Regardless of the discipline, attaining peak performance is the number one goal for all involved in the preparation of the equine athlete. This Sports Medicine column aims to provide the reader with a greater understanding of how the horse's body works during exercise. In this first article, the fundamental relationship between nutrition and exercise performance is emphasized. Subsequent articles will include the effects of exercise training, the role diet plays in maximizing the benefits of training, and conditions that impair athletic performance.
Several articles over the past few years have discussed the feeding of horses engaged in high-level athletic activity. The importance of nutrition for optimization of athletic performance (regardless of the level of competition) cannot be overemphasized. Given this intimate link, it seems appropriate that the first article in the new Sports Medicine column delves into the relationship among nutrition, energy metabolism, and exercise performance. Fundamentally, the body's ability to extract energy from food nutrients and to transfer the energy to the muscles that power the body determines the horse's (and our) capacity to run and jump. To put it another way, we can view the body as an engine that requires fuel to run, just as the engine of a car requires gasoline to drive its pistons. The fuel tank (the body's energy stores) is filled by the digestion and absorption of macronutrients (primarily carbohydrates and fats) in the diet. This stored fuel then is used to run the engine (the muscles that provide the power for movement). The faster the engine runs, the greater the amount of fuel required. Conversely, when the fuel runs out, the engine stops running!
The mammalian body uses energy from nutrients provided in the diet to run a multitude of functions, not just muscle contraction during physical activity. For example, energy is required for the digestion, absorption, and storage of food nutrients, as well as for synthesis of new chemical compounds in the body (e.g. protein structures for the building of new tissues). Even more importantly, energy is needed on a continual basis for the maintenance of virtually all of the body's functions. Nutritionists use the term "basal metabolic rate" to describe the energy used for these maintenance functions.
Although we mostly think of energy needs in terms of physical activity, in reality more than 60% of the body's daily energy needs relate to this basal metabolism. If these needs are satisfied, the body can use additional available energy for processes such as fuel storage, physical activity, growth of tissues, and lactation. On the other hand, if the energy provided by the diet is barely enough to support basal metabolism, minimal energy will be available for physical activity, and exercise performance will suffer.
What Is Energy?
To begin this discussion, we must define some concepts relating to energy. Energy exists in a number of different forms, namely nuclear, light, chemical, electrical, mechanical, and heat energy. Beyond this, energy becomes a little more difficult to define. Unlike the physical properties of matter (e.g. size, shape, weight), energy cannot be defined in concrete terms. However, we all are very familiar with the term calorie, which is a measure of energy.
By definition, one calorie is the amount of energy required to raise the temperature of one gram of water by 1.8° F. The standard international unit for expressing energy is the joule, where one calorie equals 4.2 joules. One kilocalorie is one thousand times a calorie, and one kilojoule is one thousand times a joule.
An extremely important physical principle is that energy is neither created nor destroyed. Rather, energy is transformed from one form to another without being used up. In the case of physical activity, stored fuels (carbohydrates and fats) are broken down by a series of reactions and chemical energy is released. Muscle contraction and movement are made possible by conversion of this chemical energy into mechanical energy. This energy transformation is analogous to the way in which mechanical energy is taken from water in a hydroelectric plant-the mechanical energy necessary to drive the turbines is harnessed from the flowing water. Within the horse's contracting muscles, this conversion of chemical energy into mechanical energy is extremely inefficient. At best, only 25% of the chemical energy is converted into mechanical energy, the remainder being released as heat energy.
During exercise, this release of heat energy presents a potential problem for the horse. Physical activity requires an enormous amount of energy, and, therefore, a large amount of heat energy is released during exercise. This heat energy causes a rise in body temperature. In fact, without a means to lose this heat, the horse's body temperature would reach dangerously high levels after only a short period of exercise. Fortunately, the equine body has a temperature control system that, in the face of rising body temperature, ensures that this heat is transferred to the skin, then to the surrounding air. We will return to this issue in an upcoming article.
ATP: The Energy Currency
The energy present in food cannot be used directly to drive the body's functions. Instead, the energy in food is unleashed through the energy-rich compound adenosine triphosphate (ATP). Each ATP molecule contains three phosphate bonds. These bonds are termed "high-energy" because a large amount of energy is released when a phosphate splits from the original ATP molecule. This released energy then can be used to drive energy-requiring processes in cells of the body. For example, the energy released from ATP allows muscle fibers to shorten during muscular activity.
