Dietary requirement for vitamin A
Human requirements for vitamin A are based on studies of two kinds: nutritional status studies on various population groups throughout the world and controlled depletion eA'Periments carried out on humans and other animals. Field studies have indicated that in countries where the vitamin A intake is 3000 IU to 9000 IU (900 mcg-2700 mcg RE), per person per day, vitamin A deficiency is rarely seen, whereas, in other countries, on intakes of 1000 IU to 2500 IU (300 mcg-750 mcg RE), vitamin A deficiency is known to occur in the population. Research has shovm that varying doses of vitamin A are required to relieve different symptoms: 150 mcg of retinol per day reversed the changes in the eye; 300 mcg per day of retinol resulted in normal balance as well as normal taste and smell thresholds; however, 600 mcg per day of retinol were required to clear the skin lesions. Blood levels also returned to the normal range when a 600-mcg dose was given. Twice the amount of pure beta-carotene compared to retinol was necessary to correct the symptoms.25 The overall vitamin A requirement found in this study agreed fairly well with the earlier estimates from population studies and the now classic human experiments conducted under the auspices of the Bri tish Medical Research Council,26 which have influenced the recommended allowances of vitamin A established by the United States National Research Council's Food and Nutrition Board. As a result, there has been very litte change in the vitamin A allowances over the years.
The 1980 recommended allowance of 1000 mcg RE (5000 IU vitamin A) for an adult man is in excess of the minimum to allow some reserve of vitamin A to be stored in the body. The allowance for the adult woman is 800 mcg RE (4000 IU vi tamin A) or 800/0 of the male allowance because of the usually smaller body size of women. The allowances assume that the American diet provides half of the total vitamin A activity as retinol and half as provitamin A carotenoids. When calculated as International Units, this is 2500 IU as retinol and 2500 IU as provitamin A or a total of 5000 IU. In terms of retinol equivalents (RE), it is 750 mcg retinol (1 RE = 3.33 IU retinol) and 250 retinol equivalents as beta-carotene (1 RE = 10 IU f3-carotene) for a total of 1000 RE.
Suggestion for infants are based on the amount of retinol in human milk, about 49 mcg per 100 ml. An infant consuming 850 ml of breast milk would receive approximately 420 mcg RE of retinol; this amount is set as the recommendation for infants from birth to 6 months. It is reduced to 400 mcg RE for infants 6 months to 1 year of age. Because no definite information is available regarding the actual requirements of children and adolescents, these recommendations have been interpolated from the infant and the male adult allowances and are based on body weight and growth needs. The female allowance increases to 1000 mcg RE during pregnancy and 1200 mcg RE during lactation.
The requirements of vitamin A are recommended intake of 750 mcg (2500 IU) of retinol per day for the normal adult. No additional recommendation for pregnancy was made, provided that the usual diet supplies the recommended adult intake. To cover the vitamin A secreted in milk, 1200 mcg (4000 IU) of retinol was recommended during lactation. Recommendations for infants are based on the amount of vitamin A in breast milk. Suggested levels for children range from 300 mcg (1000 IU) for the 6 month child to 750 mcg (2500 IU) for the older adolescent.
Because carotene, which is less efficiently utilized (1/6) than retinol, is often the major source ofvitamin A in the diet, the recommended intake of vitamin A is modified depending on the percent of vitamin A supplied by carotene.
