Interesting subject, but complicated. This article, if read carefully helps the average lay person understand this complex subject. If you are serious about your Equine Athlete it is a subject you need to at least have a basic knowledge of....
Physiological Functions of Equine Nutrition
(Carbohydrates and Fats)
Anne Rodiek, Ph.D.
Californa State University, Fresno
Feeding horses is both a science and an art. Horses have done well on a wide variety of diets for hundreds of years. While evolved as a roaming and nibbling eater, horses have adapted to confinement and meal feeding. While evolved on a diet of fibrous feeds, horses have adapted to highly digestible, high energy rations. While man has tried a myriad of dietary manipulations to cause horses to perform better in the service of man, horses have remained robust and still willing to serve man. From a large view, it could be summated that horses have done an amazing job at continuing to do what they do in spite of what man has fed to them, rather than because of it.
DIGESTION, METABOLISM AND FEEDS
The digestive system of the horse is designed to maximize digestion of both high and low quality feeds. The small intestine is the site of enzymatic digestion of soluble carbohydrates, proteins and fats, the components of which are absorbed into the circulation. Insoluble carbohydrates pass to the cecum and colon where they are fermented by bacteria and protozoa, similar to those of the ruminant. Prececal digestion is relatively rapid, while postileal fermentation is quite slow with total rate of passage exceeding 24 hours. Although with large variability depending on diet and physical form of the feeds. This anatomical arrangement allows enzymatic digestion by the horse before fermentation by microbes ensuring that high quality nutrients are absorbed directly by the horse and are not converted to lesser quality nutrients by the microbes.
More than 70% of the horse’s daily feed intake is used to provide energy to fuel the horse’s metabolism. Traditional horse feeds are comprised primarily of carbohydrates with very little (less than 5%) fat. However, research over the past twenty years has shown that horses absorb and utilize fat efficiently. As such, both carbohydrates and fats (and to a lesser extent protein) can be used as energy sources for all activities of horses.
Plant materials contain both structural and nonstructural carbohydrates. Structural carbohydrates are the support structures of the plant and are composed primarily of insoluble carbohydrates, largely cellulose, hemicellulose and lignin. Cellulose and hemicellulose are degradable by large intestine microbes and yield as endproducts volatile fatty acids. These are short chain fatty acids (acetate, propionate and butyrate and small amounts of others) that are absorbed from the large intestine into the blood stream and can be used by the horse as an energy source. In horses maintained on an all forage diet, volatile fatty acids can provide up to two-thirds of the horse’s total energy requirement.
Nonstructural carbohydrates are primarily starches and smaller amounts of sugars. Grains are high in nonstructural carbohydrate as are the leaves of younger plants. Nonstructural carbohydrates are digestible by the enzymes of the horse’s small intestine. These enzymes break down the carbohydrates to simple sugars, most abundantly glucose, which are absorbed from the small intestine into the blood stream. If nonstructural (or soluble) carbohydrates reach the large intestine, they are rapidly fermented by the large intestine microbes. If large amounts of soluble carbohydrates are fermented by microbes, a build up of volatile fatty acids can change the pH of the large intestine and can harm certain microbes, possibly also setting up conditions for colic or laminitis. As plants age, the ratio of nonstructural to structural carbohydrates decreases, making the plants more fibrous and causing the horse to rely more on large intestinal, microbial fermentation.
Pasture grasses grown in temperate climates can accumulate large amounts of nonstructural carbohydrate during the growing season. Among these are fructans, large molecules composed primarily of fructose that are not digested in the small intestine, but are fermented by certain species of microbes in the large intestine. Fructans have been implicated as a potential cause of laminitis in horses, similar to grain overload. Fructan accumulation is variable depending on the species of grass, and is greatest during warm days with high intensity sunshine. Fructan accumulation is also highest around mid day and lower in early morning and at night. If fructans do play a role as a causative factor in laminitis, founder-prone horses should not be grazed when plants are in a growing stage. If grazing is permitted, grazing at night is preferable over grazing during the day.
