Written By Dr. Kris Hiney

We have already discussed two of the fat soluble vitamins in a horse’s diet. This month we continue with a closer look at vitamin E, a vitamin which is commonly supplemented to horses for a variety of reasons.  It is often used for aging horses, horses which have muscle disorders and horses which undergo strenuous exercise.  But how do you know if your own horse needs more vitamin E in its diet?

First, let’s explore the role of vitamin E in your horse’s body.  Vitamin E occurs in a variety of forms (both tocopherols and tocotrienols).  Of these, there are then four subgroups, α, γ, β and δ. While γ is the most common in the natural diet, the alpha form is the most potent in activity, the most supplemented and the subject of most studies.   In their natural diet, horses receive the most vitamin E as γ tocopherol from growing forages or harvested forage that was cut at an immature state.  As the plant ages, vitamin E decreases in content.  Vitamin E concentration also decays over time in harvested forages, as much as 50% over one month.  Therefore, older hays which have been stored for some time will have little activity.  If you also feed non-processed concentrates  to your horse (such as oats, barley, corn etc.) they will also be low in vitamin E. However, most commercial equine feeds will be supplemented vitamin E, usually as α tocopherol acetate.   It can be provided as either natural α tocopherol or synthetic, with natural forms having 36% more biological activity than synthetic.  The natural form has been shown to increase plasma α tocopherol concenrations greater than its synthetic counterpart but both are effective supplements.

(This is the structure of alpha tocopherol.)

Despite its form, vitamin E’s function is most frequently thought of as an anti-oxidant.  Vitamin E can work to eliminate free radicals which are formed through the incomplete oxidation of oxygen or other molecules.  During normal metabolism some amount of free radicals are always formed.  However, stress, work, aging, poor nutrition etc can increase the amount of free radicals in the body.  These are essentially molecules which are missing an electron, making  them highly reactive.   This is an unstable condition and the free radical can remove electrons from other cell components, such as lipids, cell membranes etc.  Vitamin E, along with other anti-oxidants donates an electron to the free radical, thus stabilizing it and preventing further damage.  One oxidized, vitamin E itself must be reduced back to its active form.  This is usually accomplished through the action of other anti-oxidants in the body such as ascorbic acid or glutathione peroxidase.  As the cells of the immune system have a high amount of polyunsaturated fatty acids which are quite susceptible to damage by free radicals, vitamin E plays a vital role in the optimization of the immune system.  Furthermore, vitamin E plays a role in reproduction, gene transcription and platelet aggregation.

(Traditional concentrates such as just corn and oats may be relatively low in vitamin E content.)

Currently, vitamin E is recommended to be fed to maintenance horses and breeding horses at 1 IU/kg of body weight (not sure if your horse is a maintenance horse, see Energy Requirements).  Growing horses  and lactating mares are suggested to need more vitamin E in their diet, at double the rate of maintenance horses or 2 IU/kg body weight.  Vitamin E intake for the working horse may need to be a bit higher.  While the current recommendation for working horses is 1.8 IU/kg body weight for moderate work and 2 IU per kg body weight for heavy work, many research studies have provided Vitamin E at higher levels.  Supplementation rates from 150-250 IU/kg DM, 300 IU /kg DM or even as high as 11.1 IU kg/body weight (in a simulated endurance race) have been found to be necessary to maintain blood and muscle concentration of vitamin E in more rigorously exercised horses.  To make these values seem more familiar, if we assume we are feeding a 500 kg horse 2% of its body weight, than the range of vitamin E would be between 1500 – 5500 IU of vitamin E per day in these studies.

