Finding the Right Amount of Vitamin E for Your Horse

An equine nutritionist explains the difference between international units (IU) and milligrams (mg) and how to make sure your horse is getting enough vitamin E.
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Finding the Right Amount of Vitamin E for Your Horse
The nutrients in feeds and supplements associated with the equine diet are expressed with a range of different units. | Photo: The Horse Staff

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

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Clair Thunes, PhD, is an equine nutritionist who owns Clarity Equine Nutrition, based in Gilbert, Arizona. She works as a consultant with owners/trainers and veterinarians across the United States and globally to take the guesswork out of feeding horses and provides services to select companies. As a nutritionist she works with all equids, from WEG competitors to Miniature donkeys and everything in between. Born in England, she earned her undergraduate degree at Edinburgh University, in Scotland, and her master’s and doctorate in nutrition at the University of California, Davis. Growing up, she competed in a wide array of disciplines and was an active member of the U.K. Pony Club. Today, she serves as the district commissioner for the Salt River Pony Club.

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