Vitamin A

Nutrient values labelled "vitamin A" in different food composition tables and nutrient data bases may represent different portions of the total vitamin A activity in a food. Therefore, it is essential to know what is included in a particular "vitamin A" value before a tagname is assigned to that value.

Total vitamin A includes both retinol (also called "preformed vitamin A") and carotene (also called "vitamin A precursor"). Total vitamin A is usually calculated as the total vitamin A activity contributed by retinol, beta-carotene, and other active carotenoids. This calculated value is expressed in retinol equivalents (RE) and is sometimes referred to simply as "retinol equivalents". It is calculated according to the following formula:
total vitamin A (RE) = mcg of retinol + 1/6 mcg of beta-carotene + 1/12 mcg of other active carotenoids

Total vitamin A may also be determined directly by bioassay and expressed in international units (IU). The tagnames for identifying total vitamin A values distinguish between those values that were calculated from retinol and the active carotenoids and those determined by bioassay. A tagname is also available for use when the method of vitamin A determination is not known. These three tagnames are listed below. All are subsidiary to the <comp> structural tag.

<VITA> values are usually expressed in retinol equivalents, and <VITAA> values are usually expressed in international units. However, a value expressed in one unit may be converted to the other unit according to the following formulae:

1 mcg retinol = 3.3 IU of retinol
1 RE from beta-carotene = 10 IU of beta-carotene

Beta-carotene equivalents (also called "provitamin A carotenoids") are a measure of the vitamin A activity contributed by the active carotenoids. As indicated in the above formula for calculating vitamin A in retinal equivalents, beta-carotene has twice the vitamin A activity of the other active carotenoids. Therefore, beta-carotene equivalents are calculated according to the following formula:
mcg of beta-carotene equivalents = mcg of betacarotene + 1/2 the mcg of other active carotenoids

The tagname for beta-carotene equivalents is listed below, as are the tagnames for identifying data on the retinol and active carotenoid components that can contribute to the calculated total vitamin A value. All are subsidiary to the <comp> structural tag.