The tightly packed field of Thoroughbred racehorses, straining every muscle for a few more inches of gained ground on the final turn...the Arabian endurance horse, with 89 miles of hard, mountainous terrain behind him and 11 more to go before he can rest...the Olympic three-day event horse, summoning up all his courage, agility, and speed to answer the questions posed by a diabolical course designer on a cross-country course built to put fear into the hearts of ordinary mortals. All of these are high-octane horses: animals performing at the peak of their athletic ability. Standardbred racehorses, high-goal polo and polocrosse ponies, sprint racers (Quarter Horses, Paints, and Appaloosas), and top-level cutting horses and barrel racers also can be high-octane horses, and because they are pulling out all the stops in order to perform at their maximum, they need serious nutritional support to provide maximum energy.


Understanding how your horse converts his feed into performance can help you design not only the most appropriate fitness program for him, but an optimum diet as well.

High energy, traditionally, comes from high levels of carbohydrates--but when horses are fed a high-carbohydrate (and subsequently, low-fiber) diet, they suffer all sorts of digestive troubles, from colic and diarrhea to founder and exertional myopathy (tying up). As a result, fueling the equine athlete becomes a delicate balancing of nutrients, to provide as much energy as possible while minimizing the risk of carbohydrate overload.

Things are further complicated by the fact that not all athletes are created equal. The metabolic pathways by which an endurance horse fuels his performance may be quite different than those drawn on by a sprint racer or a draft horse used for pulling contests. Understanding how your horse converts his feed into performance can help you design not only the most appropriate fitness program for him, but an optimum diet as well.

Energy Pathways

There are three main types of athletic performance in horses:

a) Endurance activity--generally defined as two hours or more of low-intensity exertion, such as you are likely to find in endurance and competitive trail horses, working ranch or draft horses, and hard-working lesson horses.

b) Sprint activity--venue in which horses perform for a minute or less, at close to 100% of their maximum exertion intensity. Generally, in that timeframe, a horse travels 400 meters (about a quarter of a mile) or less. Quarter Horse racing, barrel racing, rodeo, and draft horse pulling contests fall into this category.

c) Middle distance activity--performance of several minutes' duration, at 75% to 95% of the horse's maximum exertion intensity. Horses will usually travel between half a mile and two miles, or 800 to 3,200 meters. Thoroughbred and Standardbred racehorses are good examples of middle distance athletes.

Of course, many horse sports are hybrids of these three broad categories. For example, showjumpers may perform as both sprinters and middle distance athletes at various times, while polo and cutting horses might sometimes fit into all three sections.

Regardless of the type of performance a horse is asked to do, there is only one source of energy which can be used to produce muscular movement: adenosine triphosphate, usually shortened to ATP. The horse's body converts feed into "storage" forms of energy (glycogen, glucose, and free fatty acids), which, when called on by an athletic effort, are converted to ATP, the fuel muscle fibers need to fire.

If the exertion is less than maximal, the horse's body will prefer to operate aerobically, a system in which oxygen (drawn in from the lungs) is added to the chemical reaction to create large amounts of ATP from glucose, glycogen, or fatty acids (and to a minor extent, protein), and the harmless byproducts of the reaction, water and carbon dioxide, are excreted through sweat and exhalation. The main drawback of aerobic metabolism is that it is a slow process, and can't fuel 100% effort. For that, the horse must use the anaerobic energy pathway, which can produce energy extremely rapidly, but only in very small amounts. Researchers estimate that the aerobic system generates over 600 times more energy from body stores than can be produced anaerobically. The anaerobic system operates without oxygen, but also produces toxic byproducts, one of which is lactic acid, a chemical implicated in muscle fatigue. Furthermore, the anaerobic system is not as versatile an energy pathway, because it can only draw from glucose and glycogen stores, not from fatty acids or protein. When glycogen and glucose become depleted, or when lactic acid buildup in the muscles starts to inhibit their utilization, the pathway shuts down--so anaerobic metabolism only supports a few minutes of intense exertion. The greater the effort, the faster muscle fatigue (or complete exhaustion) sets in.

