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Vitamin E in the Neurological Horse; Maybe More Harm Than Good?

Vitamin E often goes hand in hand with conditions such as EPM (equine protozoal myeloencephalitis) , EDM (equine degenerative myeloencephalopathy), and even PSSM (polysaccharide storage myopathy) in the horse.  This vitamin possess moderate antioxidant capabilities and can render much needed benefits to those horses afflicted with those conditions, however, not every horse responds and in fact, there is clinical evidence that some horses might actually deteriorate.  Despite rendering antioxidant properties, is there maybe more to the story, regarding vitamin E supplementation in the horse and should a broader approach be utilized for improved outcomes?

Vitamin E and the Horse
Vitamin E and the Horse

Vitamin E is a fat soluble nutrient that is needed for equine health in many regards.  In many instances where there is neurological disease, such as EPM and EDM, or even muscle related conditions including myopathies and PSSM, vitamin E is often heavily relied upon.  While this nutrient is readily available from pasture and fresh, green forage, the levels tend to rise and fall throughout different times of the year.  Stored forages will also lose their vitamin E content over time, sometimes losing as much as 50% over a one month period.

Some horses will demonstrate deficiencies, when their levels are noted to be consistently <1.5 ug/ml, with ideal levels being above 2.o ug/ml.  The reality is, based on research, that even normal horses tend to fluctuate between 1.5 ug/ml and the more normal 2.0 ug/ml, so if a low level is detected, it is recommended to re-evaluate with a second blood sample from a different time of the day. This has to do with vitamin E metabolism and recycling in the horse’s body.

While some horses are deemed deficient in vitamin E, the deficit is almost always attributed to dietary causes, which are often the culprit, but not always to blame.  Those horses that are not allowed to graze active pastures or are consuming most of their calories from older, stored hay are more prone to vitamin E deficiencies.  Outside of diet, there are health reasons which can create a relative deficiency, which are more often than not the true cause of vitamin E deficiency in the horse in my opinion, but overlooked.

Oxidative Stress, Inflammation, and Vitamin E in the Horse

Vitamin E is an antioxidant, and one of many antioxidants present within the proper diet for the horse. An antioxidant serves one major purpose and that is to negate the effects of free radicals, which are present within the horse’s body at any one time.  Free radicals are generated naturally in the horse just through the act of normal breathing and energy production.  Then, additionally, the digestive process, exercise, stress, and even chemicals found in the water or food sources, add to the free radical load. Excess minerals found in the diet, such as copper and iron, can also serve as free radical sources.  Free radicals create oxidative damage upon tissues present in the body, including fats, proteins, and DNA. These same free radicals are closely tied in with the inflammatory process in the horse as well.  Where there is inflammation, there is also usually evident oxidative stress and free radicals.

Neurological conditions in the horse are often associated with oxidative stress, which is a state where there is an imbalance between antioxidant levels and free radical levels within the body.(1)  Oxidative stress in the horse is often the same as an unhealthy horse. Many of these horses demonstrate lower levels of vitamin E and overall antioxidant levels.  Some do also demonstrate higher levels of free radicals when evaluated, while others do not.

Evaluating the horse’s blood for antioxidant and free radical levels is not routinely done, nor available, but is available in our laboratory.  When the antioxidant potential or level is evaluated, this is a reflection of TOTAL antioxidant potential, which includes both exogenous and endogenous antioxidants.  What does that mean?  The horse’s body, like ours, produces internal or endogenous antioxidants which include glutathione (GSH) and superoxide dismutase (SOD). These are natural antioxidants produced by the body to counter free radical production.  In addition, the horse takes in external or exogenous antioxidants such as vitamin E, C, and many others from the diet which add to the antioxidant reserves in the body.  The internal and external antioxidants work together to an extent, to provide protection for the horse’s health, against free radical damage and inflammation.