Therefore, Vitamin E is often part of the suggested management protocols for horses which are heavily exercising or may have muscle disorders. In fact, in a study looking at endurance horses and supplementation of Vitamin E, the authors were unable to create a control group as no riders were willing to not supplement their horses!  However,i t has been difficult to prove the effectiveness of supplementation for the enhancement of the horse’s health.  In exercised horses receiving 300 IU/kg DM of vitamin E compared to 80 IU/kg DM, or no supplementation of vitamin E, the higher rate of supplementation did increase the muscle concentration of vitamin E.  However, it did not affect the indicators of oxidative stress in the muscle following a submaximal exercise test.  Perhaps a difference would have been observed with a more aggressive exercise regimen.  More recently, horses supplemented at a rate of 3000 IU per day of vitamin E compared to 80 IU/kg DM, underwent a training protocol.  The anti-oxidant capacity of all the horses increased following training, which is a natural adaptation to exercise.  There were no differences in reduced or oxidized glutathione peroxidase at rest, or total glutathione peroxidase.  However after a standard exercise test, the horses receiving 3000 IU vitamin E did have more reduced gluthathione peroxidase, suggesting a greater anti-oxidant capacity. Horses exercised to fatigue following 8 weeks of supplementation of 3000 IU of vitamin E had less muscle oxidation as measured by myofibril carbonylation( a measure of protein oxidation).

(Heavily exercised horses may need more vitamin E in their diet than maintenance horses or lightly worked horses.)
Determining if your horse has a vitamin E deficiency may not be as straight forward as taking a blood sample.  It has been shown that the concentration of vitamin E in the horse’s blood  varies irrespective of diet.  In one study, the variation within an individual horse in a 72 hr period would have shown the same horse as more than adequate in vitamin E, to marginal as well as deficient.  Therefore, it may be more important to look at your feeding regimen and the feedstuffs your horse consumes to determine whether or not they may have a deficiency.  The diet your horse is on may also affect his vitamin E needs.  Vitamin E is protective against the peroxidation of lipids in the body, especially the polyunsaturated fatty acids.  Horses which consume diets higher in PUFAs, which is certainly recommended in many cases, may increase the need for anti-oxidants in the body to prevent lipid perodixation.  Thankfully, many sources of PUFAs may be higher in vitamin E content.

If your horse is older, they may also be a candidate for vitamin E supplementation.  As horses’ age, their body systems may not function at the same level seen in their younger years.  As in people, the immune system of our aged horses may begin to fail.  When horses over 20 years of age were vaccinated for influenza, they were unable to mount the same immune response as their younger counterparts.  Therefore, older horses may be prime candidates for supplements which are known to complement the immune system.  In older horses fed vitamin E at 15 IU/kg of body weight, the bacterial killing ability of specific immune cells was increased, along with an increase in some, but not all, of the specific types of immunoglobins (or antibody).  However, in this study, the horses were previously on a marginally deficient amount of vitamin E. Therefore, it is not known whether it was the correction of the deficiency or the over supplementation that yielded positive effects.
Horses are fairly tolerant of relatively high amounts of vitamin E in the diet.  The upper range of vitamin E intake has been set at 1,000 IU/kg of DM. To think of this in more common terms, we will do a brief example using an 1100 lb horse that consumes 2% of its body weight.  Thus this horse would typically consume 22 lbs of feed per day.  We will convert this to kg to look at our total amount of vitamin E the horse should ever safely consume.  22 lbs of feed is equivalent to 10 kg of feed. Thus, the upper range of safe intake of vitamin E is 10,000 IU per day for a 500 kg horse.

However, vitamin E should not be used without caution.  In human medicine supplementation of vitamin E has not always yielded positive results, and if fact can actually enhance the disease state.  In humans undergoing heavy exercise, vitamin E supplementation actually decreased some of the positive adaptations to exercise.  In addition, heavy supplementation has been actually linked to mortality. As always, supplementation is never the answer for a properly balanced diet.   Overzealous  supplementation may actually work against your horse’s health!  But if your horse is older, more heavily worked or has added poly-unsaturated fatty acids in its diet, you might want to examine your diet for its Vitamin E content.

Next month we will finish our discussion of the fat soluble vitamins with vitamin K.