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DRI DIETARY REFERENCE INTAKES FOR Vitamin C, Vitamin E, Selenium, and Carotenoids
A source and for the prevention and control of vitamin A deficiency in at-risk populations.
The most prevalent carotenoids in North American diets include the following: α-carotene, β-carotene, lycopene, lutein, zeaxanthin, and β-cryptoxanthin. The structures of these carotenoids are shown in Figure 8-1. Three of these carotenoids, namely α-carotene, β-carotene, and β-cryptoxanthin, can be converted into retinol and are thus referred to as provitamin A carotenoids. Lycopene, lutein, and zeaxanthin have no vitamin A activity and are thus referred to as nonprovitamin A carotenoids. Most naturally occurring carotenoids are in the all-trans-configuration; but under conditions of heating, for example, cis-isomers such as 13-cis-β-carotene (Figure 8-1) are formed.
Functions and Actions
The various biological effects of carotenoids can be classified into functions, actions, and associations. Carotenoids function in plants and in photosynthetic bacteria as accessory pigments in photosynthesis and protect against photosensitization in animals, plants, and bacteria. In humans, the only known function of carotenoids is vitamin A activity (provitamin A carotenoids only).
Carotenoids also are thought to have a variety of different actions, including possible antioxidant activity, immunoenhancement, inhibition of mutagenesis and transformation, inhibition of premalignant lesions, quenching of nonphotochemical fluorescence, and activity as a pigment in primate macula (Olson, 1999). Carotenoids have also been associated with various health effects: decreased risk of macular degeneration and cataracts, decreased risk of some cancers, and decreased risk of some cardiovascular events (Olson, 1999).
However, as described above, the only known function of carotenoids in humans is to act as a source of vitamin A in the diet. This function, as well as carotenoid actions and associations, is reviewed elsewhere (Krinsky, 1993; Olson, 1989) and discussed in subsequent sections.
Physiology of Absorption, Metabolism, and Excretion
The intestinal absorption of dietary carotenoids is facilitated by