IU? What does that mean? Is that a kind of weight measurement?

Question

It seems to be used to measure the quantity of Vitamin A in foods, but what is it equal to? Milligrams? Micrograms?

Also there seem to be TWO types of measurement for Vitamin A, one is IU, the other is "RAE". Why are there 2 different measurements, and what is RAE?

http://www.nal.usda.gov/fnic/foodcomp/cgi-bin/list_nut_edit.pl

Answer 1

It's difficult to measure Vitamin A. The body uses it as Retinol, but this is poisonous in large doses. More usually our body stores it as Carotene (not poisonous) and the liver converts it to Retinol as needed.

RAE stands for Retinol Activity Equivalents and is the equivalent amount of Retinol in micrograms. IU stands for International Units. This is a fairly arbitrary measurement equivalent to 3.3 micrograms RAE.