Introducing vitamin E
The term ‘vitamin’ comes from ‘vital amine’, after a group of unrelated compounds that were essential for good health and growth were discovered in the early 20th century (Gropper & Smith, 2013). Vitamins are essential in tiny amounts in the diet, yet a lack of any causes deficiency disease. Vitamin E is no exception – it cannot be made in the body, so is considered essential in the diet (Zeyner & Harris, 2013).
Vitamin E was discovered in the early 1920s by two scientists, H. Evans and K. Bishop, who found that rats could not reproduce on a diet of rancid lard, which is deficient in vitamin E. Giving the rats wheatgerm oil solved the problem, and vitamin E was later extracted from wheatgerm oil and given the term ‘tocopherol’ which comes from the Greek meaning ‘to bear or bring forth’ (Gropper & Smith, 2013).
Vitamin E actually describes several different compounds that are synthesised by plants. Lush green pasture is a rich source of vitamin E, as are the germs of grains and oils pressed from the germ (Kane, 2009). Unfortified dry horse diets contain little or no vitamin E and must be supplemented.
Vitamins tend to be described as the activity level of the compound, rather than the actual amount. Out of eight different types of vitamin E (including four tocopherols and four tocotrienols), only α tocopherol – in natural or synthetic forms – can meet the body’s vitamin E requirement (Gropper & Smith, 2013).
Naturally occurring tocopherols exist as the RRR stereoisomer (RRR-α-tocopherol, also called d-α-tocopherol) and are the most active out of all the vitamin E compounds. A stabilised form, RRR-α-tocopherol acetate, is used in horse supplements and feed. The synthetic form, commonly used in horse feed and supplements is called all-rac α-tocopherol (there is also a form called dl-α-tocopherol), and contains a mixture of eight isomers (slightly different structures), only one of which is RRR-α-tocopherol, and therefore is not as active as the naturally occurring form. Stabilised synthetic forms include all-rac-α-tocopherol acetate and all-rac-α-tocopherol succinate (Kane, 2009).
Synthetic all-rac-α-tocopherol acetate contains 1 IU/mg activity, whereas the natural-source d-α-tocopherol acetate contains 1.36 IU/mg activity. The natural source, therefore, has 36% greater biological activity than the synthetic source (Kane, 2009).
Vitamin E is a fat-soluble vitamin which is absorbed in the small intestine and is reliant on bile salts for breakdown. It is transported in the body and taken up into cells in association with fat carrier molecules, and is stored in body fat, liver, muscle and other body tissues, as well as being found in the blood plasma (Gropper & Smith, 2013).
Functions of vitamin E in the horse’s body
Like most vitamins, vitamin E has a variety of roles in the body, but its main one is as an antioxidant. Antioxidants protect the body from oxidative damage by ‘mopping up’ free radicals. Free radicals are reactive substances that are produced naturally in the everyday metabolic function of the body, and they can cause damage to body tissues, especially cell membranes, protein and the genetic material DNA. Damage is caused in a chain reaction, and only antioxidants substances can break this chain, by neutralising the free radical. If the chain is not broken and the free radicals overwhelm the body’s antioxidant capabilities, disease – especially degenerative disease – will result (Kane, 2009). Muscle cells are particularly active, use a lot of oxygen and are at a high risk of oxidative stress (Gropper & Smith, 2013).
Antioxidants, including vitamins and enzymes containing microminerals, break the change of damage by scavenging the free radicals and deactivating them. In this way they have an important role to play in protecting cell structure including membranes and the genetic material inside.
Within the body, vitamin E functions in close association with a substance called glutathione peroxidase, which contains selenium. The actions of these two nutrients are linked, and higher levels of one can reduce the effects of lower levels of the other. Vitamin C can regenerate vitamin E following oxidation, so it is another nutrient that affects vitamin E requirements and status (Gropper & Smith, 2013).
