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).

An IU of vitamin A has the biological equivalent to 0.3 micrograms (1/1000th of a milligram) of retinol or 0.6 micrograms of beta-carotene. Vitamin D from cholecalciferol and ergocalciferol has the same biological activity, so in each case 0.025 micrograms of either is 1 IU.

If you want to know how many milligrams of a certain form of a vitamin a feed contains, look at the feed tag’s ingredient list and guaranteed analysis. For example, when looking at vitamin E, if the ingredient list shows dl-alpha tocopherol and guarantees 250 IU of vitamin E, you would multiply the number of IU by 0.9. So in the case of 250 IU, this would be 225 milligrams of dl-alpha tocopherol.

Conversely if a product gives you the amount of vitamin E in milligrams and is using dl-alpha tocopherol and you want to compare it to other products that state IUs, you would multiply the number of milligrams by 1.1. Therefore, a product with 50 milligrams of vitamin E per serving from dl-alpha tocopherol would provide 55 IU. Now you can compare this to other products that state IU regardless of source, because if they have the same number of IUs they should have the same biological activity.

On first glance the IU system can seem arbitrary, but it allows for useful comparisons across similar compounds.