This Is What IU Means When It Comes To Vitamins

When you're reading the label on a vitamin container, you may be confused when you see the amount of the vitamin that a capsule contains listed as IU rather than milligrams. IU — the abbreviation for International Unit — is used to describe the amounts of specific substances, like fat-soluble vitamins. An IU measurement refers to the biological effect of the vitamin, rather than the weight itself. 

Certain vitamins, hormones, vaccines, and enzymes can't easily be quantified by more standard forms of measurement. For example, MedicineNet notes that one IU of vitamin E is the specific biological activity of 0.671 milligrams of d-alpha-tocopherol. 

When a vitamin is described as containing a certain number of IUs, that measurement has been defined by the International Conference for Unification of Formulae. And all IUs aren't equal: An IU of Vitamin D will differ from an IU of Vitamin A or an IU of Vitamin E.

How many IUs do you need of vitamins?

Unfortunately, there is no single correct dosage of IUs for vitamins in general. Each vitamin will have a recommended dosage, and that dosage may vary from person to person. For instance, the recommended vitamin D dosage is 600 IUs, while the recommendation for vitamin A is 5,000 IUs (via LIVESTRONG). 

You may be surprised that certain foods list the IUs of vitamins. For example, almost all U.S. dairy milk is fortified with 400 IUs of vitamin D per serving (via the National Institutes of Health). It's important to be aware of how much of certain vitamins you're consuming: Harvard Health Publishing notes that you should avoid more than 4,000 IUs of vitamin D per day unless a doctor has recommended it. Always let your doctor know about the supplements you're taking, including the dosage. 

If your vitamin container doesn't have the measurement listed in IUs, don't worry. Some vitamins, particularly those that aren't fat-soluble, like vitamin C, are listed in milligrams.