Vitamin E is an essential nutrient for horses beneficial in combating the multitude of effects of free radical production that can damage membranes and components of cells.
Vitamin E is an essential nutrient for horses beneficial in combating the multitude of effects of free radical production that can damage membranes and components of cells.
As such, vitamin E is beneficial to young, rapidly growing foals, pregnant mares, stallions, and, especially, equine athletes with no access to lush pasture.
Free radicals are unstable atoms with unpaired numbers of electrons that are formed when oxygen interacts with other molecules in all cells.
Once formed, these reactive radicals can initiate chain reactions resulting in a cascading negative effect on many other molecules within cells and cell walls resulting in oxidative stress within the animal. Free radicals are commonly produced as part of normal cell metabolism, but also can become excessive following injury or disease. Uncontrolled, free radicals can cause considerable irreparable damage to cells. They can alter the structure of cell membranes, and create havoc to polyunsaturated fatty acids (PUFA), and proteins and DNA within cells.
Molecular structure of Vitamin E
The more active the cell, the greater the potential risk of cellular damage. Excessive free radical production or oxidative stress results when the formation of free radicals overwhelms the body's ability to break the chain reactions that occur when an imbalance between production and removal of free radicals occurs. Uncontrolled oxidative stress can overpower the horse's ability to fight back and may result in tissue damage, thus possibly impairing its life.
In several species, including humans, this damage has recently been linked to degenerative diseases such as rheumatoid arthritis, cancer, cardiovascular disease, inflammatory bowel disease, renal disease, Parkinson's disease, cataracts and may have a deleterious affect on the immune system (NRC Dietary Reference Intakes for Vitamin C, Vitamin E, Selenium and Carotenoids, 2000).
Antioxidants are the horse's major defense system against the scourge of free radicals and oxidative stress keeping their damage to a minimum. Enzymatic antioxidants are synthesized in the body to neutralize free radical production. Key enzymatic antioxidants include superoxide dismutase, glutathione peroxidase, and catalase. Other major sources of antioxidants available to the horse are non-enzymatic or nutritional antioxidants. Non-enzymatic antioxidants, like vitamin E, carotenoids and vitamin C, scavenge and convert free radicals to relatively stable compounds and stop the chain-reaction of free radical damage. Therefore, all antioxidants are critically important to protect horses from tissue damage, disease, and may during these processes, enhance immunity. For the horse, the critical phases of reproduction in mares and stallions, growth of foals, and exercise of equine athletes, are all especially important. Thus, for the horse, vitamin E appears to be the most important dietary fat-soluble non-enzymatic antioxidant to assist in combating free radical production and propagation in the horse. Since the horse can synthesize vitamin C, vitamin E and possibly carotenoids, appear to be the only major antioxidant vitamins needed from dietary sources.
Figure 1: Seasonal variation in vitamin e status of horses in western canada
Vitamin E is unique among vitamins in that it is not required for a specific metabolic function. As alpha-tocopherol, vitamin E's major function appears to be the body's major fat-soluble antioxidant. Thus, vitamin E is notably essential for the proper function of the reproductive, muscular, nervous, circulatory and immune systems.
Since selenium is in glutathione peroxidase, an enzymatic antioxidant, it is often difficult to distinguish between the signs of vitamin E and selenium deficiencies. Signs representing possible deficiencies of both nutrients have been described in the foal and in adult horses (NRC, 1989, Schougaard et al., 1972; Wilson et al., 1976). Myodegeneration was common, with pale diffuse or linear areas in skeletal and cardiac muscle. Histological examination revealed hyaline and granular degeneration, as well as swelling and fragmentation of muscle fibers from several sites, including the tongue. The latter defect may interfere with normal nursing. Subcutaneous and intramuscular edema, pulmonary congestion and occasionally, steatitis were also observed.
Liu et al. (1983) reported a degenerative myelopathy in six Przewalski horses up to 14 years of age. They had been fed commercially prepared horse pellets, timothy hay, and fresh grass in the summer. The horse pellets contained 22 IU of vitamin E and 0.3 mg of selenium/kg. Plasma alpha-tocopherol concentrations were low and ranged from less than 0.3 to 0.8 Âµg/ml. Ataxia was evident in all, including uncoordinated movement of the hind limbs, and an abnormally wide-based gait and stance. No gross lesions were observed in the brain, vertebrae, or spinal cord, but histologic examination revealed degeneration of the neural processes in the ventral and lateral funiculi. Myelin sheaths were dilated and vacuolated, and a number of axons were swollen, fragmented or lysed. Neuronal degeneration, phagocytosis, and accumulation of periodic acid-Schiff-positive, xylol-insoluble lipopigment occurred in the affected neurons of the dorsal root ganglia.