Grains are also high in nonstructural carbohydrates, but unlike pasture grasses, carbohydrates in grains are found primarily in the form of starches (large molecules composed primarily of glucose). Starches are a concentrated form of easily digestible carbohydrate and, as such, are excellent energy sources for horses. Research done in Germany (Meyer et al, 1993), however, has indicated that not all starches are alike with regard to their site of digestion in the horse. Type of grain, grain processing, amount of grain intake; source, preparation and amount of roughage intake; and individual variation between horses all affect the extent of starch digestion in the small intestine. Prececal starch digestion was found to be greatest in oats, intermediate in corn and lowest in barley. Grinding and popping of grains improved digestibility of corn as processing decreased particle size and possibly disintegrated the starch granules, increasing the efficiency of small intestinal enzymatic digestion. As grain intake increased, prececal digestion decreased and as the amount of roughage fed with grains increased, prececal starch digestion decreased. As prececal digestion decreased, the amount of grain reaching the large intestine increased. This is considered to be harmful as microbial fermentation of soluble carbohydrates can lead to harmful endproducts.
Beet pulp has become popular as a specialty feed for horses because of its “soluble fiber” attributes. Beet pulp is high in pectins, carbohydrate-like molecules, which are digested more slowly than starch giving slow release of energy without the alleged “grain high”. Beet pulp is popular as a feed for horses with HYPP as it is low in potassium and is also favored for endurance horses for its proposed ability to retain water in the digestive tract of the horse. More research is needed on beet pulp to confirm or refute its nutritional attributes.
Fats are generally well digested by horses, but there is some variability depending on source. Animal tallow have been shown to be 88 to 92% digestible while plant source fats are digested at rates up to 94%. Horses tend to prefer corn oil when given the choice, but most plant source fats are acceptable. Animal source fats are usually unpalatable unless incorporated and masked in a pelleted ration.
The main purpose of adding fat to equine diets is to provide an energy-dense feed for working, performance, lactating or growing horses. From a chemical standpoint it is known that fats contain 2.25 times the energy of an equivalent amount of carbohydrate, but from a practical feeding standpoint, vegetable oils contain about three times the digestible energy of an equivalent amount of grain. On a “by-volume” basis, vegetable oil contains 3.5 times the digestible energy of an equivalent amount of corn and 6 times the digestible energy of an equivalent amount of oats.
Oils can be added at rates up to 20% of the total diet on a by-weight basis. Supplemental fat should be added gradually to rations so that the diet is accepted by horses and also to allow the digestive tract a period of time to adapt to the added fat. Fat that passes through the small intestine will not be fermented by large intestine microbes and will appear in the feces, sometimes contributing to transient diarrhea.
Fats are frequently added to the diets of growing, pregnant, lactating and exercising horses to increase the energy density of the diet. Most research trials with weanlings and yearlings have concluded that growth is not different between young horses fed fat supplemented or more traditional, grain supplemented rations. Proponents of feeding fat to growing horses believe that fat should be used as a partial replacement for grain as an energy source. The theory herein is that the high starch content of grains produce elevated blood glucose levels which are thought to cause elevated levels of insulin, growth hormone and thyroid hormone which may alter the growth rats of bones and joints, predisposing colts to developmental orthopedic disease. Research in this area has yielded inconclusive results.
