For most vitamins and minerals there’s one simple unit of measure (usually milligrams). Vitamin A is more complicated because there are different forms of the vitamin and different precursors.
Vitamin A is usually measured in micrograms (one-thousandth of a milligram) of RAE (retinol activity equivalents). However, nutrition labels on foods and supplements list vitamin A in International Units (IU). 1 IU of retinol equals 0.3 micrograms of RAE.
The Institute of Medicine (IOM) has set the recommended dietary allowance (RDA) at 700 micrograms (equal to 2,333 IU) a day for women, 900 micrograms (3,000 IU) for men. To make matters more confusing, the Daily Value for vitamin A on nutrition labels is 5,000 IU, still based on the outdated RDA from 1968.
The IOM has set the Upper Limit for vitamin A at 3,000 micrograms (10,000 IU) a day. It generally takes long-term use of much higher doses to cause clinical toxicity. Pregnant women, in particular, should not exceed that limit. Some leading researchers believe that the IOM should lower the Upper Limit.