Equine degenerative myeloencephalopathy (EDM) is a diffuse degenerative disease of the brain and spinal cord, a form of 'wobbler' syndrome. Young horses affected by EDM first show signs of incoordination and clumsiness, which deteriorates over time to outright ataxia. These horses also showed low vitamin E levels in blood.
Figure 2: Horse response to vitamin e source and levels
Mayhew et al (1987) showed that supplementing stallions with 1500 IU vitamin E per day decreased the incidence of EDM in their foals from 40 percent to 10 percent. He described a vitamin E-responsive degenerative myeloencephalopathy in Standardbred and Paso Fino horses, from 3 to 30 months of age. Symmetric ataxia and paresis, along with laryngeal adductor, cervicofacial, local cervical and cutaneous trunci hyporeflexia, were characteristic. No clinical signs were observed before 3 months of age. The onset of gait abnormality was usually abrupt. Clinical signs then remained static or progressed for weeks or months. Severely affected animals often fell while running. The mean (Â± SD) serum alpha-tocopherol concentration of 13 ataxic weanlings was 0.62 Â± 0.13 (range 0.47 to 0.84) Âµg/ml. A number of non-ataxic weanlings had similar values, although vitamin E supplementation markedly reduced the incidence of the syndrome on affected farms. Based on genetic studies, this disorder appeared to have a familial disposition.
Blythe and Craig (1993) found that when young foals showing signs of incoordination were supplemented with 6000 IU vitamin E per day appeared normal by 2 years of age.
Subclinical vitamin E deficiencies are difficult to recognize in the horse. Symptoms such as an impaired immune system or difficulty of reproduction may often go undiagnosed and may be attributed to other causes.
Low blood levels of alpha-tocopherol appear to be the first indication of reduced vitamin E status in horses. In order to determine vitamin E adequacy in horses, serum or plasma alpha-tocopherol levels can be measured using HPLC procedures.
For horses, vitamin E (alpha-tocopherol) is higher in green growing pastures (45 to 400 IU/kg D.M.), particularly in alfalfa (McDowell, 1989), and diminishes with maturation. Harvesting and length of storage diminishes the quantity of vitamin E about 10-fold below levels in fresh forages (Schingoethe et al. 1978). Vitamin E is abundant in the germ of grains and oils pressed from the germ (wheat germ oil, 1330 IU/kg). Vegetable oils, such as corn and soybean oil are relatively higher in vitamin E (50-300 IU/kg), but may also contain high levels on gamma- and delta-tocopherol, that have little or no vitamin E activity. Due to such variability in vitamin E content in processed feedstuffs, nutritionists typically do not rely on the diet to provide any vitamin E activity and depend totally upon supplementation to meet the horse's requirement. Though horses grazing lush pasture may obtain sufficient vitamin E, it is common practice to supplement horse feeds with vitamin E, especially important for those animals in confinement and consuming stored roughages during winter months.
Figure 3: Effectiveness of water-soluble tocopherol in late gestating mares
The determination of dietary requirements of vitamin E for various classes of horses comes from several research studies and mostly by action of members of the NRC committees that published the requirements.
In one of the first vitamin E studies in horses, Stowe, 1968, found that foals deficient in vitamin E required 27 Âµg of parenteral (intramuscular) or 233 Âµg of oral alpha-tocopherol/kg of body weight/day to maintain erythrocyte stability.
Roneus et al (1986) suggested that vitamin E supplements be provided daily and concluded that to ensure nutritional adequacy, adult Standardbred horses fed a low-vitamin E diet should receive a daily, oral supplement of 600 to 1,800 mg of d,l-alpha-tocopheryl acetate, equivalent to 1.5 to 4.4 mg/kg of bodyweight.
Maenpaa et al. (1988) observed seasonal differences in serum alpha-tocopherol concentrations in mares and foals kept on pasture from early June until early October, then fed timothy hay and oats during the winter. Serum alpha-tocopherol concentrations were highest in August and September (about 2.7 Âµg/ml for mares, and 2.1 Âµg/ml for foals) and reached their lowest values in April or May (about 1.5 Âµg/ml for mares, and 1.2 Âµg/ml for foals). When mares were given winter supplements of 100 to 400 mg of vitamin E/day, no significant increases in serum alpha-tocopherol concentrations occurred and seasonal differences persisted. However, when foals were given supplements of 400 mg of vitamin E/day in the winter, serum alpha-tocopherol concentrations increased significantly during late winter and early spring to approximately 2 Âµg/ml in April and May from low values of about 1.3 Âµg/ml in December. Blakley and Bell (1994) observed similar seasonal variation in plasma vitamin E (Figure 1, p. 10E).
