The Food and Nutrition Board of the Institute of Medicine revised the Recommended Dietary Allowance (RDA) of vitamin A in 2001 as 900 retinol activity equivalents (RAE) for men and 700 RAE for women. The RAE was introduced to avoid the ambiguity of international units (IU), which arises because 1IU of vitamin A (0.3 mg) and 1 IU of the vitamin A precursor (provitamin A) all-trans-[3-carotene (0.6 mg) do not have the same biological activity. Rather, 6 IU of ^-carotene and 12 IU of mixed carotenoids provide the biological activity of 1 IU of vitamin A. The RAE refers to the amounts necessary for the same degree of biological activity: 1 mg atROH = 12 mg ^-carotene = 24 mg mixed carotenoids = 1 RAE.
Liver, dairy products, and saltwater fish, including herring, sardines, and tuna, serve as dietary sources of vitamin A and its esters. Cod and halibut liver oil provide especially rich sources of vitamin A, as does the liver of the polar bear, which benefits from occupying the top of the marine food chain. Carrots, yellow squash, corn, and dark-green leafy vegetables serve as dietary sources of provitamin A carotenoids. Because less than 10% of the 600 naturally occurring carotenoids generate vitamin A, vegetable color does not necessarily indicate vitamin A value. In the United States and Europe, retinol and its esters serve as the chief sources of dietary vitamin A, but elsewhere carotenoids serve as the primary source. According to the World Health
Organization (WHO), the major inadequacies of dietary micronutrients involve vitamin A, iron, and iodine. Although vitamin A intake seems adequate in most populations in the United States, Canada, and Europe, this appears evolutionarily aberrant. The recurrent vitamin A-deficiency problem in Third World populations indicates that human diets in nonindustrialized countries are limited in vitamin A.
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