Immune system development and function depends on vitamin E; the high polyunsaturated acid content of immune cells means they are at high risk of oxidative damage (Finno & Valberg, 2012). Researchers have shown that passive transfer of immunity from mare to foal is better in mares with a high vitamin E status (Kane, 2009 cites Hoffman et al, 1999). 160 IU/kg DM daily – double the normal recommended amount – is recommended for mares during the final three months of gestation (Zeyner & Harris, 2013).
Vitamin E is also important for the function of the gonads (the testes and ovaries) so it is vital for reproduction and has been referred to as an ‘anti-sterile vitamin’ (Zeyner & Harris, 2013). In a non-oxidative role, vitamin E is involved in genetic function (Finno & Valberg, 2012). Vitamin E is also involved in the function of nervous tissue, and a deficiency results in neuromuscular dysfunction (Finno & Valberg, 2012).
The NRC (2007) recommends 50 IU vitamin E per kg dry matter (DM) intake – about 500 IU for a 500 kg horse eating 10 kg DM daily, for maintenance. For growing horses, lactating mares and exercising horses, they recommend 80 IU/kg DM (based on 2.5% BW DM intake daily). For broodmares and stallions, 50 IU/kg DM. These levels are the minimum to avoid deficiency symptoms, and not necessary enough for optimal health and performance. Zeyner & Harris (2013) recommend twice the NRC requirement for hard working horses, those on high-fat diets, and an extra 2115 mg per day for those with recurrent myopathy.
Nutritionists tend to recommend around 3-4000 IU vitamin E for hard working horses and those with recurrent myopathy (tying up), and an extra 2-3000 IU for pregnant mares during their final 3 months of gestation, and for sick or healing horses.
Deficiencies and toxicity
A shortage (deficiency) of vitamin E causes muscle and heart dysfunction and weakness, and degenerative neurological problems including ataxia and coordination loss (Gropper & Smith, 2013). In young horses, white muscle disease is a syndrome of selenium and vitamin E deficiency, which is treated with supplementation of both essential nutrients (NRC, 2007). Deficiency of vitamin E is implicated in both equine degenerative myeloencephalopathy (EDM) and equine motor neurone disease (EMND), although other factors are also involved (Finno & Valberg, 2012; NRC, 2007). Horses suffering from equine rhabdomyolysis are often given supplementary vitamin E (usually along with selenium) to boost antioxidant status, but there is no good evidence that deficiencies of either are involved in the pathophysiology (Zeyner & Harris, 2013; NRC, 2007).The muscle damage associated with rhabdomyolysis probably does increase requirements, however.
Plasma levels of vitamin E can be used to evaluate intake (Gropper & Smith, 2013; NRC 2007). Vitamin E deficiency is relatively rare, but marginal intakes that could limit optimal health and performance are quite likely.
Vitamin E is the least toxic fat-soluble vitamin and slightly over-supplementing is preferable to under-supplementation, especially for performance horses, breeding horses and ill horses. An upper tolerable level of vitamin E for horses has been set at 1000 IU/kg DM (NRC, 2007). Intakes over this level have been associated with poor bone mineralisation and problems with blood coagulation in other species (NRC, 2007).
Sources of vitamin E in the diet
Fresh green plant material is the best source of natural vitamin E for the horse, and so horses grazing growing pasture will ingest good levels (around 45-400 IU/kg DM (Kane, 2009)). Preserving forage in the hay or haylage-making process causes it to lose vitamin E, so horses on mostly preserved forage diets must be supplemented. Storage of forage also causes loss of the vitamin; as much as 50% in one month (Finno & Valberg, 2012 cite McDowell et al, 1989). A high variability in the amount in preserved forages exists, with some dehydrated green forage harvested at a young stage being relatively high in vitamin E, whereas in a late-cut hay, very low or negligible (Zeyner & Harris, 2013).
High levels of vitamin E are found in wheatgerm , and very high levels in the oil (around 1330 IU/kg (Kane, 2009)). Correctly stored vegetable oils are also rich in vitamin E (Zeyner & Harris, 2013).