Because energy from ATP powers all work within the body, ATP is termed the energy currency.
Cells store only a small quantity of ATP, and must, therefore, continually resynthesize ATP. This is a critical point because the energy needs of a muscle cell increase enormously during exercise, whereas the amount of ATP stored within the cell is barely enough to support two or three contractions.
Some energy for ATP synthesis comes from the splitting of a phosphate from another high-energy phosphate compound called phosphocreatine (PCr). Similar to the breakdown of ATP, the release of energy from PCr does not require the presence of oxygen, and this fuel reservoir provides a readily available energy source at the start of exercise. In fact, the combination of stored ATP and PCr is the major energy source for the first few seconds of exercise.
However, for "all-out" exercise-such as galloping-PCr stores are depleted after about 10 seconds of work.
So, how is the body able to maintain the supply of ATP?
This is the point where we can start to see the link between nutrition and exercise-the stores of chemical energy that are ultimately used for resynthesis of ATP come from the food that the horse consumes.
Storage Of Chemical Energy In The Body
The three macronutrients in the horse's diet are carbohydrate, fat, and protein. All three can be used as energy sources. However, protein from the diet mostly is used for synthesis of new proteins in the body, and any protein consumed in excess of these requirements will be excreted (mostly in the urine). For protein, it is definitely a case of "use it or lose it." Only under extreme circumstances, such as starvation, does protein become an important energy source. So, in terms of the energy for physical activity, carbohydrate and fat are the main sources.
The storage form of carbohydrate is glycogen, a substance consisting of a huge number of glucose molecules. Glucose from food is used to synthesize this glycogen, both in the liver and in the muscle. The stores of glycogen in the liver are used to supply glucose for all the tissues in the body, particularly the brain and other nervous tissues and the red blood cells.
Working muscles also use some of this glucose released from the liver. A complicated regulatory system controls the release of glucose from the liver so that blood glucose levels remain within a very specific range.
The vast majority of the horse's glycogen stores are present within skeletal muscle. Unlike liver glycogen, the store of glycogen within muscle is not available to the rest of the body, but rather serves as the most important source of energy for working muscle. Up to 1-2% of the muscle's weight is in the form of glycogen.
As indicated in the accompanying sidebar "Carbohydrate Stores In The Horse," total stores of glycogen in the average light breed of horse are in the range of three to four kilograms (six to nine pounds). Impressive as this might seem, this store of carbohydrate is trivial in comparison to the store of fat.
Fats, also known as triglycerides, are stored in adipose (or fat) tissue. This specialized tissue surrounds the body's major organs, such as the heart, kidneys, and intestinal tract, and some adipose (fatty) tissue is scattered throughout the muscles of the horse. The amount of fat tissue present in different horses varies greatly, but, on average, 7-8% of the horse's body weight is fat. This percentage of body fat is lower than that of the average human. It's been found that 10-20% of our bodies are fat tissue.
Nonetheless, fat is an extremely energy dense fuel and in terms of provision of energy during exercise, the horse's store of fat is virtually inexhaustible and could sustain moderate exercise for a number of days. However, as we will discuss, the horse cannot rely solely on fat for energy during exercise, meaning that the supply of carbohydrate (from liver and muscle glycogen) is critical to the maintenance of energy supply during exercise.
Fueling The Engine During Exercise
The relative contributions by carbohydrate and fat to ATP resynthesis during exercise are primarily dependent on the speed and duration of exercise. Two fundamental metabolic processes drive ATP resynthesis:
1) Aerobic metabolism--this involves the complete breakdown of carbohydrates and fats to water and carbon dioxide. These reactions require oxygen, hence the term aerobic metabolism.
2) Anaerobic metabolism--glucose (from glycogen stores) is converted to lactic acid, yielding a small amount of ATP. These reactions do not require oxygen and only involve the metabolism of glucose or glycogen. Fat cannot undergo anaerobic metabolism.