Now, taking this into consideration, we can evaluate a horse for vitamin E deficiency and note that the levels may be low.  Then, we can evaluate the antioxidant potential of their blood and also find a deficiency.  In most cases, when the blood’s antioxidant potential is low, this is usually reflective of deficits of internal production, more so than external supplementation.  This means that most times, if the antioxidant potential is low in the blood, it more often correlates with defective internal cellular production than it does over vitamin E deficiency.  This poses a problem when it comes to routine vitamin E supplementation in the neurological horse.

When we test a horse and determine they are vitamin E deficient, this may or may not be due to diet.  If the horse has a concurrent health condition, whether if diagnosed or not, or is undergoing high levels of stress, they simply may be encountering increased oxidative stress levels.  As these levels rise, the free radical level increase and may actually deplete antioxidant reserves, including vitamin E.  So, that low vitamin E level may be a secondary problem, not actually primary.  And if it is a secondary problem, meaning a high level of oxidative stress, more than likely vitamin E is not your horse’s only antioxidant deficit.

Antioxidants in the Horse and Recharging

Antioxidants are substances used to neutralize free radicals in the horse, and free radical when in excess are associated with cellular and tissue damage.  A free radical is an unstable molecule, usually of nitrogen or oxygen derivation, having an unpaired electron in their outer shell.  An antioxidant, whether if internal (endogenous) or external (exogenous), neutralizes or stabilizes the free radical by donating an electron.  In the act of donating that electron, the antioxidant then becomes unstable itself, really becoming a free radical, and must be recharged by another antioxidant. If it is not recharged, that original antioxidant can become a free radical and contribute to more damage and ill health for your horse.

This is not normally a problem in the average healthy horse on a good plane of nutrition, as both the internal production of antioxidants is intact and also, they are receiving an adequate supply via the diet.  However, in the compromised horse, such as in cases of EPM, EDM, or even PSSM, often due to their health status, the internal pathways are not functioning up to par.  In many, if you tested their blood antioxidant levels, you would find deficient supplies.  This indicates that the internal mechanism is not correct.  If this internal mechanism is not correct and functioning adequately, there is a good chance that some antioxidants may go without recharging, which can create a problem.

Let’s take for example a horse with EPM.  If we test the horse for antioxidant levels or potential, we may find that they are deficient, which is a general reflection of their internal and external antioxidant sources.  If we just supplement that horse with vitamin E, which is routinely done and often at very high doses, the vitamin E may help to increase the antioxidant potential in the short-term and benefit the horse, but in the long-term and potentially within a month or less, it can cause a problem.  If the vitamin E is not recharged, initially it will buffer free radicals, but then it will become a free radical itself and often in high numbers dependent on the dose used and the status of the horse.  As the vitamin E is converted to a free radical and not recharged, the free radical level in the horse will then rise, which is detected on laboratory analysis.  As this level rises, the oxidative stress level in the horse will also rise as will the level of internal inflammation.

This concept may actually explain why some horses respond initially to high dose vitamin E in cases such as EPM or otherwise, but soon begin to either plateau out in their recovery or get worse and relapse.  Instead of realizing what is going on, on a cellular and antioxidant level, often the solution is just to increase the dose of vitamin E to the horse.  This often makes things worse.

This is by far not always the case, but each horse is different and certainly, some are more prone to relapses and failure to recover than others.  This may be in part due to their ability to produce internal antioxidants.  In reality, if they were compromised in this area to begin with, due to various reasons, it may actually be the reason they developed EPM.  It is well known that most EPM cases are associated with an immuno-compromised horse on some level.  More than likely, this impaired immune response was associated directly with a reduced ability to produce internal antioxidants for protection or a failure for the diet to provide sufficient support on a nutrient and antioxidant level.

