Is Dietary Fat Really Healthy?
- Nov 1, 2002
Marketing claims regarding the virtues of fat in equine diets are plentiful. Statements such as "Added dietary fat for improved performance," "Increased stamina," "Calm energy," or "Improved coat and hoof condition" abound. Indeed, at times it is easy to conclude that an increase in dietary fat is the solution to anything that ails a horse--the proverbial "best thing since sliced bread." Contrast this sentiment with the prevailing attitude toward dietary fat among human nutritionists and physicians. Diets high in saturated fat and cholesterol have long been associated with 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). High-fat diets have also been blamed for the current epidemic in obesity throughout the Western world. However, this issue is hotly debated, and there now is evidence that consumption of excess sugar, rather than fat, underlies the tendency to gain weight.
Should we have similar concerns regarding dietary fat for horses? Current evidence would suggest that there are few, if any, concerns associated with the use of fat in horse diets. On the other hand, it might be argued that there has been insufficient research in horses to allow us to give a definitive yes or no answer to this question.
Sources of Dietary Fat
Unless fat (e.g., some type of vegetable oil) is added to the diet, horse rations are very low in fat, typically less than 3-4% by weight. Some fat is needed to facilitate absorption of the fat-soluble vitamins A, D, E, and K. The horse also needs small amounts of the essential fatty acids called linoleic and alpha-linolenic acids. In biochemical language, linoleic acid is termed an omega-6 fatty acid whereas alpha-linolenic acid is termed an omega-3 fatty acid. The difference between omega-6 and omega-3 fatty acids relates to their chemical structure and the position of the double-bonds in the carbon backbone of the fatty acid. There is considerable interest in the dietary ratio of omega-6:omega-3 fatty acids; in particular, diets that promote an increase in omega-3 relative to omega-6 fatty acids are advocated because of purported health benefits (more on this later).
In the last decade or so, there has been increasing use of supplemental vegetable oil in horse diets. Corn and soya oils are the most commonly used, although canola, linseed, or flaxseed oils can also be safely fed to horses. Flaxseed and linseed oils are both produced from flaxseed; flaxseed oil is cold-pressed whereas linseed oil is extracted under high temperature using petroleum-based solvents. Thus, flaxseed oil is a more "natural" feed ingredient, although rations with 4-8% linseed oil or flaxseed oil have been fed to horses without apparent problems.
Other rich sources of vegetable fat include rice bran and copra meal. Rice bran is 18-22% fat while copra meal (a by-product of coconut processing) is 8-9% fat; both are good sources of highly digestible fiber and are low in starch and sugar, and they are useful when a reduction in dietary starch is desired. Both, however, are naturally high in phosphorus and low in calcium. They should be used in limited amounts, especially if the calcium:phosphorus ratio of the total ration is not balanced. Pure rice bran oil is also available. The vegetable fats (e.g., corn or soy oil, rice bran) are quite palatable for horses. Animal tallows, on the other hand, are generally much less palatable and are not recommended.
The horse's digestive system can manage reasonably large quantities of dietary fat, although there can be digestive upsets (diarrhea) if large amounts of oil are suddenly added to the diet. In general, the quantity of oil added to the diet should be increased gradually over a two- to three-week period.
Providing there has been a gradual increase in dietary fat intake, horses are able to digest and utilize up to 20% of the diet (by weight) as oil. However, most equine rations are considerably lower in fat content, even with the use of so-called high-fat feeds (e.g., a grain concentrate that is 6-10% fat). For example, the total fat content of a ration comprised of 15 pounds (6.8 kg) of hay (less than 3% fat) and eight pounds (3.6 kg) of a 10% fat feed is approximately 5.4% fat ([0.03 x 15 pounds] + [0.1 x 8 pounds] = 1.25 pounds fat and 1.25 ÷ 23 pounds of total feed = 0.054 or 5.4% fat).
One of the main nutritional advantages of fat is its energy density. Vegetable oils have about three times as much digestible energy (DE) as oats and 2.5 times as much as corn. This feature is especially useful when managing a hard keeper or sick horse--adding fat can result in a substantial increase in caloric intake without requiring the horse to consume more food. The horse also can use fat energy in an efficient manner. When a typical hay and grain diet is fed, a horse is only able to utilize 50-60% of the energy in these feedstuffs. On the other hand, horses can utilize more than 90% of the energy contained in a vegetable oil since more of the energy is available to the horse. This is one reason why fat supplementation is useful for fattening thin animals or helping to meet the very high-energy requirements of horses in heavy training.
Healthy or Not?