The balance between aerobic and anaerobic metabolism depends on the intensity of the exercise. The endurance horse typically travels at speeds (trotting and light canter) that can be maintained almost entirely by aerobic metabolism. On the other hand, all-out sprinting (e.g. Thoroughbred and Standardbred racing, Western events such as cutting) and other forms of heavy exercise, such as Phases B and D of the speed-and-endurance test of a three-day event, involve a substantial amount of anaerobic metabolism, with accumulation of lactic acid in muscle and blood.
In addition, these sprint activities will use most of the muscles' reserves of PCr. The graph on page 52 shows the approximate percentage of energy for ATP resynthesis from the aerobic and anaerobic systems during different equine exercise activities. Although we typically refer to endurance exercise as "aerobic" and sprint exercise as "anaerobic," in reality all forms of exercise involve a bit a both. In fact, laboratory research has shown that aerobic metabolism supplies about 75% of the energy for galloping at racing speeds. This is a tribute to the tremendous capacity of the horse to transport oxygen to its muscles during exercise.
When one considers how much carbohydrate and fat are used as fuel during exercise, a general rule is that the faster or harder the exercise, the greater the reliance on carbohydrate for energy. The graph on page 54 is an illustration of the relative contributions of carbohydrate, fat, and protein macronutrients needed for energy metabolism at rest and during various intensities of exercise.
Among other factors, training status, diet, and whether or not the horse was fed before exercise will influence the actual mix of fuels used during a bout of exercise. Nonetheless, some generalizations can be made.
At rest and very slow speeds (walk and slow trot), fat provides a large proportion of the energy used, and this contribution increases with lengthening duration of exercise. However, even during light exercise, carbohydrate contributes 30-40% of the energy. This is a critical point because during endurance exercise (e.g. endurance rides), depletion of the horse's carbohydrate reserves (liver and muscle glycogen) can occur, resulting in premature fatigue. At this point nutritional management plays a key role in ensuring that a lack of fuel does not prevent the horse from completing the task asked of it.
Carbohydrate stores must be "full" before the event. During long rides and competitions, the horse must consume feeds or supplements which provide glucose that can be used directly for energy or used for restoration of the glycogen stores. In addition, the horse must be thoroughly trained for the task at hand.
It is well recognized that training enhances the capacity to burn fat during low and moderate intensity exercise-for a given running speed, the highly trained horse will use more fat and less carbohydrate, thus preserving the precious supply of glycogen (a "glycogen-sparing effect"). Although more controversial, there is some evidence that feeding a fat-supplemented diet also enhances use of fat for energy during moderate exercise.
The pattern of fuel use is much different during galloping exercise of only a few minutes duration. Virtually all of the energy is provided by carbohydrate-mostly muscle glycogen.
Why is it that the horse does not use fat during such strenuous exercise?
First, the reactions involved in the mobilization and utilization of fat are quite slow. During these short bursts of activity, these reactions have insufficient time to kick into gear. Second, as mentioned, sprinting exercise and other explosive movements such as jumping involve a substantial amount of anaerobic metabolism. In short, the body relies heavily on muscle glycogen to generate the power needed to run at high speed.
As shown in the graph on page 55, the faster the horse runs, the greater the rate of muscle glycogen use. At speeds greater than about 600-700 meters per minute, there is a dramatic increase in glycogen breakdown. At that point, the additional energy demands for movement can be met only by anaerobic metabolism of glucose.
Anaerobic metabolism is 12 times less efficient than aerobic metabolism in terms of the amount of ATP produced. When glycogen is aerobically metabolized to water and carbon dioxide, 36 ATP molecules are produced, but when glycogen is metabolized to lactate (anaerobic metabolism), there is resynthesis of only three ATP molecules. The other disadvantage of this anaerobic process is that the lactic acid formed can have negative effects of muscle cell function. At such high rates of metabolism, the lactic acid accumulates in the cell, resulting in a fall in pH.
Normally, the pH of the muscle cell is between 7.2 and 7.4 pH units. Studies in horses have shown that muscle pH can fall as low as 6.5 during sprint exercise. At that level of muscle acidity, the contractile ability of the muscle is impaired and the muscle begins to fatigue. In part, this explains why horses are not able to sustain very high running speeds for more than two to three minutes.
We already have discussed how carbohydrate availability can be a limiting factor during prolonged exercise tasks. Whether this also is the case during intense exercise is less clear. For racing and other forms of exercise involving a single, short bout of exertion, the availability of muscle glycogen is probably not a limiting factor, as only 20-30% of the total stores are used during these types of activities. More likely, the accumulation of lactic acid and other metabolic by-products contributes to development of fatigue.
