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The difference between mg and IU

Measuring up

Q: I’m confused about some supplement recommendations I’ve read. Some are recommended in milligrams and some in IU. How do they differ and are they convertible? Thanks for your help.

Dr. Wright: IU stands for “international unit.” An IU is the amount of a particular substance required to produce a certain biological effect.

But unlike milligrams and other units of measure, the IU measurement is based on the potency of the nutrient, so 1 IU of vitamin E is not the same as 1 IU of vitamin A. And since each substance’s IU value is different, it’s impossible to have a single equation that will convert IUs into another measurement, such as milligrams.

All substances measured in international units are officially defined by the International Conference for Unification of Formulae.