What Are International Units (IU)?
An International Unit (IU) is a unit of measurement for the biological activity or effect of a substance, rather than its mass. The IU system was established by the Expert Committee on Biological Standardization of the World Health Organization (WHO) to standardize the measurement of vitamins, hormones, vaccines, and other biologically active substances.
Unlike mass-based units (grams, milligrams, micrograms) which measure how much of a substance is present, IU measures how much biological effect is produced. This distinction is critical because different chemical forms of the same vitamin can have vastly different levels of biological activity per unit of mass.
For example, 1 mcg of retinol (preformed Vitamin A) has much greater biological activity than 1 mcg of beta-carotene (provitamin A), even though both are forms of Vitamin A. The IU system accounts for these differences, making it easier for healthcare providers to prescribe and patients to take the correct dosage.
Why Different Substances Have Different Factors
Each substance has a unique IU conversion factor because the relationship between mass and biological activity varies depending on the chemical structure, bioavailability, and metabolic pathway of each compound:
- Vitamin A (Retinol): Retinol is the most biologically active form of Vitamin A. It is directly usable by the body, so a small mass (0.3 mcg) provides 1 IU of activity.
- Beta-Carotene: As a provitamin, beta-carotene must be converted to retinol in the body. This conversion is inefficient (roughly 50%), so twice as much mass (0.6 mcg) is needed for 1 IU.
- Vitamin D: Cholecalciferol (D3) and ergocalciferol (D2) are extremely potent; just 0.025 mcg provides 1 IU of activity, meaning 40 IU per mcg.
- Vitamin E: Natural d-alpha-tocopherol has greater biological activity than the synthetic dl-alpha-tocopherol form, which is a mixture of stereoisomers. The body preferentially uses the natural form.
Conversion Factor Reference Table
| Substance | 1 IU Equals | IU per mcg (or mg) | Base Unit |
|---|---|---|---|
| Vitamin A (Retinol) | 0.3 mcg | 3.33 IU/mcg | mcg |
| Vitamin A (Beta-Carotene) | 0.6 mcg | 1.667 IU/mcg | mcg |
| Vitamin D (D2/D3) | 0.025 mcg | 40 IU/mcg | mcg |
| Vitamin E (Natural) | 0.67 mg | 1.49 IU/mg | mg |
| Vitamin E (Synthetic) | 0.9 mg | 1.1 IU/mg | mg |
| Retinyl Palmitate | 0.55 mcg | 1.82 IU/mcg | mcg |
| Retinyl Acetate | 0.34 mcg | 2.94 IU/mcg | mcg |
IU Conversion Diagram
Conversion Formulas
MCG to IU
For substances measured in mg (Vitamin E):
IU to MCG
For Vitamin E:
Recommended Daily Intakes
| Vitamin | RDI (Adults) | In IU | Upper Limit |
|---|---|---|---|
| Vitamin A (Retinol) | 900 mcg RAE (men), 700 mcg RAE (women) | 3,000 IU / 2,333 IU | 3,000 mcg (10,000 IU) |
| Vitamin D | 15 mcg (600 IU) | 600 IU | 100 mcg (4,000 IU) |
| Vitamin E | 15 mg | ~22.4 IU (natural) / ~33.3 IU (synthetic) | 1,000 mg (1,500 IU natural) |
Worked Examples
Example 1: Vitamin D 400 mcg to IU
This is 400 mcg of Vitamin D, which equals 16,000 IU. Note that the typical daily supplement dose is only 10–25 mcg (400–1,000 IU).
Example 2: 5,000 IU Vitamin A to mcg
Example 3: 200 IU Vitamin E (natural) to mg
Frequently Asked Questions
Why do supplement labels use IU instead of mcg?
Historically, IU was the standard unit on supplement labels because it measures biological potency rather than mass. However, the FDA has been transitioning labels to use mcg RAE (Retinol Activity Equivalents) for Vitamin A and mcg for Vitamin D. Many labels now show both units during the transition period.
Are IU the same for all forms of a vitamin?
No. The IU value accounts for differences in biological activity between forms. For example, 1 IU of natural Vitamin E (d-alpha-tocopherol) equals 0.67 mg, while 1 IU of synthetic Vitamin E (dl-alpha-tocopherol) equals 0.9 mg, reflecting the lower bioavailability of the synthetic form.
Can I convert IU of one vitamin to another?
No. IU values are substance-specific and cannot be compared across different vitamins. 1,000 IU of Vitamin D is a completely different biological effect than 1,000 IU of Vitamin A.
What is the difference between mcg and mcg RAE?
mcg RAE (Retinol Activity Equivalents) is a newer measurement system that accounts for the different bioactivities of various Vitamin A sources. 1 mcg RAE = 1 mcg retinol = 12 mcg beta-carotene = 24 mcg other provitamin A carotenoids. This system is replacing IU for Vitamin A on food and supplement labels.