The vitamin-A content of a food is sometimes measured in International Units (IU) and at other times in "micrograms of retinol equivalents," or "micrograms RE." There is no direct comparison between an IU and a microgram RE, but you can get a crude estimate if you know whether the food is of animal or plant origin.
· To convert IUs of vitamin A into micrograms RE: multiply the number of IUs by 0.1 if the food is of plant origin, by 0.2 if it's of animal origin. The result is the approximate number of micrograms RE in the food.
· To convert micrograms RE to IUs: multiply the micrograms RE by ten for a food of plant origin, by five for a food of animal origin. The result is the approximate number of IUs in the food.
In the case of vitamin A supplements, the conversion may be more precise. First, check the label to see which source of vitamin A the supplement contains. Then:
· If the source is retinol: multiply the number of IUs by 0.3 to get the number of micrograms RE, or multiply the number of micrograms RE by 3.3 to get the number of IUs.
· If the source is beta-carotene: multiply the number of IUs by 0.025 to get the number of micrograms RE, or multiply the number of micrograms RE by 39.6 to get the number of IUs.
(Some supplements may contain both sources. If the label specifies an amount for each, do the math for each and add the two together.)