For the most part, the word "fat" has bad connotations in our society today--fat often is used to describe an overweight or obese state. When we think of dietary fat and the proportion of calories in our diet that is derived from various sources of fat, we typically are admonished to reduce fat intake, particularly intake of foods high in saturated fats and cholesterol. Of course, a major concern is the relationship between a diet high in fat and cholesterol and the development of coronary heart disease (when deposits of fat and cholesterol cause a narrowing of the arteries that supply blood to the heart, resulting in damage to the heart muscle). Do we have the same concerns when feeding fat to horses? Fortunately, the answer to this question is no. Indeed, there are a number of reasons why you should feed your horse a fat-supplemented diet, and in recent years, it has become common, and recommended as beneficial.
In this article, we will examine the various sources of fat that can be used in horse diets, then review some basics on the digestion and metabolism of fat in horses, and conclude with a discussion on the advantages of fat-supplemented diets for athletic horses.
Fats--What Are They?
Dietary fats also are known as triglycerides--there are three (tri) fatty acid molecules attached to one glycerol molecule. Some of you are aware that fatty acids can be saturated or unsaturated. There can be considerable variation in the size or length of the fatty acids. These chemical features are important because they affect the physical properties of the fat or triglyceride--at room temperature, fats are in either a solid or liquid form. Fats with unsaturated and/or short fatty acids tend to be in the liquid state at room temperature and are referred to as oils. Conversely, longer chain fatty acids that are saturated usually are solid at room temperature and are referred to as fats.
Unless fat (e.g. some type of vegetable oil) is added to the diet, horse rations are very low in fat, typically less than 2%-3%. However, horses are able to digest and absorb dietary fat quite well. In fact, it is important that horse diets contain at least some fat or oil--it is needed to facilitate absorption of the fat-soluble vitamins A, D, E, and K. The horse also needs a small amount of a linoleic acid, a special type of fatty acid, in its diet. For this reason linoleic acid is termed an essential fatty acid.
Fats can be of veg-etable or animal origin. Vegetable origin fats include corn and soy oil, rice bran or oil, sunflower seeds or oil, flax meal, and linseed oil. Animal fats sometimes are used in the preparation of horse rations. Vegetable oils are higher in unsaturated fatty acids than animal fats.
Unsaturated fats become rancid during storage--conditions that promote development of rancidity include high temperature and relative humidity (i.e., a warm, moist environment) and prolonged storage. Rancid fats cause a number of problems. Most importantly, the fat becomes less palatable so that the horse might refuse to eat the ration containing it. As well, rancid fats interfere with absorption of some of the vitamins in the horse's diet and do not provide its linoleic acid requirements.
Fortunately, commercially available fat sources and horse diets supplemented with fat contain antioxidants such as vitamin E that prevent or delay development of rancidity. Even so, it is important to store fat-supplemented feeds in a cool, dry area.
Fat Digestion And Metabolism
Dietary fats are digested and absorbed in the small intestine. As mentioned, within their natural environment, horses normally do not consume large quantities of fat. Nonetheless, the horse's digestive system can manage reasonably large quantities of dietary fat--studies have shown that diets providing 30%-35% of calories from fat are tolerated by horses. However, more typically a "high-fat" diet for horses is one that provides about 20% of calories from fat. More on this issue later in the article.
Fat primarily is added to the diet because it is an excellent energy source. Compared to an equivalent weight of carbohydrate (provided by the starch in cereal grains), fat provides about 2 1/2 times as much energy. The horse can use this energy in an efficient manner. In a typical hay and grain diet, a horse only is able to utilize between 50% and 60% of the energy. On the other hand, horses can utilize 85% to 90% of the energy contained in vegetable oil. This is one reason why fat supplementation is useful for putting weight on thin animals or helping meet the very high energy re-quirements of horses in heavy training.
Within the body, fat is used either directly for energy, or stored. For storage, fatty acids are repackaged as triglycerides within adipose tissue. Most of this fat is stored in adipose tissue that lies beneath the skin (subcutaneous) or surrounds the intestinal organs. In addition, a smaller amount of fat is stored within muscle, where it can be utilized for energy during muscular work.
Although it is difficult to assess body fat in horses, it is safe to say that horses in average body condition have huge fat stores. Recall that carbohydrate and fat represent two main sources of energy for the horse during exercise (see January 2000 Sports Medicine article). Body carbohydrate stores (glycogen stored in the liver and muscle) are quite limited--these stores are usually adequate for sustenance of short-term exercise such as racing, but alone cannot support longer duration exercise such as endurance racing. Indeed, depletion of muscle and liver glycogen reserves contributes to fatigue during prolonged exercise tasks. It is therefore imperative that fat provide a substantial proportion of the energy.