However, if the horse is asked to sprint several times within a relatively shortperiod (e.g. a cutting horse working over an afternoon), it is possible that low glycogen content of the muscles will impair that animal's performance.
The answer to this question also depends on how full the tank is when exercise is started. Researchers at The Ohio State University recently demonstrated that low muscle glycogen stores (about 50% of their normal levels) impair high-intensity sprint exercise in horses. Thus, regardless of the intensity and duration of exercise, it is extremely important that muscle glycogen stores are replete.
Re-Fueling The Tank
After a hard workout, the horse needs fuel to replace the expended carbohydrate reserves. Dietary energy can be provided by four different sources-starch, fat, protein, and fiber. Of these, starch is the dietary energy source of choice for replenishment of glycogen stores.
Fat is a great energy source, but it cannot be used for the synthesis of glycogen. Starch, a carbohydrate composed of a large number of glucose molecules, is the primary component of cereal grains, comprising 50-70% of the grain's dry matter.
Of the grains commonly fed to horses, corn has the highest starch content. In the small intestine, the starch is broken down to individual glucose molecules that are absorbed into the bloodstream. Within about 60 minutes of a grain meal, there is a marked increase in blood glucose concentrations. In addition, the increase in blood glucose stimulates release of insulin, a hormone that stimulates entry of glucose into muscle cells. Insulin activates the enzymes responsible for synthesis of new glycogen molecules in muscle.
Studies in human athletes have shown that consumption of a starch meal within 30-90 minutes of the completion of hard, glycogen-depleting exercise is optimal in terms of replenishing glycogen stores. On the other hand, research studies in horses have shown that glycogen re-synthesis occurs relatively slowly after hard exercise, and 24-48 hours might be required for complete replenishment. Researchers at the University of Sydney demonstrated that administration of glucose (three grams per kg of body weight, administered via stomach tube) soon after hard exercise did not have a significant effect on the rate of muscle glycogen re-synthesis in the 24 hours after exercise. However, an intravenous infusion of a larger amount of glucose (total dose of six grams per kg body weight) did result in an increase in glycogen re-synthesis rate.
Unfortunately, it is not safe to administer this amount of glucose to a horse orally (or to allow the horse to eat the equivalent amount of starch in a single grain meal) because of the risk for development of gastrointestinal disturbances and laminitis. So, in terms of the overall health of the horse, post-exercise feeding management should not stray from the norm-plenty of high-quality forage (at least 50% of the total daily ration) and provision of high-energy nutrients from grain and fat supplements.
Feed, including a small grain ration, should be made available to the horse 60-90 minutes after exercise, particularly following work that will have depleted the horse's carbohydrate stores. A second grain ration should be fed two- to three-hours later. Providing the horse is not required to perform hard, exhausting exercise on a number of consecutive days, glycogen reserves should remain at the appropriate level throughout periods of training and competition.
Further studies are needed to determine the best method for restoration of glycogen reserves following exercise in the horse.
About The Association For Equine Sports Medicine
The Association for Equine Sports Medicine (AESM) is a group of equine veterinarians, trainers, sports physiology researchers, and other persons with interest in the understanding and/or application of scientific knowledge in the area of equine sports medicine. This non-profit organization is dedicated to the advancement of scientific knowledge and care of the athletic horse. The AESM was founded in 1981 and holds an annual meeting where members and others interested in new advancements in equine sports medicine are invited to present papers and reports of interest to the membership. The annual meeting typically involves both classroom-style lectures and discussions and hands-on experience covering the latest advancements in diagnostic and therapeutic techniques.
For more information about AESM, including membership and annual meetings, or to contact an AESM member in your area, contact Holly Greene, Executive Director of the AESM, 3579 East Foothill Blvd #288, Pasadena, Calif. 91107; 909/869-4859, fax 909/869-6788.