To evaluate whether or not dietary fat is "healthy," we need to consider the impact of fat on body functions. The first consideration is gastrointestinal function, particularly in performance horses requiring large amounts of high-energy feeds for maintenance of body condition. Traditionally, heavy grain feeding was used to meet these energy requirements. The major concern with this approach is an increased risk of gastrointestinal problems--with large grain intakes (more than 11-15 pounds, or 5-7 kg, per day for a 1,100-pound, or 500-kg, horse) some of the grain passes undigested into the large intestine, where it ferments. Excessive grain fermentation can disrupt the normal microbial population of the hindgut and predispose the horse to colic and other digestive upsets.
Adding fat (and a highly digestible fiber source such as beet pulp) to the ration allows for a substantial reduction in the quantity of grain (and starch) in the diet. This approach can help stabilize the hindgut environment and prevent digestive disturbances. Score 1 for fat.
A higher-fat diet also might favorably alter behavior. Compared to a traditional hay and cereal grain diet, a diet containing a combination of corn oil and soy lecithin has been shown to reduce spontaneous activity and reactivity to noise and sudden visual stimuli.1 Clinical experience also indicates that many horses exhibit a calmer disposition--being less hot-headed--when fed a higher-fat ration compared to a more traditional starch- and sugar-based diet. Score 2 for fat.
Speaking of hot-headed horses, it is well known that a nervous temperament is one of the factors contributing to tying-up episodes in horses with recurrent exertional rhabdomyolysis (RER), a condition most common in racing Thoroughbreds and Standardbreds. It is also well established that higher-fat and -fiber feeds (compared to a grain-based ration) help in the management of horses with RER, possibly because of the "calming effect" of the higher fat diet. Score 3 for fat.
A higher-fat, lower-starch and -sugar diet is also crucial in the management of horses with polysaccharide storage myopathy (PSSM), another form of chronic tying-up. PSSM is associated with abnormal storage of excess glycogen (the body's storage form of glucose) in skeletal muscle, likely because these horses are more sensitive to the effects of insulin, the hormone responsible for the clearance of glucose from the bloodstream. Therefore, the starches present in grain "fuel the fire" by providing the extra glucose necessary for the synthesis of glycogen in muscle. Grain should be removed from the diets of horses with PSSM (and from the diets of draft-breed horses with a similar condition called equine polysaccharide myopathy, or EPSM). An increase in fat intake, together with the restriction in dietary starch and sugar, is also important in the management of horses with PSSM or EPSM, because it can provide the calories normally provided by the grains without stimulating insulin release. Score 4 for fat.
Finally, additional oil in the diet might benefit skin and hoof appearance. Clinically, the coat and hooves take on a shiny, healthy look when horses are fed a fat-supplemented diet. Another point for fat.
Based on our current understanding, there is little to indicate that feeding fat to horses is harmful. In fact, the weight of evidence suggests that a higher-fat diet is a much healthier approach than a more traditional high-grain diet. However, there are no data on the long-term effects of feeding rations containing over 7-10% fat to horses. It is currently recommended to limit the total fat intake to 10% or less.
The Omega Story
As mentioned earlier, all mammals (including horses) have a dietary requirement for the essential fatty acids. The omega-3 essential fatty acids are derived from alpha-linolenic acid, whereas the omega-6 essential fatty acids are derived from linoleic acid. The ratio of omega-6:omega-3 fatty acids varies among the different oils. Flaxseed (or linseed) oil contains approximately 50% alpha-linolenic acid (omega-3) and about 15% linoleic acid (omega-6). Soya oil also has a high omega-3 fatty acid content, whereas the predominant essential fatty acid in corn oil is linoleic acid (omega-6).
Here's where it gets a bit complicated, with way too many unpronounceable words! Once in the body, the essential fatty acids are further metabolized to other types of fatty acids--arachidonic acid (AA) is the predominant product of linoleic acid metabolism (omega-6 fatty acid) whereas eicosapentanoic acid (EPA) is a major product from alpha-linolenic acid (omega-3 fatty acid). In turn, these fatty acids can be metabolized to substances called eicosanoids, which are important mediators of inflammation and blood coagulation.
Most importantly, the eicosanoids produced from EPA are less potent inflammatory mediators than those resulting from the metabolism of AA. Recognition of this fact has generated much interest in the effects of diets high in omega-3 fatty acids. There is some evidence from human and animal studies that dietary manipulation of the omega-6:omega-3 fatty acid ratio (via an increase in dietary omega-3 fatty acids) can dampen the body's response to inflammatory stimuli. This might be useful for a horse with overactive immune system responses, such as horses with heaves, equine recurrent uveitis, etc.
