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

Continue reading “Why Vitamin D is Measured in “IU””