Carbohydrate Stores In The Horse
Horses rely on stored fuels to provide energy during exercise. Although a small amount of protein (as amino acids) can be used for energy, by far the largest amount of energy is supplied by carbohydrate and fat stores in the body. Glycogen, a substance comprising very long chains of glucose molecules, is the storage carbohydrate found in muscle and liver. The accompanying figure illustrates that a 500 kg (1,100 pound) horse stores approximately 3.8 kg (8.4 pounds) of carbohydrate. Of this reserve, 90% is muscle glycogen, 8% is liver glycogen, and less than 1-2% is plasma or blood glucose (the latter is produced by the liver). Because each gram of carbohydrate (either glycogen or glucose) contains 4 kcal of energy, there are approximately 15,000 kcal of carbohydrate energy. This energy store is quite small relative to the amount of fat stored in the body, and diet can markedly alter its quantity. If a horse is fasted for a 24-hour period, liver glycogen reserves will be greatly reduced because of continual use of glucose by tissues such as red blood cells and the central nervous system. Similarly, in a horse in moderate to heavy exercise training, a poor quality diet or one deficient in energy will result in sub-optimal muscle glycogen stores.
Aerobic vs. Anaerobic Energy
The relative contributions of aerobic (blue) and anaerobic (red) energy metabolism depend on the intensity of exercise. This schematic illustrates the relative amounts of aerobic and anaerobic energy used at rest and during different speeds of exercise. The numbers at the end of each bar column designate the relative effort associated with each activity--2% for rest and 100% for a horse running a six-furlong race at a speed of 1,000 meters per minute. The intersection between the red and blue bars indicates the so-called anaerobic threshold; at speeds above this threshold, most of the additional energy required to reach the higher speed is provided by anaerobic breakdown of glucose. Note that even for the highest intensity of exercise (a six-furlong race), aerobic metabolism provides most (70-75%) of the energy.
Energy From Dietary Macronutrients
The dietary macronutrients--carbohydrate, fat, and protein--ultimately provide the energy (in the form of ATP) for muscular work during exercise. The two main macronutrient sources that provide energy for ATP resynthesis are 1) muscle and liver glycogen, and 2) fatty acids derived from triglyceride stores in muscle and in adipose tissue. In addition, a small amount of protein can be used for energy during exercise. In general, the fuel mixture that powers exercise depends on the intensity and duration of work, and the horse’s fitness level and diet. The accompanying figure provides an overview of the relative contribution to energy provision of carbohydrate, fat, and protein during rest and different athletic activities. Note the reliance on carbohydrate during heavy exercise.
Muscle glycogen is by far the major source of energy for active muscles during exercise. As shown on this graph, the rate of muscle glycogen usage increases with the intensity of exercise--dramatically so for running speeds greater than 600-700 meters per minute (speeds attained during racing and other forms of sprint exercise). The very rapid increase in glycogen breakdown at speeds above 600 m/min reflects the increasing contribution by anaerobic metabolism to total energy provision. Glucose (from glycogen) is the only fuel that can be used for anaerobic metabolism.
Aerobic--Occurring in the presence of oxygen. Aerobic metabolism is that which is fueled by oxygen. Opposite of anaerobic, meaning without oxygen.
Anaerobic threshold--The point at which the horse can no longer function by aerobic metabolism alone.
ATP--The only source of energy which can be used by muscles and other body tissues. Energy from carbohydrates, fats, or proteins is converted into ATP to allow transfer to the body tissues for uses such as muscle contraction or brain function.
Glycogen--A storage form of glucose, present primarily in the muscles and liver.
IU--International Units, a measure used to describe quantities of vitamins in the diet.
Lactic Acid--Produced as a byproduct when glucose or glycogen are used for energy in the absence of oxygen. Once produced in the exercising muscle, it must be removed and transported back to the liver or kidney for utilization. If production exceeds the removal rate, it accumulates in the muscle and contributes to the development of fatigue.
Macronutrients--nutrients needed in relatively large quantities in the diet (protein, fat, carbohydrate).
Metabolism--Chemical reactions in the body, including the utilization of nutrients following their absorption from the intestine.
pH--A measurement of acidity or alkalinity, ranging from 0 to 14. A pH level of 7 is neutrality, below that is acidic (has a high hydrogen ion concentration), and above 7 is alkaline or basic (has a low hydrogen ion activity concentration).
About the Author
Ray Geor, BVSc, PhD, Dipl. ACVIM, is professor and chairperson of Large Animal Clinical Sciences at the College of Veterinary Medicine at Michigan State University
POLL: University Equine Hospitals