Vitamin A belongs to a class of compounds referred to as the retinoids, which consist of four isoprenoid units joined in a head-to-tail manner.20 All double bonds in the isoprenoid units are implied to be in the E (trans) configuration unless stated otherwise. Currently, the term retinoid is applied to retinol and its naturally occurring derivatives plus synthetic analogs, which need not have vitamin A activity. Vitamin A is currently used as a generic descriptor for all retinoids that exhibit the biological activity of retinol, the original substance identified as vitamin A. The term vitamin A1 and all-trans retinol have been used to refer to retinol whereas vitamin A2

was used to refer to 3,4-dehydroretinol. Retinoate analogs are used to refer to "compounds that control epithelial differentiation and prevent metaplasia, without possessing the full range of activities of vitamin A."21 Retinal (formerly all-trans retinal), the form of vitamin A that takes part in the visual cycle, and retinoic acid (formerly all-trans retinoic acid), the form that takes part in cell differentiation, are the two active forms of vitamin A (Fig. 28.1). About 60 ft-carotenes, out of 700 carotenoids that have been identified, exhibit vitamin A activity in the body and are termed the provitamin As or provitamin A carotenoids. The important carotenoids regarded as the provitamin As are a-, ft-, and y-carotenes and cryptoxanthin (Fig. 28.2). ft-Carotene is the most important provitamin A carotenoid. The transformations of preformed vitamin As and ft-carotene are shown in Figure 28.1.

Vitamin A activity has been historically expressed as United States Pharmacopeia (USP) units, international units (IU), retinol equivalents (RE), or ft-carotene equivalents. The USP units and IU are equivalent. Each unit expresses the activity of 0.3 ¡g of retinol. Thus, 1 mg of retinol has the activity of 3,333 units. Other equivalents are listed in Table 28.4. This system does not take into account the incomplete absorption and rate-limited bioconversion of the provitamin A carotenoids into the active form, retinal. One RE represents the biological activity of 1 ¡ g of retinol, 6 ¡ g of ft-carotene, or 12 ¡ g of a-carotene or ft-cryptoxanthin. The RE was used

TABLE 28.4 Weight of Various Vitamin A Sources Equivalent to 1 International Unit




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