LETTER TO THE EDITOR – FEBRUARY 1999

I was reading some of the articles and I've forgotten what IU stands for. Is this International Units and if so, what has IU's got to do with a vitamin being so many milligrams?

Wanda B., ON, Canada

Editor: Yes, IU stands for International Units. IUs are used to standardize the expression of potency when there is more than one form of the vitamin. For example, natural vitamin-E comes in two forms, d-alpha-tocopherol and d-alpha-tocopherol acetate. 100 mg of the d-alpha-tocopherol corresponds to 149 IU and 100 mg of d-alpha-tocopherol acetate corresponds to 136 IU. The potency of vitamin-D is also usually given in IUs; here 400 IU corresponds to 10 micrograms. The potency of most other vitamins and supplements is given in milligrams or micrograms except for vitamin-A and beta-carotene which are usually given in IUs.