IU

International unit (IU) is a unit of measurement for certain substances that are not measured by their mass, but by their function or activity. Examples include vitamin A and D3, but also medicinal preparations such as heparin and insulin. For each substance, the ratio of the quantity equivalent to one IU differs. For many substances, these ratios are chosen arbitrarily. The World Health Organization defines them.

Here is an example for vitamin D3: 1 IU vitamin D3 ≙ 0.025 µg vitamin D3 ≙ 65.0 mmol vitamin D3. For vitamin A, a standard conversion of 400 IU vitamin A from 1 mg ß-carotene is used.