VDC test kit slider

Why is vitamin D continuing to be measured in international units?

Posted on: October 30, 2017   by  John Cannell, MD


While approximately 2-3 % of Americans take 5,000IU/day of vitamin D3, many individuals are supplementing with inadequate doses of vitamin D. This problem is even worse in Europe, where recommendations of 400 IU/day are very common. According to this recommendation, a three-hundred-pound football player, an elderly man, a pregnant woman, a 4 year old child and a newborn infant should all be taking the same dose. Why such an illogical regimen?  How could a large football player and an infant have the same needs?

One of the culprits is that vitamin D is measured in IUs, or International Units. Instead of using the metric system, we are holding on to an antiquated way of measuring mass. IU values may make vitamin D seem dangerous because these units are not commonly used to measure micronutrient mass, making the actual mass of vitamin D difficult to reference. Many individuals will not take 10,000 IU/day, but they may take 250 mcg/day. As readers know, 10,000 IU and 250 mcg are the same amount (1 IU = 0.025 mcg).

Why is vitamin D not measured by the metric system? International Units are actually a standardized measurement of biological activity when differing measurements of the same compound give varying reports of the activity of that agent, which is no longer the case with vitamin D. Many biological agents exist in different forms or preparations (e.g. vitamin A in the form of retinol or beta-carotene). The goal of the IU is to be able to compare these, so that different forms or preparations with the same biological effect will contain the same number of IUs. There used to be a need to use IUs when measuring vitamin D, but this is no longer the case.

A close friend of mine recently had a total knee replacement at MGH (Harvard). He took 1,250 mcg/day for ten days before the surgery and ten days after. The Chairman of orthopedics at MGH did the surgery and told my friend that he had never seen a case of a total knee replacement that healed as well and where the patient gave up his cane so quickly. Was it the vitamin D?

My friend told the orthopedist that he was taking vitamin D. He asked how much and my friend said “1,250 mcg/day.” The orthopedist said, “Oh that’s fine, I thought you were going to say 5,000 IU/day.” Can you imagine what he would have said if my friend told him, “50,000 IU/day?”


John Cannell, MD. Why is vitamin D continuing to be measured in international units? The Vitamin D Council Blog & Newsletter, October, 2017.

1 Response to Why is vitamin D continuing to be measured in international units?

  1. HannahMC

    Yep. I was trying to convince my father in law to up his dose, but saying 5,ooo-10,000 IU sounds like a bizarrely high number to tell someone. Even my husband was alarmed

Test Your Vitamin D Levels at Home!

Our in-home vitamin D test kit is easy, affordable, and an accurate way to find out your Vitamin D status.

order NOW

We need your help!

We're spreading awareness on Vitamin D Deficiency
Donate NOW
Latest Articles
4 tips for writing an essay in English

Writing an essay in English can be a challenge, especially if you are beginning to learn the language or do not yet fully master it. But don’t worry, did you know that there are strategies that can make this task easier for you? To help you meet this challenge, we leave you a series of […]

Weekly Newsletter

Our Sponsors

December 21st is DDAY. Click here to celebrate the day with us!