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?

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. 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

Karen’s Fit Tip:  NEVER use milligrams interchangeably with International Units. For example, 2,000 mg of vitamin D is equivalent to 40,000,000 IU!

Karen Owoc

Karen Owoc is a certified Clinical Exercise Physiologist specializing in cardiopulmonary rehabilitation and lifestyle medicine. Her science-based approach to longevity, nutrition, and muscle health has made her the go-to source for health seekers and medical professionals alike. Karen's best-selling book on functional longevity, "Athletes in Aprons: The Nutrition Playbook to Break 100", and her transformative perspective have mended many minds, hearts, and spirits.