Vitamin E Dosing and Effectiveness in the Horse

Vitamin E comes in several forms offered in supplementation form for the horse.  There are also 4 different isoforms being α,β,γ,and δ.  The α-tocopherol is the most abundant form in the horse’s diet. In supplements, there are different configurations offered including a synthetic racemic blend of both the dl-α-tocopherol or the d-α-tocopherol.  When it comes to synthetics, the d-α-tocopherol is the most readily available form for the horse.  Then, there are natural vitamin E supplements which offer the form RRR α-tocopherol, which is the most ideal form to use in supplementation.  So, bottom line, the d-α-tocopherol is good, but the natural RRR form is better.  Ideally, we try to steer clear of the racemic blend of both the d and l form which is noted in the dl-α-tocopherols due to low availability by the body.

Now, when it comes to supplementation, you always need to keep in mind that there is the variable of digestive health and absorption.  So, you could provide the most optimal form of vitamin E, but yet, your horse fails to respond.  This could be due to absorption issues, or due to antioxidant levels being low in the blood as a whole, as mentioned above.  In our Cur-OST supplements for horses, we provide the d-α-tocopherol form along with several other antioxidants, including herbs which possess extremely potent antioxidant capabilities.

In the healthy horse, general recommendations are to supplement 500 iu of vitamin E per day, if the diet is not adequate for various reasons.  It has not been noted to be of any advantage to increase the dose, as increased protection is not noted.  In some human research studies, increased doses of various antioxidants, as isolated nutrients, has actually been associated with a worsening of outcomes rather than improvement.  Likely, this is due to the reasons mentioned above and increasing oxidative stress within the patient.

The NRC upper safety recommendations on supplementation of vitamin E are at 10,000 iu/500 kg or 1000 lbs.  As you will note in many cases of neurological disease, many veterinarians will utilize doses ranging from 1,500 iu of vitamin E to doses well over 15,000 iu per day.  This exceeds the recommend safe limits and potentially could lead to problems for the horse, especially if a readily absorbable form is used such as d-α-tocopherol or the natural form, RRR α-tocopherol. (2)

How to Supplement Vitamin E for Equine Health and Recovery?

That is the million dollar question for everyone as no one really knows.  In most cases, it is really difficult to determine if a horse is responding clinically to the vitamin E supplementation or not, despite checking repeat vitamin E levels.  The horse may recover clinically, which is the end goal and fantastic, but did the vitamin E play a role?  Not really sure.  Then there is the matter of the EPM, PSSM or otherwise neurological horse that fails to recover, or relapses and succumbs to the condition again.  Are they encountering increasing oxidative stress damage as a result of the vitamin E supplement?

Routine testing of the horse’s blood for free radicals and antioxidant potential is not available to the average veterinarian, outside of the research setting.  In our equine patients, often involving rehabilitation cases with refractory health conditions, testing becomes a mandatory practice.  In fact, all of our horses, even if recovering from a tendon, hoof, or joint condition receive this testing along with fecal cultures.  These two testing modalities help me tremendously, as their veterinarian, to gauge the overall status of the patient and it helps me to direct my therapy options more effectively.  This often then leads to a better outcome for the patient in a shorter period of time.

When it comes to vitamin E supplementation, to be honest, I do not use it, at least as a solitary agent in the horse’s recovery.  When I do use vitamin E, it is always in combination with other herbs which possess antioxidant properties, which is what is found in our Cur-OST equine formulas at very adequate doses.  These herbs not only serve as primary antioxidants, but also serve to trigger and support natural internal antioxidant production in the horse.  This benefits the patient on many levels.  Then, we monitor their progress via repeat antioxidant testing, more so than relying on vitamin E blood levels. Synergism between vitamin E and the herbs present in our formulas is what is often critical for recovery.

Remember the phrase pushed into our veterinary brains upon graduation….”First do no harm.”


Author:  Tom Schell, D.V.M, CVCH, CHN



  1. Mohammend, OH et al. Association of oxidative stress with motor neuron disease in the horse. Am J Vet Res. 2012 Dec;73(12):1957-62
  2. Finno, CJ, Valberg SJ. A comparative review of vitamin E and associated equine disorders. J Vet intern Med, 2012; 26:1251-1266


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