Finding the Right Amount of Vitamin E for Your Horse

Q. I’m looking for a vitamin E supplement for my horse. My vet has recommended that I give 2,500 IU per day. I have found some supplements that state the amount in international units (IU), but others state the amount of vitamin E in milligrams (mg). How can I compare?

A. The nutrients in feeds and supplements associated with the equine diet are expressed with a range of different units. For example, macrominerals such as calcium and phosphorus are typically expressed as percentages or grams while trace minerals such as copper and zinc are given in milligrams or parts per million (milligrams per kilogram). The fat-soluble vitamins A, D, and E are most commonly stated as IU. However, as you have found, they can also be given as milligrams.

These international units are a standard unit of potency as defined by the International Conference for the Unification of Formulae. The unit of potency is based on a bioassay that determines the relative strength of a substance by comparing its effect on a test organism against that of a known standard. Therefore, different forms of a vitamin can have different potencies and, thus, receive different IUs. 

One milligram of the synthetic form of vitamin E, DL-α-tocopheryl acetate, has a potency of 1 IU, while 1 milligram of the same type of vitamin E in the alcohol form (DL-α-tocopherol) provides 1.1 IU. The natural forms of vitamin E, D-α-tocopheryl acetate and D-α-tocopherol, are 1.36 IU per milligram and 1.49 IU per milligram, respectively. The natural forms of vitamin E are absorbed better and are more bioactive, earning them a greater potency and higher IUs per milligram. Therefore, you need more milligrams of the synthetic form to exert the same response as a lower amount of the natural D-α-tocopheryl.

Running these kinds of comparisons requires an ability to identify the form of vitamin E in the products being considered. This means carefully reading the ingredient lists to determine the type of vitamin E being provided. It also requires that the manufacturer clearly discloses the vitamin E sources being used rather than using vague terms such as “vitamin E supplement,” which tells you nothing of the form of tocopheryl used.

Researchers have noted increases in serum α-tocopherol when feeding the natural-source D-α-tocopheryl acetate at about one-third the amount of synthetic DL-α-tocopheryl acetate. So even if synthetic vitamin E provides more IUs, its effect on serum values might be less than a lower number of IUs of the natural form vitamin E. This is why it’s always best to give horses a natural form of supplemental vitamin E, even if the two products provide the same number of IUs per serving.

Micellized or water-soluble forms of natural vitamin E appear to be even more effective at raising serum levels. If a horse tests low in vitamin E, these forms are considered to be the most effective at rapidly raising vitamin E status.

With an ability to identify the type of vitamin E a product provides and an understanding of how milligrams convert to IUs and vice versa, you can set about making comparisons and choosing the best products for your horse’s needs.