Vitamin E and Selenium Status in Horses

Vitamin E and selenium both function as antioxidants in the cell. Selenium is part of the enzyme glutathione peroxidase, which breaks down peroxides. Vitamin E is a fat-soluble vitamin that inserts itself into the cell membrane and neutralizes free radicals. Both selenium and vitamin E can offset a deficiency of the other, as long as both are not deficient.

Selenium deficiency or selenium-responsive conditions have been associated with several problems in horses, including white muscle disease, sporadic tying-up, retained placenta,
reduced fertility, masseter (jaw) muscle myopathy, and heart failure or difficulty swallowing in foals.

The best sample to assess selenium adequacy is whole blood. Because erythrocytes cannot produce more glutathione peroxidase, whole blood selenium concentration reflects long-
term selenium status over the lifespan of the sampled red blood cells (120 to 150 days). It is recommended to use royal blue top (trace mineral) tubes when measuring blood selenium concentration. Horses require 0.1 ppm of selenium per kg of dry matter intake (mg/kg); this equates to approximately 1 mg of selenium per day for a 1,000-lb (454-kg) horse.

Serum is the most commonly used sample for measuring vitamin E. The vitamin E requirement is approximately 1 IU/kg body weight/day, or 500 IU/day for an 1,100-lb (500-kg) horse. The source of vitamin E is very important, especially when treating horses for equine motor neuron disease, equine degenerative myelopathy, or other vitamin E responsive conditions. Natural (d-α-tocopherol), water-soluble sources of vitamin E are better absorbed and preferentially used by the horse. Synthetic vitamin E sources (d,l-α-tocopherol) contain a mixture of eight chemically different sources of vitamin E, only one of which is d-α-tocopherol. Studies have shown that natural vitamin E is more bioavailable than synthetic vitamin E and significantly raises both serum and cerebrospinal fluid concentrations of α-tocopherol compared to synthetic vitamin E.

Vitamin E deficient myopathy is a newly recognized cause of muscle loss and weakness. It is unknown if this is a unique problem or an early, reversible stage of equine motor neuron
disease. Affected horses have low serum (<2 µg/ml) and muscle concentrations of vitamin E. Supplementation with 5,000 IU natural vitamin E/day for six weeks has been shown to restore serum and muscle vitamin E concentrations to normal and most horses make a gradual return to normal exercise.