What Does "IU" Mean in Vitamins?

Tip

According to Merriam-Webster, the precise international unit or IU definition is "a quantity of a biologically active substance (such as a vitamin) that produces a particular biological effect agreed upon as an international standard." Or, to put it another way, international units are a way of quantifying the biological effect you can expect from a vitamin dose.

Why Use the IU Measurement?

The vitamins you take may be available in multiple forms. For example, vitamin D exists as cholecalciferol and ergocalciferol, while vitamin A can be available as retinol or beta-carotene, and vitamin E might be sold as d-alpha-tocopherol or dl-alpha-tocopherol.

Recommended IU Vitamin Intake

Although you can convert IU measurements to units of weight, in most cases you don't need to do that conversion, which in turn would force you to be aware of which form of the vitamin you're consuming. Remember, the "IU" unit was created specifically to make standardized dosage effects easier.

So for fat-soluble vitamins, certain hormones and some enzymes, the recommended daily dosage and commercially available supplements are all usually labeled with IU. The fat-soluble vitamins you'll typically see labeled with IU, and their recommended daily intake for adults, is:

Vitamin A: 5,000 IU (upper limit of 10,000 IU)
Vitamin D: 600 IU
Vitamin E: 22.4 IU

Other vitamins, such as vitamin C, are measured in milligrams, or mg, because they don't require the additional standardization imposed by the IU unit. Vitamins that are usually present or consumed in very small quantities, such as vitamin B12, are measured in micrograms.

Converting to/From International Units

As Dr. William C. Shiel Jr. explains on MedicineNet, "The definition of an international unit (IU) is generally arbitrary, technical, and eminently forgettable." And the specific definition varies between vitamins, hormones or whatever else is being measured.

But, every once in a while, you might find yourself in a position to convert between the IU measurement and milligrams, micrograms or grams. To make those calculations, you need to know which form of the vitamin you're dealing with and the appropriate conversion factor. If you're converting from IU to weight (mcg, mg or g), you'll divide by the conversion factor. If you're converting from weight to IU, you'll multiply by the conversion factor.

Converting Vitamin A From IU

For example, if you're dealing with vitamin A as retinol, the appropriate conversion factor is 3.33. If you have 8,000 IU of vitamin A and want to know what that is in weight, you'd use a table or an online search to look up the appropriate conversion factor (3.33 IU/mcg) and the usual unit of weight for measuring vitamin A (mcg). Then you'd divide the quantity of IU by the conversion factor:

So, 8,000 IU of vitamin A as retinol is equivalent to 2,402 mcg of retinol.

But don't forget: Different forms of the same vitamin won't have identical conversion factors. If you're dealing with vitamin A as beta-carotene, the conversion factor is 1.66 IU/mcg. Once you do the math, this gives you:

So, 8,000 IU of vitamin A as beta-carotene is equivalent to 4,819 mcg of beta- carotene.

Converting Vitamin D to/From IU

Vitamin D is another fat-soluble vitamin that is almost always expressed in IU, although some manufacturers list both IU and mcg on their supplement labels. If the vitamin D is present as cholecalciferol or ergocalciferol, the conversion rate is 40 IU/mcg.

So, if you have pills that give you 2,000 IU of vitamin D, divide by the conversion rate to find the equivalent in mcg:

If you want to convert the other way, simply multiply by the conversion factor instead of dividing. For example, if you have 25 mcg of vitamin D as cholecalciferol and want to know how many IU that's equivalent to, you'd multiply:

So 25 mcg of vitamin D as cholecalciferol is equivalent to 1,000 IU.

Converting Vitamin E From IU

Read more on: vitamins