Most forms of prolonged physical activity, at less than top speeds, are fueled primarily by the aerobic pathway. As the degree of exertion goes up, so does the rate of ATP utilization, until eventually it exceeds the rate at which aerobic activity can resynthesize more ATP. At that point, the anaerobic system takes over so that the horse can continue to perform. As a rule, this "anaerobic threshold" is thought to be in the 140 to 150 heartbeats per minute range (though of course it varies somewhat from horse to horse). This is also considered to be the rate at which lactic acid begins to accumulate in the muscles, triggering fatigue. In racing, for example, a horse can only run at top speed for about one minute (during which he is likely to cover about 1,000 meters, or 5/8ths of a mile). After this first minute, his speed decreases because anaerobic energy production can't keep up, and the aerobic system isn't fast enough to provide energy for running at 100%. As some pundits have described it, it isn't a matter of which horse is going the fastest at the finish--it's more a matter of which horse is slowing down the slowest.

In the real world, no form of exercise is fueled exclusively by one pathway or the other; there is always an element of both in play.

Twitching Muscles

The horse's body type plays a role in determining which energy pathways will tend to be used. Three types of muscle fibers are found in horses, each of which has differing capacities for storing energy substrates (primarily glycogen and fatty acids), for utilizing glycogen, fat, and oxygen, and for supporting athletic endeavors. Type I muscle fibers are also described as "slow twitch" fibers. They are the smallest of the three types and have an affinity for fat storage, a moderate ability to store glycogen, and the capacity to deplete glycogen fastest of the three types with exercise. They have the lowest strength of the muscle fiber types, but because they utilize oxygen well and store fat well, they can support prolonged muscular activity with minimal fatigue. Horses which specialize in endurance events, especially Arabians, have a high proportion of slow twitch muscle fibers.

The larger, bulkier "fast twitch" muscle fibers are better at supporting athletic tasks requiring strength, speed, or power. Type IIA (fast twitch high oxidative) fibers and Type IIB (fast twitch low oxidative) fibers have high strength but low oxidative capacity compared with Type I fibers. Type IIA fibers have a good capacity for storing glycogen and a moderate capacity for storing fats, while Type IIB fibers have the best ability to store glycogen of the three, and the slowest glycogen depletion with exercise, though their ability to store fat is negligible. Muscular, powerful breeds like the Percheron and Quarter Horse have an abundance of bulky, fast twitch muscle fibers, which give them a talent for pulling weights and short, top-speed sprints. (See the accompanying chart on page 77 for the run-down on different muscle fiber types in various equine athletes.) The relative percentages of different muscle fiber types are largely the result of genetics, and horses will be predisposed towards athletic talent in a certain sport according to their muscle fibers.

All of this is important because, depending on the sport in which you'd like your horse to excel, you can support his athletic efforts by providing the right nutrients to fuel the pathway he'll most likely be drawing from. Now let's talk specifically about those nutrients, and how you can manipulate the diet to provide maximum energy per pound.

Feeding Considerations for High-Octane Horses

Most high-octane horses, performing at the top of their game, need serious calories to support their endeavors. When we assess these athletes according to the basic rule of thumb that says, "feed between 1.5% and 3.0% of the horse's body weight in total feed (hay plus grain) per day," the vast majority of high-performance horses will need to be at the 3.0% end of the scale. (Some easy keepers--for example, Quarter Horses and drafts, may not need quite this much.) But even when we want to deliver a high-octane diet, it's a mistake to alter the diet so that less than 50% of it is forage (pasture or hay). Elite equine athletes are still grazing animals, designed to digest and run on fibrous plants, and to ignore that evolutionary outcome is to court disaster. Studies of young Thoroughbred racehorses have demonstrated an increased risk of gastric ulcers in horses fed less than 50% fiber in their daily diets, and colic is also a dangerous possibility.

But because forages are not high-energy feeds, the athletic horse's diet needs to be supplemented in order to provide enough energy for him to perform at peak capacity. Traditionally, this is done by feeding grains, which are rich in carbohydrates (the "raw material" for glucose and glycogen stored in the muscles). But sometimes, even when you're feeding the maximum 50% of the diet as grain (1.5% of the horse's bodyweight in grain every day), an equine athlete still doesn't have enough energy for the work he's doing. How, then, can you boost the energy content without feeding so much grain that you cause digestive overload?

