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

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

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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?”

Citation 

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

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