International Units and Retinol Equivalents

The obsolete International Unit (iu) of vitamin A activity was based on biological assay of the ability of the test compound to support growth in deficient animals (1 iu = 10.47 nmol of retinol = 0.3 fg of free retinol or 0.344 fg of retinyl acetate).

13-Q's-retinol has 75% of the biological activity of all-frans-retinol, and retinaldehyde has 90%. Food composition tables give total preformed vitamin A as the sum of all-frans-retinol + 0.75 x 13-ds-retinol + 0.9 x retinaldehyde (Holland et al., 1991).

To take account of the contribution from carotenoids, the total vitamin A content of foods is expressed as micrograms of retinol-equivalents - the sum of that provided by retinoids and from carotenoids. Because of the relatively low absorption of carotenes and incomplete metabolism to yield retinol (Section 2.2.2), 6 fig of p-carotene is 1 fg of retinol-equivalent - a molar ratio of 3.2 mol of p-carotene equivalent to 1 mol of retinol. p-Carotene is absorbed much better from milk than from other foods; in milk, 2 f g of p-carotene is 1 f g of retinol-equivalent (1.07 mol equivalent to 1 mol of retinol). This is still far from the theoretical yield of 2 mol of retinol per mol of p-carotene. Other provitamin A carotenoids yield at most half the retinol of p-carotene, and 12 fg of these compounds = 1 f g of retinol-equivalent. On this basis, 1 iu of vitamin A activity = 1.8 fg of p-carotene or 3.6 fg of other provitamin A carotenoids.

The U.S./Canadian Dietary Reference Values report (Institute of Medicine, 2001) introduced the term retinol activity equivalent to take account of the incomplete absorption and metabolism of carotenoids; 1 RAE = 1 f g of all-frans-retinol, 12 fg of p-carotene, and 24 fg of a-carotene or p-crypto-xanthin. On this basis, 1 iu of vitamin A activity = 3.6 fg of p-carotene or 7.2 fg of other provitamin A carotenoids.


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