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see Vitamin A under vitaminvitamin,
group of organic substances that are required in the diet of humans and animals for normal growth, maintenance of life, and normal reproduction. Vitamins act as catalysts; very often either the vitamins themselves are coenzymes, or they form integral parts of coenzymes.
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(also called axerophthol or vitamin A), an isoprenoid whose chemical formula is C20H30O. Retinol is soluble in nonpolar organic solvents, insoluble in water, and has a melting point of 63°–64°C. In animals and man, it is converted from the carotene in food and stored mainly in the liver; it is particularly abundant in the liver of whales and fish and in fish oil.

The most important biological function of retinol is its participation in the form of retinal in the visual process. A retinol deficiency results in impairment of twilight vision (night blindness, or nyctalopia) and in injury to epithelial tissue, as in xe-rophthalmia. An excess of retinol causes a variety of metabolic disturbances, an accumulation of retinol in the hydrophobic fraction of cell membranes, and destruction of these membranes. Retinol is commercially synthesized from β-ionone. Retinol is sometimes called vitamin A1; its dehydro derivative C20H28O is called vitamin A2. Retinal1 and retinal2 are distinguished accordingly.


References in periodicals archive ?
Encouraged by these findings, we decided to evaluate whether oranges and tangerines, the primary food sources for beta-cryptoxanthin in most of the world, could be used to prevent vitamin A deficiency.
Providing vitamin A supplements or vitamin A-rich foods to people with vitamin A deficiency seems to decrease the severity of diseases such as influenza and diarrhea, and reduce infant and postpartum maternal mortality by about 30% (Black et al.
Some national governments run vitamin A supplementation programs; they are cost effective, but reaching most people at risk of vitamin A deficiency has proved challenging.
The amounts eaten must not differ greatly: the food must prevent vitamin A deficiency in most people but not cause toxicity in those who eat more than average amounts.
Direct sequelae of vitamin A deficiency, including disability-adjusted life years (DALYs), as well as mortality associated with measles, diarrhoeal diseases and other infections, and mortality and DALYs associated with malaria in children and all-cause maternal mortality.
8% of all DALYs in South Africa in 2000 were attributable to vitamin A deficiency.
The vitamin A supplementation programme for children and the recent food fortification programme introduced in South Africa in 2003 should prevent future morbidity and mortality related to vitamin A deficiency.
The dramatic progress is the result of a global partnership launched in 1997 between UNICEF, The World Health Organisation, and the governments of a number of countries including the United Kingdom, Canada and the United States, as well as national governments in countries where Vitamin A deficiency is a problem.
Just five years ago, only six of the 72 countries with Vitamin A deficiency had adequate supplementation programmes," said Ms.
The absence of such cells is not only the first symptom of developing xerophthalmia, but also an indicator of otherwise unapparent vitamin A deficiency, explains Keith P.
Until now, there has been no practical or sustainable process which would alleviate vitamin A deficiency," says Dr.