How Vitamin A Is Measured
For: New Advice About Vitamin A, July 2002
In our article on vitamin A and bone fractures in the July 2002 issue of the Wellness Letter, we used International Units (IU) as the unit of measurement for the vitamin, since this is what the FDA uses for the Daily Values—and thus what you will see on the labels of foods and supplements.
However, the international standard way of expressing vitamin A activity is retinol equivalents (RE) on a weight basis (micrograms). This is the unit of measurement used in studies, such as the Harvard one we discussed, and by the Institute of Medicine, which determines the Recommended Dietary Allowances (RDAs). Retinol equivalents (RE) are used not only for preformed vitamin A (retinol), but also for "pro-vitamins" (carotenoids, notably beta carotene), which the body can convert to vitamin A.
| SUBSTANCE | AMOUNT (in micrograms) |
RE | IU (FDA usage) |
| Retinol (vitamin A standard) |
1 | 1 | 3.3 |
| Provitamin A (beta carotene) |
12 | 1 | 3.3 |
| Other carotenoids | 24 | 1 | 3.3 |
Thus, one IU vitamin A = 0.3 mcg retinol or RE.
Another way to show the relationships:
|
One IU =
|
0.3 mcg retinol |
| 3.6 mcg beta carotene | |
|
7.2 mcg other carotenoids |