Just how much fat can be utilized or "burned" during exercise depends on a number of factors. These include the intensity and duration of exercise, the breed and training history of the horse, whether or not the horse is adapted to a fat-supplemented diet, and the food consumed before exercise.
At rest, fat provides more than 50% of energy requirements. In horses, much of this energy is derived from volatile or short-chain fatty acids that are produced by microbial fermentation in the large intestine. During exercise, however, the fatty acids stored in adipose tissue and muscle are the primary source of fat energy. In general, fat is an important energy source, providing the exercise intensity is below 75% to 80% of maximum aerobic capacity (VO2max). Above this exercise intensity, carbohydrates provide most of the energy.
Consider that horses undertaking endurance racing are working between 30% and 60% of VO2max--at that point, fatty acids are an important energy source. On the other hand, fat does not supply significant energy during Thoroughbred or Standardbred racing that involves exercise at an intensity well above VO2max.
One of the reasons for this limitation in fat use during intense exercise is the time required to generate energy (as adenosine triphosphate or ATP) from the breakdown of fatty acids. The breakdown of muscle glycogen can produce ATP up to six times faster compared to the breakdown of an equivalent amount of fat. Because intense exercise requires very rapid generation of ATP, this energy must be provided by carbohydrate rather than fat.
The breed or type of horse also affects the amount of fat that can be used during exercise. Type I muscle fibers have the highest capacity for utilization of fat, while type II fibers are more adapted for use of carbohydrate (glycogen). Horses bred for sprinting activities (e.g., Thoroughbreds and Quarter Horses) have a greater proportion of type II fibers and are therefore more geared toward utilization of carbohydrates. On the other hand, Arabs and other breeds accustomed to more prolonged exercise tasks are adapted to higher utilization of fat, probably because of a higher percentage of type I muscle fibers. Regardless of breed, exercise training results in a shift in metabolism such that there is a greater use of fat and a concomitant sparing of the more limited glycogen stores.
Diet and feeding strategy can markedly influence the rate of fat utilization during exercise. It generally is believed that increased fat use will decrease glycogen breakdown and enhance performance. Thus, there has been considerable interest in the effects of fat feeding, particularly for horses engaged in endurance-type exercise. Let's delve into this topic further as we consider the advantages of fat-supplemented diets.
Pros And Cons
There are several reasons for adding fat to an athletic horse's diet. As already mentioned, fat is an energy-dense nutrient--adding fat can result in a substantial increase in caloric intake without requiring the horse to consume more food. This is an important issue because the energy needs of horses in heavy training is up to twice that of horses at maintenance (i.e., not in training). Traditionally, this increased energy requirement was met by adding grain to the diet.
The major concern with that approach is an increased risk of gastrointestinal problems--with large grain intakes (more than 5-7 kg/day for a 500-kg horse), some of the grain passes undigested into the large intestine, where it undergoes fermentation. Excessive grain fermentation can disrupt the normal microbial population of the hindgut and predispose the horse to the development of colic or laminitis.
Adding fat or oil to the ration allows for a reduction in the amount of grain in the diet, hopefully reducing the risk of colic. We also know that high-grain diets are problematic for horses prone to two different types of the muscle condition known as tying-up or exertional rhabdomyolysis (see the June 2000 Sports Medicine article). For Quarter Horses and related breeds with the disorder polysaccharide storage myopathy (PSSM), the problem arises because of a propensity to store excess glycogen in their muscles. The starches present in grain "fuel the fire" by providing the glucose necessary for the synthesis of glycogen in muscle. For horses with PSSM, grain must be removed from the diet. Fortunately, these horses often are "easy-keepers," and many will thrive on a diet of hay, pasture, a small amount of a fat supplement (such as one to two pounds of rice bran per day), and a vitamin-mineral supplement.
The other form of tying-up that can be worsened by high-grain diets is recurrent exertional rhabdomyolysis (RER), a condition most common in racing Thoroughbreds and Standardbreds. Recent research by Stephanie Valberg, DVM, PhD, and colleagues at the University of Minnesota has shown that reducing the amount of grain fed to affected horses, and adding fat to replace those calories, is an important component of the overall management of horses afflicted with RER. Because those horses usually have very high-energy requirements, it is not possible to completely remove grain from the diet. As a general rule, no more than five pounds of grain per day should be fed, with additional energy provided by fat and highly fermentable fiber feeds such as beet pulp or soybean hulls.
Let's return to the issue of fat-supplemented diets and athletic performance. Research in other species (albeit mostly in rats) has demonstrated that very high-fat diets can be beneficial because, after a period of adaptation to that diet, there is increased use of fat as a fuel during exercise. This increased fat utilization, in turn, reduces use of a more precious commodity--liver and muscle glycogen. It must be emphasized, however, that these alterations in metabolism will not necessarily result in improved performance. In fact, studies in man have shown the complete opposite!