Vitamin and mineral-fortified compound feeds usually contain vitamin E, as does a wide range of supplements. Many vitamin E and selenium combination supplements are available, as well as straight vitamin E, and all in both synthetic and natural-source forms.
Feeding vitamin E
All horses not grazing lush green grass 24/7 need to have vitamin E supplemented in the diet. Horses and ponies who need to have grass intake restricted including good doers, laminitics, those with equine metabolic syndrome (EMS) or insulin resistance (IR) – and especially those fed soaked hay and/or straw – are at particular risk of suboptimal vitamin E intakes since they are usually not fed the full recommended amount of a fortified compound feed.
There is evidence that higher levels than those recommended by the NRC (2009) are beneficial for hard-working horses (Rey et al, 2013; Kane, 2009), and rather than 80 IU/kg dry matter intake, a total of 2-4000 IU daily are recommended (Kane, 2009). Due to the higher activity of the natural form, it could be fed at the lower end of the recommendations for a similar effect.
Race horses fed an extra 1400 IU of natural vitamin E (d-α-tocopherol) over their regular diets had a higher plasma vitamin E concentration, and were able to maintain their antioxidant defences, unlike those in the control group (no extra vitamin E supplementation) (Rey et al, 2013).
Vitamin E requirements of most species rise when intake of polyunsaturated fatty acid intake rises (Gropper & Smith, 2013).Therefore, extra vitamin E is sometimes recommended when substantial amounts of vegetable oil is fed to horse. The higher than recommended levels usually found in the diet of exercising horses, along with the high levels of vitamin E usually found in such vegetable oils usually mitigates this need (NRC, 2007). Zeyner & Harris (2013) have proposed that 1-1.5 mg of extra vitamin E is fed per 1 ml of vegetable oil fed, which equates to around 3-400 mg daily.
In conclusion, horses who should be fed extra vitamin E – over the NRC published requirements – include:
- Hard-working horses
- Horses who suffer from rhabdomyolysis (tying up)
- Horses fed supplementary vegetable oil
- Breeding mares and stallions
- Mares during the final 3 months of gestation
- Sick horses and those undergoing healing (including laminitics)
- Horses with a compromised immune system
Vitamin E is an essential micro-nutrient that cannot be synthesised by the horse so has to be supplied in the diet. It is the primary antioxidant that protects cell membranes and genetic material (Kane, 2009) and has roles in immune, nervous and reproductive funcion. Horses fed limited green forage are likely to be deficient in vitamin E unless it is supplemented. In certain circumstances – including horses in hard work, those who tie up, laminitics, mares in their final trimester, horses fed high levels of dietary fat, sick and immunocompromised horses – higher than normal levels of vitamin E are recommended.
Duberstein, K. J. et al, (2009) Effects of vitamin E supplementation and training on oxidative stress parameters measured in exercising horses. Comparative Exercise Physiology, 6 (1), 17-25.
Finno, C. J. & Valberg, S. J. (2012) A comparative review of vitamin E and associated equine disorders. Journal of Veterinary Internal Medicine, 26(6): 1251-1266.
Gropper, S. S. & Smith, J. L. (2013) Advanced Nutrition and Human Metabolism. 6th edition, Wadsworth, Australia.
Kane, E. (2009) Vitamin E: An essential nutrient for horses? In Pagan, J. D. (Ed), Advances in Equine Nutrition IV, Context, Leicestershire.
NRC (2007) Nutrient Requirements of Horses. 6th ed., The National Academies Press, Washington DC.
Rey, A. L. et al, (2013) Short- and long-term effect of oral administration of micellized natural vitamin E (D-α-tocopherol) on oxidative stress in race horses under intense training. Journal of Animal Science, 91, 1277-1284.
Zeyner, A. & Harris, P. (2013) Vitamins. In: Geor, R. J et al, Equine Applied and Clinical Nutrition, Saunders Elsevier, Edinburgh.