August 10, 2023
If you’ve gone down the vitamin aisle recently, you may have noticed a slight change to some of the Supplement Fact labels. What used to be called “IU” is now being referred to as “mg” or “mcg.”
So what is IU, how does it compare to “mg” and “mcg,” and how do you know if you’re taking the right dosage? In this article, we’ll help clear the confusion on IU vs. mg for your vitamins.
IU stands for International Unit and it’s typically used to measure fat soluble vitamins such as vitamins A, D, E, and K. Though it can also be used to measure hormones, insulin, vaccines, and blood products.
The IU measurement is an international standard that you can see all over the world. Because it’s based on other international medication standards, you’ll be able to find consistent information from country to country. The Committee on Biological Standardization of the World Health Organization defines all international units (1).
However, starting in 2020, the United States Food and Drug Administration (FDA) required that supplement facts labels begin swapping any “IU” terminology to metric units like mg and mcg (2). This was due to a few reasons (3):
This is why you now see milligrams (mg) or micrograms (mcg) where IU used to be, though many supplement facts labels continue to list the IU as well to ease the transition.
This can get quite mathematical, so I’m going to do my best to describe it as simply as possible.
The system of international units (IU) was introduced for vitamins due to their varying forms and strengths. Take Vitamin A, for instance, available as retinol or beta-carotene.
Despite similar health benefits, these forms often differ in potency, requiring different doses to achieve equivalent effects. Standardizing doses based on weight isn't feasible, as it fails to account for this potency difference. IU resolves this by providing an agreed-upon measure for consistent dosing. This diminishes any concerns one may have about excessive intake.
Note that IU is suitable for comparing equivalent forms, like apples to apples, not apples to oranges. For instance, 1,000 IU of Vitamin D equates to 25 mcg, while 1,000 IU of Vitamin A equates to 300 mcg (4). This approach ensures precision in the diverse world of vitamin supplementation.
The IU measurement is primarily used for fat-soluble vitamins like vitamins A, D, E, and K.
Vitamin A is important for normal vision, the immune system, reproduction, and growth and development. Vitamin A also helps your heart, lungs, and other organs work properly (5). There are two widely available preparations of vitamin A: retinol and beta carotene.
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.
Vitamin E is a powerful antioxidant and is needed for immune health and cellular signaling in your body (7). Vitamin E is available in two formats: d-alpha-tocopherol (a natural source) and dl-alpha-tocopherol (a synthetic source). Vitamin E is now milligrams alpha-tocopherol. This helps standardize the measurement across natural and synthetic vitamin E (8).
Although IU has been phased out, this change doesn't affect the vitamin amount in the product. However, it's still important to consult your doctor to ensure you're taking the right dosage.
Kat Kennedy is the Fitness and Nutrition Editor at NativePath. With a NASM CPT, NCSF CPT, and NCSF Sports Nutrition Certification, she has a passion for giving people the tools they need to feel healthy, strong, and confident.
This content is for informational and educational purposes only. It is not intended to provide medical advice or to take the place of such advice or treatment from a personal physician. All readers/viewers of this content are advised to consult their doctors or qualified health professionals regarding specific health questions. Neither Dr. Chad Walding nor the publisher of this content takes responsibility for possible health consequences of any person or persons reading or following the information in this educational content. All viewers of this content, especially those taking prescription or over-the-counter medications, should consult their physicians before beginning any nutrition, supplement, or lifestyle program.
