1Retinol Equivalents. 2Retinol Activity Equivalents.

1Retinol Equivalents. 2Retinol Activity Equivalents.

to convert all dietary sources of vitamin A into a single unit for easy comparison and accounts for inefficiencies in absorption and rate-limited bioconversion of the provitamin A carotenoids.22 The current unit used to express vitamin A activity is the retinol activity equivalents (RAE). This came about from the finding that the absorption of the provitamin A carotenoids is 50% less than originally thought. The only difference in this system is that the provitamin A carotenoids are one half the activity assumed when using REs (see Table 28.5).

The stereochemistry of vitamin A and related compounds is complex, and the study of the structural relationships among vitamin A and its stereoisomers has been complicated by the common use of several numbering systems (Fig. 28.3). The first numbering system (A) is the one currently recommended by IUPAC and is used throughout this text. The second system (B) places emphasis on the conjugated w system, whereas the third (C) is used by the USP Dictionary of USAN and International Drug Names.

The conjugated double-bond systems found in vitamin A and j-carotene, as well as the attached ring in the form of j-ionone or dehydro-j-ionone found in 3,4-dehydroretinol, are essential for activity. When any of these is saturated, activity is lost. The ester and methyl ethers of vitamin A have a biological activity on a molar basis equal to that of vitamin A. Retinoic acid (vitamin A acid) is biologically active but is not stored in the liver.

For steric reasons, the number of isomers of vitamin A most likely to occur is limited. These are all-trans, 9-cis, 13-cis, and the 9,13-di-cis. A cis linkage at double bond 7 or 11 encounters steric hindrance. The 11-cis isomer is twisted as well as bent at this linkage; nevertheless, this is

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