Vitamin A: Carotenoids and Retinoids
Vitamin A is required for the maintenance of normal mucous membranes and for normal vision. It occurs naturally only in foods of animal origin, such as liver, butter, whole milk, and egg yolks, but the body converts certain carotenoids, especially β-carotene, to vitamin A. Only 50 of the more than 500 naturally occurring carotenoids have provitamin A activity (Isler et al., 1971; Olson, 1983, 1984). Carotenoids are present in dark-green, leafy vegetables and in yellow and orange vegetables and fruits. In addition, skim milk, margarines, and certain breakfast cereals are fortified with vitamin A. From food composition tables, one can estimate only the total vitamin activity (vitamin A value) but not the quantity of specific carotenoids or retinoids in foods (Beecher and Khachik, 1984). However, researchers are now beginning to add carotenoid values to those in the U.S. Department of Agriculture (USDA) data bank.
In 1980 the Recommended Dietary Allowance (RDA) for males 11 years of age and older was 1,000 retinol equivalents (RE), or 5,000 International Units (IU), and for females, 800 RE (4,000 IU) (NRC, 1980). By definition, 1 RE is equal to 1 µg of retinol, 6 µg of β-carotene, or 12 µg of other provitamin A carotenoids. One IU of vitamin A activity is defined as 0.3 µg of retinol and as 0.6 µg of β-carotene. One RE is equal to 3.33 IU of vitamin A activity from retinol and to 10 IU of vitamin A activity from β-carotene. Many food composition tables and most food labels still list vitamin A activity in IU, although the official unit is now RE.
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The availability of vitamin A in the food supply rose from 7,300 IU per capita in 1967-1969 to 9,900 IU in 1985, an increase of 37% (see Table 3-3). This increase was due chiefly to new varieties of carrots containing higher amounts of carotenoids.
The 1977-1978 Nationwide Food Consumption Survey, based on 3-day dietary intake reports, indicated that total vitamin A intakes averaged 133% of the RDA. More than two-thirds of the population had intakes of at least 70% of the RDA. Intakes were higher for adults over 64 years of age and for children under 8 than for other age groups. People above poverty levels were more apt to reach the RDA for vitamin A than were those below. Intakes were highest in the western United States and lowest in the South (DHHS-USDA, 1986). According to the 1976-1980 National Health and Nutrition Examination Survey (NHANES II), total mean dietary intake of vitamin A in the U.S. population, excluding infants, was approximately 1,000 RE. Carotenoids and preformed vitamin A contributed 25 and 75%, respectively, of the total intake. Mean serum vitamin A levels measured in NHANES II were within normal ranges for all race, sex, and economic groups.
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