The benefits of feeding fat to performance horses have been extensively researched recently. The high energy density of fat supplemented diets may be beneficial when energy demands for training and competition are great. Fat supplementation may decrease the need for large amounts of grain thus decreasing the risk of colic or laminitis. Fat has also been touted for its lower heat increment, meaning that fat digestion causes less metabolic heat than the digestion of grains or hay. This may have a small but significant effect in horses working hard in warm environments. Many studies have addressed the glycogen sparing effects of fat. Horses acclimated to fat have been shown to have higher muscle glycogen levels at rest and/or after exercise compared to horses fed grain. The practice of feeding fat during training (for several weeks or months) may increase the efficiency of fat utilization for work performance to the point that fat is catabolized “preferentially” over glycogen/glucose. The net effect of this is a “glycogen sparing” effect in which muscle glycogen stores and/or blood glucose levels remain higher and/or longer during exercise. The implications of this are: 1) more total energy is present as fat than as glycogen so energy depletion will be delayed, 2) greater amounts of glycogen/glucose are available for use during anaerobic work (where glucose is the only metabolic fuel) , and 3) less lactic acid may accumulate when fat is utilized in place of glucose. As lactic acid accumulation has been implicated as a source of fatigue during intense work, fat utilization may help delay fatigue. From a theoretical standpoint, it appears that fat supplementation may be most beneficial for horses performing submaximal work of long duration. The benefits for horses performing maximal work of short duration are less clear. While some research has shown that glycogen stores are preserved during maximal work in horses fed fat, other research has not. The benefits of fats as a fuel for maximal work are uncertain as during maximal work ATP is largely derived from anaerobic glycolysis. The only substrate capable of yielding ATP from glycolysis is glucose. Glycolysis, while an inefficient system of energy production based on number of ATPs derived from glucose, can produce large number of ATPs very rapidly during maximal exercise. As such, an energy yielding mechanism that preferentially utilizes a substrate that requires oxygen when none is present (horses acclimatized to utilizing fat for energy rather than glucose) may not be the most effective way to fuel maximal exercise. Whether the horse is able to switch easily and efficiently between aerobic and anaerobic energy production despite the influence of diet during training has not yet been elucidated. Another confounding issue is the effect of lactic acid on work performance. Anaerobic glycolysis produces lactic acid as an endproduct which accumulates in the muscles, decreasing muscle pH and enzyme activities which ultimately decreases the rate of muscle contraction. Lactic acid accumulation has also been implicated as a cause of the feeling of pain during intense muscular work. An unanswered question about the limiting factors of intense work regards the ultimate cause - do horses slow during maximal running because of substrate depletion or because of endproduct accumulation? Fat supplementation may provide an energy source that does not produce lactic acid as an endproduct, but if it cannot be utilized under anaerobic conditions, how much energy for fast running can it provide? On the other hand, if glucose/glycogen availability is enhanced for energy production during maximal work, will lactic acid production increase to the point that fatigue is caused sooner? These questions remain unanswered.
POTENTIAL FEEDING STRATEGIES
Frequency of feeding
Evolving as nibblers as opposed to meal eaters, the horse’s digestive tract is not well adapted to meal feeding regimes such as those imposed in many stables where horses are fed two distinct meals per day, sometimes of highly digestible as opposed to highly fibrous feeds. Meal feeding leads to digestive perturbations including rapid changes in blood volume, increased rate of passage of digesta and changes in large intestinal microflora and pH (Clarke et al., 1990). The first response to ingestion of a concentrated meal is a large outpouring of saliva to lubricate, liquify and buffer the feed. Further fluids are added to ingesta by digestive juices from the pancreas, liver and walls of the small intestine. This large production of digestive juices impact the fluid balance of the entire body and can lead to decreases in blood volume of as much as 15% within one hour of feeding. This hypervolemia is transient and reversed within about 2 hours after feeding, but a second, although smaller, bout of hypervolemia occurs again about 6 hours after eating when digesta reaches the large intestine. This second episode of hypervolemia is reversed within 10 to 12 hours after eating, but another wave of feed and associated bouts of hypervolemia ensue as the next meal is fed. Meal feeding also increases rate of passage of digesta through the gastrointestinal tract as the sudden presence of a large amount of digesta stimulates increased contractions of the small intestine. Meal feeding can cause digesta to reach the cecum within 2 hours after eating. This increased rate of passage decreases the length of time available for enzymatic digestion in the small intestine and may increase the amount of soluble carbohydrate which reaches the large intestine. Soluble carbohydrates in the large intestine are fermented at a furious rate by microbes resulting in increased production of volatile fatty acids and lactic acid (endproducts of microbial fermentation). Acid accumulation decreases colonic pH which kills some bacterial species. The decreased pH also damages the large intestine mucosa, allowing toxins from dying bacteria to enter the bloodstream. These toxins may contribute directly to laminitis.
