Medically Reviewed

IU vs mg for Vitamins: What's the Difference? (And Why it Matters for You)

Stack of vitamins

The FDA recently phased out the use of IU on supplement facts labels. Let’s dive into the history and use of the IU and review the label changes.

What does IU mean?

You may have seen IU as the unit of measure for a dose of a medication or supplement. But what does IU actually mean and why is it used instead of milligrams or other units of measurement?

IU stands for International Units. IU may be used for medications, vitamins, hormones, and enzymes. They measure the biological effects of a substance, so 1 IU of a substance has a certain expected effect on the human body.

Why use the IU measurement?

IU measures the effect of a substance, instead of using other measurements like mass (milligrams, micrograms, etc.) or volume (milliliters, etc.). This is useful because it allows for comparison between different forms of the same substance.

But you can’t compare apples to oranges when it comes to IU. This means that you can’t compare 1 IU of vitamin E to 1 IU of vitamin A.

Is IU the same as mg or mcg?

IU is not the same as mg or mcg. As discussed, IU measures the biological effect of a substance while milligrams (mg) and micrograms (mcg) measure the mass of a substance.

The benefit of IU is that it provides an international standardization for substances. But you can’t use IU to compare different substances. On the other hand, mg or mcg can be used to compare the mass of one substance to another.

1,000 IU of vitamin E equals 670 mg of d-alpha-tocopherol (natural vitamin E)

Is 1,000 IU the same as 1,000mg?

When measuring substances, 1,000 IU is not the same as 1,000 mg. For example, 1,000 IU vitamin D equals 0.025 mg. But 1,000 IU of vitamin E equals 670 mg of natural vitamin E.

Intrigued by IU? Grab your multivitamin bottle and let’s get nerdy!

What does IU stand for in vitamins

IU provides an international standard for the dosing of vitamins. IU was historically used to measure vitamin A, vitamin D, and vitamin E.

However, the United States Food and Drug Administration (FDA) required that supplement facts labels change IU to metric units in 2019-2020. This is why you will now see micrograms (mcg) or milligrams (mg) where there was once only IU. At this time, many supplement facts labels continue to also list the IU to help with the transition.

Types of vitamins measured in IU

If you look at a vitamin supplement facts label in the United States, you will see the amount of each vitamin per serving and the percent daily value that the vitamin provides. The daily value (DV) is the recommended amount of the vitamin to consume each day. The percent daily value on the label (sometimes listed as % DV) tells you how much the vitamin is contributing to your daily needs.

Let’s review how the measurements and percent daily values have changed since the 2019-2020 update by the FDA.

Vitamin A

Instead of IU, vitamin A is now measured in micrograms of retinol activity units (mcg RAE). RAEs provide standardization across different sources of vitamin A. For example: 1 mcg RAE = 1 mcg retinol 1 mcg RAE = 2 mcg supplemental beta-carotene 1 mcg RAE = 12 mcg beta-carotene

You can see an example of vitamin A on a supplements facts label here.

Vitamin E

The DV unit for vitamin E is now milligrams alpha-tocopherol. This helps standardize the measurement across natural and synthetic vitamin E.

On a supplement facts label, 1 mg vitamin E (listed as alpha-tocopherol) equals 1 mg of natural alpha-tocopherol or 2 mg of synthetic alpha-tocopherol.

You can see an example of vitamin E on a supplements facts label here.

Vitamin D

The DV unit for vitamin D has also changed. Vitamin D is now measured in micrograms (mcg).

Since 1 IU of vitamin D equals 0.025 mcg, an 1,000 IU vitamin D supplement will now be listed as 25 mcg per serving.

Unlike vitamin A and vitamin E, the IU conversion of vitamin D does not differ by form. Most supplements use vitamin D3 (cholecalciferol) in their products.

At this time, you will often see both IU and mcg for vitamin D on a supplement facts label. You can see an example of a vitamin D label here.

Final takeaways

Using IU is a helpful way to standardize the measurement of a substance based on biological activity. If you’re used to seeing IU on a supplement facts label, it’s important to know that IU has now been phased out. While this shouldn’t change the amount of a vitamin in the product, you should still check to make sure you are taking the desired dosage.

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