Janssen and Ullrey (unpublished research, 1986) observed that the capture and restraint of zebra and Przewalski horses for hoof trimming resulted in temporary to persistent muscle soreness and lameness when diets provided approximately 50 IU of vitamin E/kg. Plasma alpha-tocopherol concentrations were commonly below 1 Âµg/ml. When dietary vitamin E concentration was increased to approximately 100 IU/kg of dry matter, no clinical signs of muscle pathology were seen, and plasma alpha-tocopherol concentrations increased to 1.5 to 3 Âµg/ml.
Butler and Blackmore (1983) reported that the mean (Â± SD, the standard deviation) plasma or serum alpha-tocopherol concentration for 140 samples from stabled Thoroughbreds in training was 3.3 Â± 1.29 Âµg/ml. A dietary concentration of 100 IU of vitamin E/kg of dry matter provides the equivalent of about 2 mg of d,l-alpha-tocopherol acetate/kg of body weight/day.
Figure 4: Vitamin E levels in horses fed an antioxidant supplement (myo-guard) containing natural vitamin E
The National Research Council's requirement for vitamin E is 50 IU per kg of dry ration or 1 IU per kilogram body weight for maintenance (NRC, 1989). Young, growing horses, pregnant and lactating mares, and performance horses should be fed 80-100 IU per kilogram of dietary dry matter. Based on most studies 1000-2000 IU per horse per day is the general recommendation.
Not only preventing deficiency, but also optimizing intake to prevent free radical damage and enhance immunity confounds the requirement. The amount of vitamin E required for optimum immune function has not been thoroughly studied in horses. Baalsrud and Overnes (1986) supplemented a basal diet daily with 600 IU of vitamin E, 5 mg of selenium, or both. They observed an improved humoral immune response to tetanus toxoid or equine influenza virus in adult horses supplemented with selenium alone or selenium plus vitamin E. No improvement was seen with vitamin E alone, although an insufficient amount of supplement may have been used. Vitamin E requirements may exceed 50 IU/kg of dry diet or 1 IU/kg of body weight. More data are needed; therefore it may be prudent to ensure that diets contain 80 to 100 IU of vitamin E/kg of dietary dry matter in the total diet for foals, pregnant and lactating mares and working horses.
As horses naturally obtain vitamin E from pasture, those without access to this natural source need to obtain the most efficient intake of the vitamin. In nature, most plants and oils naturally contain a mixture of tocopherols (alpha-, beta-, gamma- and delta-) that show variable vitamin E activities. Of the four tocopherols, alpha-tocopherol has the highest biological potency. In fact alpha is the only tocopherol to be recognized to have vitamin E activity (Traber, 1999).
The two commercial sources of vitamin E are natural-source (d- or RRR- alpha tocopherol) and synthetic-source (d,l- or all-rac alpha tocopherol). Usually synthetic vitamin sources are, for the most part, equal in efficacy and structure to the natural source of that vitamin. Not so for vitamin E. The source of vitamin E with the highest biological activity is natural vitamin E (d-alpha-tocopherol) isolated from seed oils (Figure 4, p. 14E). Synthetic vitamin E (dl-alpha-tocopherol) is made from petrochemicals. The difference between natural and synthetic is in the chemical structures. Natural vitamin E is a single entity, one isomer, the RRR isomer. Study results (Ingold, et al, 1987) have shown preferential uptake and transport of the natural isomer. Synthetic vitamin E, on the other hand, consists of eight stereoisomers, with only one isomer identical to the natural RRR isomer. The body preferentially transports and incorporates the natural isomer, while synthetic vitamin E is not as biologically potent.
Once fed, horses need to be able to efficiently absorb fat-soluble nutrients. Since natural and synthetic vitamin E are antioxidants, in order for them to be included into rations, and not lose their potency, esters are added to provide stability. In order for supplemental vitamin E-ester to be used, two steps are necessary. The ester has to be removed and the alpha-tocopherol has to be made water-soluble by the action of bile salts (Gallo-Torres, 1980).