Dietary fiber and feed processing
As much as possible, fibrous feeds (hays, pastures, possibly silages) instead of concentrates should be fed to meet nutrient requirements. The NRC recommends that at least 50% of the ration or at least 1% of body weight should be roughage to ensure constant fill in the digestive tract and to promote health of the large intestine. Long stem hay is preferred over pelleted hay as ground hay (the form of hay in pellets) increases the rate of passage of feed, which decreases the efficiency of digestion and does not provide sufficient volume of fill in the digestive tract. The digestibility of hay, which varies depending on species and maturity, may also affect the amount of fill it creates in the large intestine. Immature hays with a higher ratio of soluble:insoluble carbohydrates will be digested to a greater extent enzymatically and will provide less gut fill. Mature, high fiber hays will be digested almost entirely microbially and will be digested more slowly, retaining gut fill for longer periods of time
Gut fill has been described as both beneficial and nonbeneficial. A beneficial effect of fiber in the colon and cecum is the fiber’s ability to retain electrolytes and water which may be needed during exercise in hot environments. However, gut fill is also extra weight which may contribute to additional exertion during exercise.
While reduction of particle size of forages is generally not recommended, processes that open the seed coat and reduce particle size of concentrates are generally accepted as benefiting prececal, enzymatic digestion. Grinding, dry rolling, steam rolling and extrusion are commonly done with grains with the net effect of increasing the surface area available for enzyme digestion. Processes that involve both heat and particle reduction are thought to improve digestion by causing partial disintegration of the starch granules. Improved enzymatic digestion in the small intestine decreases the amount of soluble carbohydrate that reaches the large intestine which is thought to be harmful as discussed above.
Supplementation to potentiate athletic performance
Glycogen loading in horses is a potentially dangerous practice with probably low potential for the improvement of athletic performance. Glycogen loading protocols in both human and equine athletes involve muscle glycogen depletion via a combination of strenuous exercise and low carbohydrate (high protein, high fat) intake followed by glycogen repletion and hopefully supercompensation by a short period (few days) of light work in combination with high carbohydrate intake right before the event. The theory of glycogen loading is that glycogen depletion will set the stage for glycogen supercompensation as muscle glycogen stores are refilled and possibly overfilled by the abundant carbohydrate in the diet during the repletion phase. Creating low carbohydrate diets for horses is difficult as most diets are primarily carbohydrate in composition. High protein-high fat diets for horses usually involve alfalfa hay and oil, which may or may not be palatable to horses. High carbohydrate diets may increase risk of colic or laminitis if grain becomes the primary component of the diet. One of the most well-read studies on glycogen loading in horses (Topliff et al, 1983) used a combination of alfalfa, casein and oil for the depletion diet and corn (85%) and grass hay (15%) for the repletion diet. The depletion diet was accompanied by severe exercise in the form of both aerobic and anaerobic work while the repletion diet was accompanied by only hand walking of the hroses. In this study, muscle glycogen was depleted (a decrease of 71% compared to initial glycogen concentration) and then supercompensated (36% above initial) by the work/diet treatments. Work performance (time to fatigue on a treadmill exercise test) was decreased by the glycogen depletion phase but was not increased by the repletion phase.
The type of glycogen loading protocol described above is considered by most to be too risky for valuable horses in training for a particular event. However, a modified glycogen loading program may be beneficial if tailored to the horse’s need. An increase in grain (soluble carbohydrate) in the diet in combination with lighter levels of work before an event may increase muscle glycogen levels somewhat and possibly enhance energy availability for certain types of athletic performance. The light work before an event may also help relieve muscle or other types of soreness not related to nutrition that may impact performance. Certainly, many factors must be considered, but giving muscles an opportunity to reach the maximum levels of glycogen appropriate to the individual horse may be beneficial.
