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