Horse Vitamins: What’s an IU?

International units are used to quantify similar biologically active substances such as vitamins and hormones. Our equine nutritionist explains.
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Horse Vitamins: What
International units (IU) allow for comparison of different preparations of substances with the same biological effect. | Photo: Alexandra Beckstett/The Horse

Q. I notice on feed and supplement labels that vitamins are typically quantified in “IUs.” What is an IU, and why is it used for vitamins?

A. IU stands for international units and is a metric system of measure. An IU is used to quantify similar biologically active substances such as vitamins and hormones. They allow for comparison of different preparations of substances with the same biological effect. The exact measurement of one IU varies depending on the compound in question, meaning an IU of vitamin A is not the same as an IU of vitamin D. Essentially, an international agreement exists as to how much of a given compound constitutes an IU.

For example, vitamin E comes in several forms, including alpha tocopherol and dl-alpha tocopherol. These two forms of vitamin E have similar biological activity, but slightly different amounts are needed to have the same biological effect. One IU of vitamin E is the biological equivalent of 0.67 milligrams of alpha tocopherol or 0.9 milligrams of dl-alpha tocopherol. This means that to get the same biological activity you will need more vitamin E if you are using the dl-alpha tocopherol compared to the alpha tocopherol form. Note that this is why we say natural vitamin E (alpha tocopherol) is more “bioavailable” than synthetic vitamin E (dl-alpha tocopherol)

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Written by:

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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