The jury also is still out regarding the effects of fat-supplemented diets on equine athletic performance. Here we need to draw one important distinction--a so-called high-fat diet for a horse is nowhere near equivalent to a high-fat diet for a human or a dog. Realistically, we can at most provide 20% to 25% of calories from fat when it is added to the diet of a horse. By comparison, our typical diet provides 35%-40% of calories from fat, and a high-fat diet is in the 60%-70% range.
Despite these differences, there is evidence that a period of fat supplementation in horses results in metabolic adaptations that favor increased use of fat during endurance-type exercise (see Kronfeld et al. 1994; Orme et al. 1997). The general consensus is that a two- to three-month period is required for these adaptations to be complete, and that the diet should provide 20% to 25% of calories from fat.
Another advantage of fat adaptation is a reduction in metabolic heat generation, both at rest and during exercise (see Kronfeld 1996). For horses training and competing in hot, humid climates, this reduction in heat load can provide a competitive edge. The lower heat load lessens the need for evaporative heat loss (by sweating), thus reducing water loss during exercise.
Finally, studies have shown that fat supplementation might alter a horse's behavior--compared to a traditional hay and cereal grain diet, a diet containing a combination of corn oil and soy lecithin reduced spontaneous activity and reactivity to noise and sudden visual stimuli (see Holland et al. 1996).
Practically speaking, excitable and fractious horses (and their handlers!) can benefit from an increase in fat intake and a reduction in grain intake.
Enough of the theory--just how do you deliver fat to your horse? There are several options, and the approach taken will depend on cost and availability of ingredients, among other factors. The simplest method calls for the "top-dressing" of corn or soy oil onto other components of the diet. For a 500-kg horse (1,100 pounds), up to 600 ml of oil (one pint or two cups) can be added, although it is more common to add one to two cups. For horses fed hay cubes, one method that works is to mix the hay cubes (first moistened by the addition of water), grain, and oil into a single meal. The mixture looks a little messy, but providing the amount of oil is increased gradually, most horses readily adapt to this feeding method.
Another option is to purchase a commercial ration that contains fat. Typically, these rations are 8% to 10% crude fat, although there are some with a higher fat content. Note that use of these feeds is not ideal for horses with tying-up problems (PSSM or RER) because intake of cereals will still be too high. In these horses, fat intake should be increased by use of vegetable oil or rice bran. Note that rice bran is only about 20% fat (by weight), so a larger quantity must be fed (e.g., feeding four pounds of rice bran will be the equivalent of one cup of corn oil).
A concern often raised with rice bran is its "reversed" calcium to phosphorus ratio-- if large amounts of rice bran are fed, the calcium-phosphorus ratio of the total diet might be inappropriate. On the other hand, some rice bran products (e.g., EquiJewel) contain added calcium to correct this potential imbalance.
Regardless, it always is a good idea to consult with your veterinarian or nutritionist when instituting a major dietary change such as the addition of rice bran or vegetable oil to the diet. He or she can review the total diet and ensure that your horse's intake of essential vitamins and minerals is adequate, and balanced.
The amount of fat in the diet should be increased gradually. For example, if the goal is to feed two cups of oil per day, it is best to start with one cup per day. Over the next two to three weeks, the amount of oil can be increased slowly (while the amount of grain is reduced). Closely monitor the horse's appetite and the character of his manure. Some horses might get a little "loose" during the early phases of oil feeding, but this problem usually is self-limiting. Finicky eaters might be a little slow to adapt to feeding fat, but by-and-large, horses readily accept fat in their diets. Persistent feed refusal should alert you to the possibility of a rancid fat source.
As mentioned, the maximum benefits from fat supplementation occur two to three months after the start of feeding fat. Therefore, while fat feeding can be started at any time, it is best to institute fat supplementation early in the year (perhaps in late winter/early spring) as the horse begins training for the season. This will allow sufficient time for metabolic adaptation to occur and ensure that the benefits of fat supplementation are realized when they are needed most.
Kronfeld, D.S. Dietary fat affects heat production and other variables of equine performance under hot and humid conditions. Equine Veterinary Journal Supplement 22: 24-34, 1996.
Kronfeld, D.S., Ferrante, P.L., Grandjean, D. Optimal nutrition for athletic performance, with emphasis on fat adaptation in dogs and horses. Journal of Nutrition 124: 2745S-2753S, 1994.
Orme, C.E., Harris, R.C., Marlin, D.J., Hurley, J. Metabolic adaptation to fat-supplemented diet by the Thoroughbred horse. British Journal of Nutrition 78: 443-458, 1997.
Holland, J.L., Kronfeld, D.S., Meacham, T.N. Behavior of horses is affected by soy lecitin and corn oil in the diet. Journal of Animal Science 74: 1252-1255, 1996.
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: Winter Horse Care Concerns