Researchers at the University of Georgia have studied the effects of linseed oil supplementation in horses (8% of the diet, a huge amount of linseed oil) on the response to endotoxin, a bacterial substance that causes severe inflammation and illness during some forms of colic. They found that blood cells collected from horses supplemented with linseed oil produced lower quantities of harmful prostaglandins when exposed to endotoxin in a test tube. However, when the horses themselves were exposed to endotoxin, there was no difference in response between supplemented and control horses.
The jury is still out on the effects of omega-3-rich oils in horses. Marketing pieces from horse feed and supplement manufacturers often make a big deal about the inclusion of omega-3-rich oils in various products, but in reality little is known regarding the effect of this form of fat supplementation. A recent study of high-level supplementation with flaxseed oil (10% of the diet) in horses found no change in the omega-6:omega-3 ratio.2 More research is needed to determine the health benefits (if any) in horses associated with the manipulation of the dietary ratio of omega-6:omega-3 fatty acids.
What is the best way to add fat to your horse's diet? The three main options are:
- Adding a vegetable oil to the existing diet;
- Feeding a commercial concentrate containing various levels of added oil; or
- Feeding an oil-rich supplement such as rice bran or copra meal.
The simplest approach is to add vegetable oil to the ration, and this is reasonable when small quantities (one cup or less per day) are added. However, nutritional imbalances can occur when feedstuffs such as balanced concentrates are substituted by two more cups of straight vegetable oil. Remember that pure vegetable oils (e.g., corn, soya, or flaxseed oil) contain no minerals or vitamins.
For this reason, the preferred approach is to feed a commercial high-fat concentrate that is properly formulated to ensure that the right balance of nutrients is delivered. If you take the non-commercial approach, then vitamin E should be added to the diet at a rate of 200-250 international units (IU) per cup of oil. This is very important, because higher-fat diets increase the horse's requirements for antioxidant nutrients such as vitamin E. This should not be necessary when commercial high-fat feeds are used, however, as these products contain extra vitamin E fortification. However, vitamin E activity is rapidly destroyed by exposure to air and humidity. If feeds are to be stored for more than a few weeks in hot, humid climates, some supplementation might still be necessary.
What is the ideal amount of fat supplementation? Although there is no clear-cut answer to this question, it has been recommended to feed no more than 3.5 ounces (100 grams) of fat (oil) per 220 pounds (100 kg) body weight per day. For a 1,100-pound (500 kg) horse, this equates to a maximum of 17.5 ounces (500 grams) of oil per day. Of course, oil should be gradually introduced into the diet, and this level of feeding would be reached only after a two- or three-week adaptation period. Also, the daily amount is best divided into two or three feedings.
Note that one standard measuring cup holds about eight fluid ounces or 200 grams of oil. Therefore, 500 grams of oil per day is a little more than two measuring cups. Each cup provides approximately 1.6 Mcal of DE. Therefore, to maintain the same daily calorie intake, at least one pound/day of grain should be removed from the diet for every cup of oil added to the ration (i.e., oats are 1.1-1.2 Mcal DE per pound).
Most commercial high-fat concentrates are between 5-10% fat. Daily fat intake will depend on the amount of concentrate fed. For example, 10 pounds of an 8% fat product will deliver about 365 grams of oil, or the equivalent of almost two cups of vegetable oil. As with the addition of oil to an existing ration, these high-oil feeds should initially be mixed with the current ration, with a gradual increase in the amount of the new feed while decreasing the old over a two- to three-week period.
So, the answer is that yes, fat is safe to feed if done so within certain parameters. Start adding fat slowly to the diet to avoid digestive upset. Increase vitamin E when increasing fat. Use vegetable sources and not animal sources. Beware of adding too much, even of a good thing.
1 Holland, J.L.; Kronfeld, D.S.; Meacham, T.N. Behavior of horses is affected by soy lecithin and corn oil in the diet. Journal of Animal Science, 74, 1252-1255, 1996.
2 Hansen, R.A.; Savage, C.J.; Reidlinger, K.; et al. Effects of dietary flaxseed oil supplementation on equine plasma fatty acid concentrations and whole blood platelet aggregation. Journal of Veterinary Internal Medicine, 16, 457-463, 2002.
5 TIPS: Feeding Fat
- Vegetable oils have about three times as much digestible energy (DE) as oats and 2.5 times as much as corn.
- The preferred approach to feeding more fat is to feed a commercial high-fat concentrate that is properly formulated with the right balance of nutrients.
- It has been recommended to feed no more than 3.5 ounces (100 grams) of fat (oil) per 220 pounds (100 kg) of body weight per day. For an 1,100-pound (500-kg) horse, this is 17.5 ounces (500 grams, or just over two measuring cups) of oil or less per day.
- If supplementing fats separately (not in a commercial feed), use vegetable sources of fat and not animal sources (they're more palatable).
- Increase vitamin E when increasing fat.
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: Managing Working Horses