Vitamin A, requirement

The award will be made to the investigator (or group of investigators) who (1) determines the clinical value of vitamin A (if any) in human medicine, or (2) determines the vitamin A requirements of human beings, or (3) determines whether vitamin A in amounts more than obtained in a well-balanced diet is of benefit in human physiology. 

Biesalski, FI. K., and Weiser, H. (1993). p-carotene supplements can not meet all vitamin A requirements of vitamin A-deficient rats. NY Acad. Sci. 691, 216-219. 

The analysis of vitamin A requires a multistep process. In order that you should be able to follow the step-by-step procedure, a flow chart is provided here ... 

Milk fat supplies the diet with a substantial proportion of its daily P-carotene and vitamin A requirements. Dietary p-carotene is converted to retinal in the intestinal epithelium and in the liver by the enzyme p-carotene-15-15 -dioxygenase. The retinal formed is further metabolized to retinoic acid (vitamin A). 

Very few direct studies have been performed to determine human vitamin A requirements. In the Sheffield study (Hume and Krebs, 1949), 16 subjects were depleted of vitamin A for 2 years only three subjects showed clear signs of impaired dark adaptation. One of these subjects was repleted with 390 /rg of retinol per day, which resulted in a gradual restoration of dark adaptation the other two subjects received /3-carotene. On this basis, the minimum requirement was presumed to be 390 /rg, and the reference intake was set at 750 /rg. 

Hume E and Krehs H (1949) Vitamin A Requirements of Human Adults. Report of the Vitamin A Sub-committee of the Accessory Foods Factors Committee. London Medical Research Council, His Majesty s Stationery Office. 

The exact mechanism leading to toxicity is not known. Both acute and chronic toxicity may occur. Daily vitamin A requirements range from 1500 international units (lU) to 4500 lU for children, 5000 lU for adults, and 5000 lU for pregnant women. 

Vitamin A is an essential substance for healthy vision and skin, but sometimes too much of this good thing creates problems. A relationship between excessive vitamin A intake during pregnancy and birth defects has been recognized for some time. However, the minimum amount of vitamin A required to produce harmful effects is now believed to be much lower than was previously thought. According to a recent study, the consumption of vitamin A at or above 10,000 international units per day was linked to birth defects. A vitamin international unit (lU) is a measure of vitamin activity, determined... 

With, T. K. 1940. Absorption, Metabolism and Storage of Vitamin A and Carotene, with some Remarks on the Vitamin A Requirement. Munksgaard, Copenhagen. Oxford. Press, London and New York. 

Herbage of this type supplies about 100 times the vitamin A requirement of a grazing cow when eaten in normal quantities. 

The FAO/WHO Expert Group on Vitamin A requirements (FAO/WHO, 1967) did not recommend an increased intake of vitamin A during gestation (750 RE daily), although they recognized the increased need during this period to support fetal development. This decision was based on the assumption that the... 

E. M. Hume and R. A. Krebs, Vitamin A Requirement of Human Adults An... 

I. U., and hence vitamin A requirement was considered to be 4 tig. per kilogram of body weight. The new international standard for vitamin A defines one unit as equal to 0.34 jug. of vitamin A acetate or 0.30 jug. of the alcohol.The vitamin A requirement would therefore be 6 tig. per kilogram of body weight. 

There have been relatively few studies of vitamin A requirements in which subjects have been depleted of the vitamin for long enough to permit the development of clear deficiency signs. Current estimates of requirements are based on the intakes required to maintain a concentration of 70 Jmol retinol per kg in the liver, as determined by measurement of the rate of metabolism of isotopically labelled vitamin A. This is adequate to maintain normal plasma concentrations of the vitamin, and people with this level of liver reserves can be maintained on a vitamin A-free diet for many months before they develop any detectable signs of deficiency. 

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