One simple approach is to use more energy-dense grains. Hulled grains, such as oats, have a significant fiber content, which makes them less likely to cause digestive upset but also less energy-dense than hull-less, starchy grains such as corn. Substituting an equal weight of corn for the oats you previously fed will deliver significantly more energy, pound for pound. In order to keep digestive difficulties to a minimum when you make a switch like this, feed small amounts, often, so as not to end up with large quantities of starches fermenting in the cecum. It's also helpful to feed a cracked or rolled grain, rather than whole grains; breaking the seedcoat means the grain will be chewed and digested more easily.

Another solution, which has become increasingly popular with the horse-owning community in recent years, is the introduction of added fats to the diet. Fats are not a natural part of a horse's daily rations, but it turns out that they are nonetheless easily digested by equines, and are excellent energy sources, albeit less versatile ones than carbohydrates. Fed at a level of six to 10% of the total diet, fats (usually vegetable oils such as corn or soy oil, though rice bran and sunflower seeds are two other good sources) are converted to free fatty acids in the horse's system. These can be converted to ATP by the aerobic energy pathway. (In contrast, when carbohydrates are converted to glucose and glycogen, they can be burned by both the aerobic and the anaerobic pathways.)

A high-fat diet (and by "high fat," we mean up to a maximum of 15% of the total daily ration, unlike our own "high fat" diets!) provides several benefits for athletic performance. For one thing, fats are about 2.25 times as energy dense, pound for pound, as carbohydrates--so they deliver more dietary energy without a proportionate increase in feed intake. This is a particular advantage if you have a horse who is already receiving close to the maximum amount of feed he can eat on a daily basis, but is still underperforming or underweight.

Horses produce less body heat when they burn fat as a fuel, and that decreases the horse's heat load and increases the amount of energy available for physical activity and glycogen storage. In one study, feeding added fat resulted in a 14% decrease in the test horses' total heat production--and as a result, though the energy intake was unchanged, over 60% more energy was available for physical activity. What happens with all that extra energy? If it's not needed for performance, it can be efficiently stored by the body (perhaps putting some much-needed weight on a skinny equine athlete). Or if your horse is already in good weight, feeding fat may mean you can cut down on the amount of grain you feed. Doing so may help reduce the risk of digestive upset, as fats seem to cause less disruption in the intestinal tract than do carbohydrates.

Another benefit: a high-fat diet (in this case, 10 to 12%), if fed for a sufficient period of time, will increase muscle glycogen content and allow greater utilization of stored glycogen, with no change in blood glucose concentrations during anaerobic (sprint-type) activity. During aerobic activity, there is evidence of muscle glycogen sparing (which helps delay the onset of fatigue).

So while fat can only directly fuel aerobic metabolism, indirectly, it benefits both major pathways. In one study, a diet which included 15% added soy oil was better than either a high-carbohydrate (40%) or a high-protein (25%) diet for both high-speed and moderate-speed exercise. And other research has found that horses fed high-fat diets can run faster at a constant heartrate and have a delayed anaerobic threshold. Even more interesting, when a group of Thoroughbred racehorses was fed a fat-supplemented diet, 14 of the 15 in the group produced faster race times.

Why does fat work to enhance performance? Research suggests that the body "learns" to use fatty acids as an energy source, and after several weeks on a fat-supplemented diet, will burn fatty acids before it burns glucose and glycogen. The result is delayed glycogen depletion, a delayed switch-over to anaerobic metabolism when the horse is exercising strenuously, and less lactic acid buildup in the muscles. Full metabolic adaptation to a high-fat diet has been shown to be achieved in about 11 weeks. Just why it takes this long is not yet clear; all we know is that if you want to see performance enhancement from feeding fat, you must be prepared to wait for the results.

Protein for Performance

One nutrient requirement which doesn't change much with exercise is the need for protein. Studies have confirmed that a horse's protein needs don't increase significantly with hard exercise; and indeed, excess protein intake is suspected of contributing to higher heart and respiratory rates, heavier sweat loss and dehydration, and decreased performance over time.

What about protein as an energy source? Although the horse's body can, in a pinch, burn protein to produce energy, it's definitely not the ideal solution, for several reasons. First, as ingredients go, protein is very expensive. Second, when the horse's system burns protein, it produces from three to six times more heat than is produced in the utilization of carbohydrates or fats. This contributes to excess body heat as well as increased water loss due to sweating-and eventually, to electrolyte deficits. More water and electrolytes are lost in the urine, as the horse tries to excrete the excess nitrogen generated from burning protein. And the nitrogen, excreted in the form of urea and ammonia, affects the air quality in the horse's stall and might indirectly contribute to respiratory problems. Several studies have demonstrated detrimental performance effects from feeding horses more protein than they needed, and none have shown any benefit.

