IU vs mg for Vitamins: What's the Difference?

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.

What Does IU Mean?

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):

  • Inconsistent Measurements: Since IU measurements vary from nutrient to nutrient, it’s difficult to compare different nutrients based on their IU values.
  • Varying Potencies: Different forms of the same nutrient can have different potencies, leading to confusion when trying to determine the actual amount of the nutrient in a supplement.
  • Avoid Consumer Confusion: The change to milligrams (mg) or micrograms (mcg) help consumers better understand the quantity of a nutrient they are consuming, so they can make more informed decisions about their supplement dosage.
  • Align with International Standards: This change aligns with international measurement standards, making it easier to compare supplements across different countries.
  • Improve Regulation: Using standardized measurements allows regulatory agencies like the FDA better regulate and monitor dietary supplements for accuracy and safety.
  • Simplify Dosing: Nutrient recommendations from medical doctors or registered dietitians are often given in mg or mcg, making it more practical for individuals to follow dosage recommendations when the measurement units are familiar.

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.

IU vs. mg: What’s the Difference?

This can get quite mathematical, so I’m going to do my best to describe it as simply as possible.

IU (International Unit)
mg (milligram)
Measures the potency or activity of something.
Measures how heavy things are.
Used for vitamins, hormones, and other important substances.
Used for solids, liquids, and other things.
Not directly interchangeable between different substances.
Consistent across different substances. Standardized and universally recognized.

Why Was IU Used In the First Place?

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.

Vitamins Previously Measured In IU

The IU measurement is primarily used for fat-soluble vitamins like vitamins A, D, E, and K.

Vitamin A

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 D

Vitamin D helps the body absorb and retain calcium and phosphorus; both are critical for building/protecting your bones (6). The most commonly available vitamin D preparation is vitamin D3 or cholecalciferol. Vitamin D is now measured in micrograms.

You can see an example of a vitamin D label here.

Vitamin E

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).

Vitamin K

Vitamin K helps to make various proteins that are needed for blood clotting and the building of bones (9). It’s typically available as vitamin K1 (phylloquinone) and vitamin K2 (menaquinones). Vitamin K is now measured in micrograms.

You can see an example of a vitamin K on a label here.

Will This Label Change Affect My Dosage?

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
Article by

Kat Kennedy

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.

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    Medical Disclaimer

    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.

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