diet (muchsmaller percentages of fat are actually present in forages and cereal grains) are less well digested, between 40-50% for forages and 50-75% for grains. Addition of fat to the diet does not alter digestibility of other components of the diet, unless the amount of lipid exceeds 22% of the total diet. However, typically this is not a concern, as acceptability and practicality of such diets make them improbable. There are some published studies which do report a lowered fiber digestibility in horses fed soy oil, however, these horses were also rapidly introduced to the fat in the diet. Ideally horses should be gradually transitioned onto a higher fat diet in order to adapt and increase the necessary fat digesting enzymes in their system. This should take place over one to two weeks, depending on how much fat is being added to the diet.
Acceptability
Palatability of fat added feeds is quite good, especially if supplied by vegetable oils. Typical vegetable oils include corn oil, soybean oil, canola oil and linseed oil. Horses will consume animal fats and fish oil, but typically not as readily as vegetable sources. The acceptability of fats in the diet is good up to about 15% of the diet. After that consumption rates do drop off.   There are commercially available feeds which have a higher percentage of fat, but these are typically extruded feeds which are more acceptable. Again, these are fed at a smaller percentage of the diet, such that 15% of the total diet is never exceeded. When feeding fat added feeds, it is important to realize that they do have a shorter shelf life than non-fat added feeds. This is due to the peroxidation that takes place, especially in polyunsaturated fats. These feeds then develop an off taste and flavor. If your feed smells rancid, it is best to avoid feeding it. Storing feeds in a cool, dry area will help to preserve their shelf life as well. These feeds often have anti-oxidants added to them to aid in protection against oxidation. Some products, such as Omega Horseshine, specialize in stabilized fats with a prolonged shelf life, up to 12 months.
Benefits to feeding fat
The most readily realized benefit to adding fat to the diet is in order to help meet the animals’ caloric needs. Fat is very readily digestible as already stated, and is much more energy dense than other components of the horse’s diet. Compared to proteins and non-structural carbohydrates which contain 4 Mcal/kg, fat is 2.25 times more energy dense at 9 Mcal/kg. Thus inclusion of fat allows a horse to gain weight much more readily or conversely, need to consume less feed to obtain the same amount of calories. Lowering the total amount of feed may be advantageous to horses working in hotter climates as it lowers the total heat production associated with digestion. Furthermore, fat itself is a relatively cool feed, as there is no fermentation and thus heat production associated with its digestion. Replacing high energy cereal grains with fats is an additional benefit, as less digestive risk is associated with feeding fats. Horses fed large amounts of cereal grains over time are at greater risk for ulcer formation, potential development of  stereotypies such as cribbing, laminitis and insulin resistance. This does not mean that starch needs to be eliminated from normal equine diet (the exception are horses with metabolic disorders which render them more sensitive to starch in the diet), but fat can make a very useful substitution. Another benefit to replacing starch in the diet with fats appears to be a calming effect on the horse. Horses fed fat added diets compared to typical sweet feeds have been found to be  less reactive to novel stimuli. Therefore, there is a second reason that fat is a cool feed, not only does it produce less heat during digestion, but it appears to “cool” the hot minded horses. Now obviously it is not a substitute for proper training and exercise!
Essential fat and fatty acids
 Horses must also consume some amount of fat for normal body functio. Lipids are used in the synthesis of steroid hormones, and  all of the fat soluble vitamins (ADEK) are contained within the fat portion of the feed.  However, the exact amount of fat necessary in the diet of the equine has not been determined. Additionally, the horse, like all other animals, must consume its essential fatty acids, linoleic (18:2 omega 6) and linolenic acid, (18:3, omega 3) from the diet. They lack the enzymes necessary to produce these particular fatty acids within the body. Important sources of these fatty acids include pasture grasses, canola oil and linseed oil or flax seed.
Practical guidelines for feeding fat to horses.
As stated previously, most fats in horse feed actually come from vegetable oils. The oils can either be extracted and purified, or the actual oil seed can be fed. Examples of common oilseeds include cottonseeds, soybeans, canola and flaxseeds. If these seeds are referred to as meal, such as cottonseed meal, the fat has already been extracted and then they are being fed typically for their high protein content, not for additional fat. Thus, feeding linseed meal provides a much diferent percentage of fat compared to feeding flax, despite it being the product of the same plant! Pure vegetable oils can also be fed to horses as a top dressing to their feed. One cup of vegetable oil provides as many calories as 1.5 lbs of oats or 1 lb of corn, allowing you to decrease the amount of cereal grains fed.   If feeding a fat added feed, typically these feeds will allow you to feed less concentrate for a similar work class of horse, due to the increased caloric density of the feed. The benefit of feeding a fat added feed, rather than top dressing, may be in its simplicity, as well as the fact that these rations are rebalanced with the knowledge that the horse may consume total less feed. However, if you are just top dressing fat to existing feeds, and thereby decreasing the total amount of feed, be sure that the total diet still meets the horse’s other nutritional requirements.
In the next part, we will discuss the potential for performance enhancing effects of feeding fat beyond merely an easy way to supply calories.