There is one group of equine athletes which requires a higher-than-average protein content, however: young racehorses. Because Thoroughbred, Standardbred, and sprint-type racehorses are performing at likely the highest levels demanded of any equine athletes, at a time when they are still young and growing, they need to take in enough protein to support both healthy growth and the constant remodeling of bone that comes with repeated pounding on a racetrack. A total crude protein level of 12 to 14% in their diets might assist in keeping a young racehorse's bones and joints healthy, and might even reduce the risk of breakdown. (If you are feeding a low-protein grain mix and grass hay, you may have to add a protein supplement, such as soybean meal, to the rations.) Once a racehorse reaches relative maturity, however (at four or five years for Thoroughbreds and Standardbreds, or three for the faster-maturing Quarter Horses and Paints), he need not receive more protein than any other mature horse.

Perhaps the most crucial nutrient for working horses is also the one that we're most likely to overlook. For frequent or prolonged physical activity of any kind, the most important nutrient a horse must take in is WATER. Under severe exertion, a horse can lose up to half of his body's total protein, and virtually all of his fat stores, before he is in trouble, but a loss of only 15% of his internal water supply can be fatal. When an exercising horse loses water through sweating, he also loses electrolytes (salts and trace minerals which help support many of the body's normal functions). The loss of both of these helps the horse to dissipate the heat generated by converting energy substrates (glycogen, fatty acids, and so forth) into ATP. Without water and electrolytes in the system, the horse can't perform this job of internal cooling effectively, and so he overheats and performance stops. Access to clean, fresh water and free-choice salt should be a first priority, then, for any horse.

To some extent, eating forage (especially hay) helps maximize the amount of water and electrolytes in the gastrointestinal tract, making them available to replenish those lost during prolonged exercise. Forage helps "trap" water and trace minerals in the gut, so endurance riders sometimes feed hay at frequent intervals before a competition, to help their horses maintain a good fluid balance. However, sometimes a full stomach can be a disadvantage to performance. It's estimated that the equine GI tract can hold enough material and water to be equal to a full quarter of the horse's bodyweight--and that can mean extra weight that the horse has to lug around when he's performing. For short-duration or near-maximum intensity activities, it's probably better not to fill the gut immediately beforehand, as there is no need or time for the horse to absorb nutrients or utilize water from his meal.

For maximum performance in many sports, including racing, it's actually best to fast your horse for four to five hours before the competition. Carbohydrate intake within a couple of hours of a competitive event will result in a surge of blood glucose, which will be closely followed by a release of the hormone insulin. When insulin peaks in the bloodstream, it does an effective job of exacerbating fatigue, by decreasing the utilization of fat as an energy source and worsening the fall in blood glucose. To a lesser extent, hay and forage will also trigger this insulin reaction. So although it may feel cruel to do it, you might want to consider the value of letting your horse go without his breakfast on the day he's expected to perform to his utmost (or, if that seems too cruel, offer hay only, not grain). Afterwards, he can be gradually fed his missing meal in small, repeated servings.

Mineral Support

When a horse sweats during exercise, he loses not only water but also sodium, chloride, potassium, and to a lesser extent calcium and magnesium, all of which are considered electrolytes. Under most conditions, he's able to replace these trace minerals fairly easily through his diet, but in extreme circumstances (especially when it is very hot and humid), he may end up with a significant electrolyte deficit. At similar sweating rates, a horse may lose three times more sodium and chloride, and five to 10 times more potassium than his human counterpart.

A sweat-induced drop in blood chloride levels often has serious consequences for a hard-working horse. It results in alkalosis, a condition that inhibits respiration and thus makes it harder for the horse to expel carbon dioxide or bring in enough oxygen. Without oxygen, the aerobic system cannot continue to function, and so lactic acid builds up, glycogen is depleted, and the horse quickly becomes fatigued. Meanwhile, if blood potassium levels also drop, muscle strength and tone are compromised. And both sodium and potassium deficits can contribute to loss of appetite and thirst, which further decreases the water and food intake necessary to correct the imbalances and the energy depletion.

