activity of purified β-carotene in oil. This change in bioconversion means that a larger amount of provitamin A carotenoids, and therefore darkly colored, carotene-rich fruits and vegetables, is needed to meet the vitamin A requirement. It also means that in the past, vitamin A intake has been overestimated.

The median intake of vitamin A ranges from 744 to 811 μg RAE/ day for men and 530 to 716 μg RAE/day for women. Using μg RAE, approximately 26 and 34 percent of vitamin A activity consumed by men and women, respectively, is provided from provitamin A carotenoids. Ripe, colored fruits and cooked, yellow tubers are more efficiently converted to vitamin A than equal amounts of dark green, leafy vegetables.

Although a large body of observational epidemiological evidence suggests that higher blood concentrations of β-carotenes and other carotenoids obtained from foods are associated with a lower risk of several chronic diseases, there is currently not sufficient evidence to support a recommendation that requires a certain percentage of dietary vitamin A to come from provitamin A carotenoids in meeting the vitamin A requirement. However, the existing recommendations for increased consumption of carotenoid-rich fruits and vegetables for their health-promoting benefits are strongly supported (see Dietary Reference Intakes for Vitamin C, Vitamin E, Selenium, and Carotenoids [IOM, 2000]).

Vitamin A is a fat-soluble vitamin that is essential for humans and other vertebrates. Vitamin A comprises a family of molecules containing a 20 carbon structure with a methyl substituted cyclohexenyl ring (beta-ionone ring) (Figure 4-1) and a tetraene side chain with a hydroxyl group (retinol), aldehyde group (retinal), carboxylic acid group (retinoic acid), or ester group (retinyl ester) at carbon-15. The term vitamin A includes provitamin A carotenoids that are dietary precursors of retinol. The term retinoids refers to retinol, its metabolites, and synthetic analogues that have a similar structure. Carotenoids are polyisoprenoids, of which more than 600 forms exist. Of the many carotenoids in nature, several have provitamin A nutritional activity, but food composition data are available for only three (α-carotene, β-carotene, and β-cryptoxanthin) (Figure 4-1). The all-trans isomer is the most common and stable form of each carotenoid; however, many cis isomers also exist. Carotenoids usually contain 40 carbon atoms, have an extensive system of conjugated double bonds, and contain one or two cyclic structures at the end