Feeding fat at the rate of about 10% of the total diet has also been shown to improve muscle glycogen concentrations at the start of exercise. The effect of feeding fat on glycogen utilization, however, have been inconclusive.
The composition and timing of a meal before an athletic performance has been shown to alter the availability of energy substrates and could, potentially, alter athletic performance. Glucose levels peak 90 to 120 minutes after feeding with return to prefeeding levels within 5 to 6 hours. As expected, grains, with their high soluble carbohydrate content, produce higher blood glucose levels than hays. Fats fed with carbohydrates dampen the glycemic effects of the carbohydrate. In all cases, insulin responses mirror the height and time course of the glucose response curves.
When horses were fed isocaloric amounts of corn, alfalfa or offered no feed one or four hours prior to a strenuous but submaximal exercise test, blood glucose levels were highest in the horses fed corn at the start of exercise (Stull and Rodiek, 1990). However, the corn fed horses showed a precipitous decline in glucose during exercise, followed by a large post-exercise glucose rebound. Horses fed corn one hour before exercise showed greater pre-exercise glucose levels and greater during-exercise glucose declines than horses exercised four hours after feeding. The horses fed alfalfa or no feed started exercise with lower blood glucose levels and showed no decline in glucose during exercise. In fact the alfalfa fed or fasted horses showed a steady increase in blood glucose during exercise and into early recovery. Free fatty acid levels rose during exercise with all three dietary treatments but were highest in the fasted horses, lowest in the corn fed horses and intermediate in the alfalfa fed horses. These results indicate that corn-fed horses used glucose to a much greater extent to fuel the exercise performed than alfalfa-fed of fasted horses, which relied more on free fatty acid mobilized from adipose tissue. Although not significant, lactic acid levels tended to be higher in the horses fed corn.
The implications of these findings are not entirely clear. It appears that energy substrate utilization can be affected by the timing and composition of the last meal before an athletic event. The type of exercise as well as the timing and composition of the last meal must be considered when creating a feeding regime for pre-exercise treatment.
Carbohydrates and fats are more than just a source of calories for horses. The digestion and metabolism of carbohydrates vary depending on chemical composition. Soluble and insoluble carbohydrates vary in their site of digestion, endproducts of digestion and metabolism of absorbed components. The ratio of soluble to insoluble carbohydrates in the diet must be carefully considered to provide not only optimum energy, but also to optimize digestive tract health and avoid potentially dangerous metabolic conditions.
Fats are a highly concentrated source of calories that can be well digested and metabolized in horses. Fats may be especially beneficial to the performance horses to increase energy density of the diet and provide readily available energy substrates for work performance. Both diet and the duration and intensity of the exercise affect the biochemical pathways for energy metabolism.
Physical form of the diet and the frequency and timing of feeding are important management consideration for carbohydrate and fat nutrition. Small, frequent meals are preferred to large meals infrequently offered. Physical form of hays and grains should be considered as well for optimum digestion.
Clarke, L.L., Roberts, M.C. and Argenzio, R.A. 1990. Feeding and digestive problems in horses: Physiologic responses to a concentrated meal. In The Veterinary Clinics of North America. Equine Practice. H.F. Hintz, guest editor. W. B. Saunders Co. Philadelphia.
Meyer, H. Radicke, S, Kienzle, E, Wilke, S and Kleffken, D. 1983. Investigations on preileal digestion of oats, corn and barley starch in relation to grain processing. Proc. 13th Equine Nutrition and Physiology Symposium. Gainesville, FLA.
Stull, C.L. and Rodiek, A.V. 1990. Effects of postprandial interval and feed type on substrate availability during exercise. In Equine Exercise Physiology 4. Newmark, Suffolk, United Kingdom, Equine Vet. Journal Ltd.
Topliff, D.R., Potter, G.D., Dutson, T.R., Kreider, J.L. and Jessup, G.T. 1983. Diet manipulation and muscle glycogen in the equine. Proc. 8th Equine Nutrition and Physiology Symposium. Lexington, KY.