Research has shown that water-soluble, unesterifed alpha-tocopherol is better used than acetate-esters. Of vitamin E products tested in horses, the most efficient one was a water-soluble source of d-alpha-tocopherol (ElevateÂ®, Kentucky Performance Products, Versailles KY). The manufacturing process creates tiny microscopic water-soluble droplets (optimally 0.1 to 0.4 microns). This process dramatically enhances the ability of horses to absorb vitamin E. By being water soluble, absorption is greatly enhanced. It does not require the acetate-ester to be hydrolyzed prior to absorption and secondly there is no need for micellization by bile salts. Horses fed water-soluble natural vitamin E had higher plasma vitamin E levels than horses fed either natural or synthetic vitamin E acetates at levels of 500 IU to 8000 IU per horse per day (Figure 2, p. 12E) (Kane et al, unpublished).
It is beneficial to provide supplemental vitamin E to breeding horses. Green pasture forages, high in vitamin E, are not always available to gestating and lactating mares that often foal during winter months. It is advantageous to feed mares in late pregnancy and early lactation higher than normal levels of vitamin E, (i.e. > 150 IU per kilogram feed). It has been suggested (Harper, 2002) that mares known to have poor quality colostrum, be poor milkers, or have had foals that had failure of passive transfer of immunity in previous years, should be supplemented with vitamin E at twice the required level for at least a month before and after foaling. It is also recommended that pregnant mares fed lower quality hay or that graze endophyte- infected fescue should also be given vitamin E supplementation a month before and after foaling. Gard and Balinge (2003) showed that mares supplemented with water-soluble natural vitamin E (1500 IU d-alpha-tocopherol per day) for 21 days before foaling showed higher plasma alpha-tocopherol at foaling, as did their foals at 12-36 hours after birth (Figure 3, p. 12E). The higher serum tocopherol levels in foals of supplemented mares could have been due to greater colostrum transfer.
Mares supplemented with vitamin E have shown increased passive transfer of antibodies to foals. This greatly enhances the immune system (Hoffman et al, 1999). This study showed an advantage of feeding vitamin E to late-pregnant and early lactating mares. Serum and colostrum IgG levels were greater in mares supplemented with 160 IU of vitamin E per kilogram of feed (along with a mixed grass hay and grain ration), than those receiving 80 IU vitamin E per kg diet. The foals from all mares had similar levels of IgG, IgA, and IgM at birth. After nursing, foals from mares fed the higher level of vitamin E, had higher serum levels of IgG and IgA, which were reflected in their dam's colostrum.
Decreased vitamin E status is implicated in the etiology of exercise-induced muscle damage in horses, i.e. "tying up" (Snow & Valberg, 1994). Siciliano et al, 1997 showed that supplemental synthetic vitamin E levels at 80 IU and 300 IU per kg dry matter were necessary to maintain blood and skeletal muscle concentrations in horses undergoing exercise conditioning. Serum vitamin E levels did not affect the integrity of skeletal muscle following repeated submaximal exercise, as measured by changes in creatine kinase (CK) and aspartate amino transferase (AST) activities, nor did vitamin E level affect thiobarbituric acid reactive substances (TBARS).
Exercising horses supplemented daily for six weeks with natural vitamin E acetate (2000 mg/kgBW) had higher plasma alpha-tocopherol levels pre- and post-exercise compared to horses receiving an equal quantity of synthetic vitamin E acetate. The horses were fed a diet of grass/alfalfa hay and sweet feed plus a vitamin-mineral supplement devoid of supplemental vitamin E. (Pagan et al, 2002a, unpublished).
In a study looking at exercise-induced muscle damage, exercising horses fed an antioxidant supplement (Myo-GuardÂ®, Kentucky Performance Products, Versailles KY) containing natural vitamin E at various levels showed significantly higher plasma alpha-tocopherol levels pre- and post-exercise compared to unsupplemented horses. (Figure 4, p. 14E, Pagan et al 2002b, unpublished).
Vitamin E is the primary lipid-soluble antioxidant that protects the cell membranes of many systems of the horse's body. Vitamin E is not a "typical" vitamin because the deficiencies do not always produce overt symptoms like other vitamin deficiencies. The need to supplement horses with vitamin E is dependent on whether they graze lush pastures or kept in confinement and fed diets low in vitamin E. The requirements appear to be higher when enhanced immunity is expected, and greater in newborn foals, breeding animals, and performance horses. Not all forms and sources of vitamin E are equally used by horses. Natural vitamin E appears to be better used than synthetic. New areas of research have shown that water-soluble natural vitamin E (ElevateÂ®) is the most effective method to increase vitamin E status in horses. For adult horses, water-soluble natural vitamin E should be administered from 1000 to 4000 IU (2-8 ml) daily top-dressed onto feed. An increased amount should be given if a horse is actively exercised or worked. For orphaned foals 500 IU (1 ml) in milk replacer is recommended.