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.


5 Ways To Get Rid Of The Baby Fat

5 Ways To Get Rid Of The Baby Fat

Many women who have recently given birth are always interested in attempting to lose some of that extra weight that traditionally accompanies having a baby. What many of these women do not entirely realize is the fact that breast-feeding can not only help provide the baby with essential vitamins and nutrients, but can also help in the weight-loss process.

Get My Free Ebook

Post a comment