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)