Extensive sweating, especially in hot and humid conditions, might also trigger a decrease in plasma calcium and magnesium. If severe enough, this can cause anxiety, increased neuromuscular excitability (causing muscle twitching and tension), incoordination, tying up, and synchronous diaphragmatic flutter, often known in the endurance world as "thumps."

Sweat-induced mineral losses are usually the result of prolonged, endurance-type activity. By contrast, horses who perform in sprint-type activities don't have time to sweat excessively. Instead, they may suffer acidosis (the opposite of alkalosis) when lactic acid production in their muscles increases. Their plasma potassium also tends to rise (though it returns to normal levels within four to five minutes after exercise stops).

Preventing electrolyte losses is difficult because there are no body stores of these minerals, other than those carried in the gastrointestinal tract. Excesses from the diet are rapidly excreted in the urine. But you can prevent the deficiencies from becoming severe by replacing both water and electrolytes as they are lost, offering clean drinking water at frequent intervals, and providing at least five pounds of hay daily for potassium, a salt block for sodium and chloride, and, if necessary, administering an electrolyte solution or paste if symptoms of dehydration (pale mucous membranes, slowed capillary refill, and a slow response on a "skin pinch" test performed on the shoulder) indicate it's necessary. Some endurance competitors feel it's useful to substitute alfalfa for grass hay during a competition, on the theory that the extra calcium in legume hays may help counteract any calcium losses from sweating. As for salt, free-choice is best; if you prefer to add loose salt to your horse's grain ration, be careful not to exceed 0.5% of the total diet. Not only does excess salt reduce the palatability of the rest of the meal, it also increases the rate at which water and potassium are lost.

If your horse is going to be working hard over a period of many hours (as in a 100-mile endurance race), he may benefit from the periodic administration of more electrolytes, mixed in some applesauce and syringed into his mouth. You can use a commercial preparation (which may have added glucose for a little extra energy), or mix up two parts of table salt, two parts of "lite" salt (a potassium chloride/sodium chloride mix) and one part of dolomite (calcium and magnesium carbonate). A single dose, approximately 28 grams, can easily be stored in a 30-gram (one ounce) 35mm film canister. As long as your horse has enough water available to drink until he's satisfied, there is little risk that you can overdose him on electrolytes.

What about vitamins? Studies of young Thoroughbreds and Standardbreds under racing stress indicate that they may have a reduced ability to convert beta-carotene (from green plants and hay) into vitamin A, and that they may also have trouble synthesizing enough B vitamins, especially folacin, in the cecum. There is some indication that supplemental levels of vitamin C may also provide a minor performance advantage, and vitamin E and selenium levels need to be high to help prevent tissue damage caused by high levels of oxidation during exercise. Administering a general vitamin supplement might be a good idea, particularly if your horse is still young and growing.

One more nutrition tip for hard-working horses: probiotics, or live yeast cultures, seem to help horses digest their feed more thoroughly and better extract the nutrients they need. While this only has an indirect bearing on athletic performance, yeast supplements are inexpensive and harmless to feed, so they may be worthwhile. We'll have more on probiotics in an upcoming issue.


Feeding a high-octane horse is rather like taking care of a high-performance race car. The workings are delicate, and the balance fragile, and you can't expect it to run on cut-rate fuel. But the results, if you get everything working correctly, may be spectacular.

Breed or Type

Type I
(slow twitch high oxidative) (%)
Type II A
(fast twitch high oxidative) (%)
Type II B
(fast twitch low oxidative) (%)

Type I Type II A Type II B
Endurance horse
40 55 5
6 54 40
20 40 40
Heavy foxhunter
30 35 35
Standardbred racehorse
20 50 30
20 50 30
Thoroughbred racehorse
12 53 35
Quarter Horse
8 50 42
Human middle-distance runner
60 35 5
Equine Muscle Fiber Types
And Their Concentrations
In Various Types Of Equine Athletes
taken from Lon Lewis's
Feeding and Care of the Horse, 2nd ed. 1996.

About the Author

Karen Briggs

Karen Briggs is the author of six books, including the recently updated Understanding Equine Nutrition as well as Understanding The Pony, both published by Eclipse Press. She's written a few thousand articles on subjects ranging from guttural pouch infections to how to compost your manure. She is also a Canadian certified riding coach, an equine nutritionist, and works in media relations for the harness racing industry. She lives with her band of off-the-track Thoroughbreds on a farm near Guelph, Ontario, and dabbles in eventing.

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