Why Vitamin D is Measured in “IU”

Vitamin DYou see it on your bottle of vitamin D… 2,000 IU. You see IU on food labels too. What’s an IU anyway? And why IU instead of mg like vitamin C and calcium supplements?

What’s the Difference?

Gram (gm), milligrams (mg), and micrograms (mcg) are units of weight. To visually represent the relationship between the three units, think of a microgram as a miniscule grain of rice, a milligram as a small bite of rice, and a gram as the whole bowl of rice.

  • 1 gm  = 1,000 mg
  • 1,000 mg = 1,000,000 mcg (µg is the short unit symbol for microgram)

International Unit (IU) is a unit of measurement but NOT a measure of weight. It’s the quantity of a specific biologically active substance that produces a particular biological effect. IU is most commonly used for medications, vaccines and some vitamins. However, converting an IU to a unit of weight isn’t a simple equation. That’s because an International Unit is based on the potency or concentration of the substance which varies from substance to substance.

Vitamin D: Converting Biological Activity to Weight

Vitamin D exists in a couple of different forms — cholecalciferol (vitamin D3) and ergocalciferol (vitamin D2). There are common vitamin mass equivalents for vitamin A, C, D, and E with each vitamin having a different biological equivalent. See Vitamin Conversion Chart. To get the equivalents of other substances, you would need to ask a pharmacist.

One IU of vitamin D* is the biological equivalent of 0.025 mcg cholecalciferol or ergocalciferol.

How Many IU of Vitamin D Do You Need?   

supplement capsule

The recommended dietary allowance (RDA) for vitamin D is 600 IU (15 mcg). If you’re over 70 years old, the RDA increases to 800 IU (20 mcg). 

Since vitamin D is a fat-soluble vitamin, excesses are stored in your body whereas water-soluble vitamins like vitamin C are excreted in your urine.  A “megadose” of vitamin D is 10 times the daily recommended allowance, i.e., 6,000 IU for adults under age 70 and 8,000 IU if 70 and over would be considered megadoses of vitamin D. According to the Institute of Medicine, up to 4,000 IU is considered a safe tolerable upper limit of this vitamin.

Vitamin D aids the absorption of calcium and regulates how much calcium your body needs. Start gulping down megadoses of vitamin D and you may end up with vitamin D toxicity (hypervitaminosis D). The result? Excessive calcium in your blood (hypercalcemia).  This condition can cause adverse effects, such as:

  • Poor appetite
  • Nausea, vomiting
  • Constipation
  • Confusion
  • Abnormal heart rhythm
  • Weakness
  • Dizziness
  • Increased risk of falls and fractures
  • Ringing in the ears
  • Frequent urination, bladder infections, and kidney impairment

Apple c heart symbol_40x54Fit Tip:  NEVER use milligrams interchangeably with International Units. For example, 2,000 mg of vitamin D is equivalent to 40,000,000 IU!

12 thoughts on “Why Vitamin D is Measured in “IU”

  1. Karen,
    Thank you for the information about vitamin D. Now I understand the differences between ,IU,mcg and mg.
    So is about 6,000 IU for me?
    I have not been in class because I was sick with some kind of cold.
    Also hurt my back! Bummer!
    As soon as I can I will Be back.
    Miss you and everyone.
    with love and sincerity, Elena


    1. Hi Elena, oh no… how did you hurt your back? We miss you. We had expected to see you by now since your return from the beach. Did you hurt your low back?

      I’m glad you found this information helpful. Are you taking 6,000 IU of vitamin D per day? Did your physician prescribe this amount? ~Karen


  2. Hey Elena, I wondered where you went. Figured you moved:) Hope you recover and can get back.
    Karen, I was taking 4000 iu of D3 and had my D3 level checked. I was at 36. I figured that was low end of normal so increased my intake to 6000. It will be interesting to see where it is next time:)


    1. Hi Bob! 6,000 IU of D3 is a lot. It’s 10 times the RDA for adults under age 70 and considered a “megadose”. Vitamin D is a fat-soluble vitamin and promotes the absorption of calcium. Unlike water-soluble vitamins, like vitamin C, where excesses are excreted through the urine, excesses of vitamin D are stored in your body and can be toxic. Vitamin D toxicity can cause excess calcium in your blood (hypercalcemia) which can lead to abnormal heart rhythms as well as many other adverse effects (listed in the article).

      Your level of 36 ng/dL (nanograms per deciliter) 25-hydroxyvitamin D is within normal range. A level of ≥20 ng/dL is generally considered adequate for bone and overall health in healthy individuals. Some clinical guidelines state that you need >30 ng/dL to maximize the effect of vitamin D on calcium, bone and muscle metabolism. Either way, you are within range. If you were not within range, you would need to consult with your physician. An adult therapeutic dose of supplemental vitamin D is typically between 1,500-2,000 IU/day. So taking 6,000 IU is excessive. It’s always a good idea to talk to your doctor before taking megadoses of supplements. And remember… to check that your supplements have the USP-verified mark on them. Read Why Supplements Can Make You Sick


      1. You’re very welcome, Bob. Be sure your doctor orders a periodic blood test to see if you’re within normal range. No need to take a high dose of D if it’s not necessary.

        Try to get more vitamin D from natural food sources. You’ll benefit from the additional powerhouse nutrients. Try eating 3.5-oz servings of oily fish (e.g., salmon, sardines) twice a week with a side of wild mushrooms! The wild ones (like chanterelles, morels, porcini, or matsutake) contain more vitamin D than the commercially grown mushrooms. Also, Vitamin D-fortified foods like milk, yogurt and some orange juices will give you a boost of vitamin D as well as protein and calcium.


  3. Hahaha, 2000 IU is probably good enough even without more salmon. I don’t know about the mushrooms, but more salmon would be good 🙂


  4. Amazing post. I read your posts fairly often and you always do a good
    job explaining the whatever topic you’re blogging about.
    Btw, I shared this on Twitter and my followers loved
    it. Keep up the great work!


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