It influences growth of the body. It is also necessary for the maintenance of normal epithelial tissues of the organs including cornea and conjunctiva.
It serves as a carrier of specific proteins to places for special processes such as in the formation of visual purple which in the light is converted into visual yellow.
It is also essential for specific synthesis of the tissue of the nerve cells and fibres and for maintaining the integrity of the structure and functions of the reproductive organs.
Vitamin A is yellow viscous oil. Chemically, it is related to carotene, a vegetable pigment. Carotene C40H56 is a common pigment found in abundance in carrots.
Ordinary carotene was shown to consist of three isomers: ?-carotene (15%), m.p. 184°?-carotene (85%), m.p. 184°c, and y-carotene (trace), m.p. 177°c. By the addition of two molecules of water ?-carotene forms two molecules of vitamin A as it is the most active of the three carotenes, while a-carotene and y-carotene can yield only one vitamin A molecule.
The conversion of ?-carotene into vitamin A occurs partly in the liver but mainly in the wall of the small intestine.
For this conversion a normally functioning thyroid gland is necessary Carotene is, thus, called as provitamin A.
It is fairly thermostable in the absence of oxygen but it is oxidized and loses its biological activity when it is heated in the presence of oxygen. In a vaccum, it can be distilled.
Vitamin A occurs in two forms, e.g., A1 and A2 which are very closely related. Each form is derived from a separate carotene.
Vitamin Ai is formed from a-carotene, while A, from /3-carotene. The two carotenes differ in spectrum and colour tests. Vitamin A, has one more conjugated double bond as shown below
Occurrence and availability:
Vitamin A and provitamin A, the so called carotene, are found in considerable amounts in butter, liver, egg yolk, milk and especially in fish liver oil and in certain vegetables such as carrots, tomatoes and spinach, etc.
The minimum daily requirement of vitamin A recommended by Food and Nutrition Board, National Academy of Sciences, National Research Council, is about 5000 units for men and women.
For pregnant and nursing women, from 6000 to 8000 units daily are recommended. Infants and growing children require 15C0 to 5000 units daily, depending on age. The one unit represents 0.000344 mg or 0.344 /*g.
Vitamin A deficiency or avitaminosis:
It is believed that a vitamin A deficiency is associated with a loss of weight and inhibition of growth in young animals.
Its deficiency also causes morbid changes in the epithelium of the cornea, the respiratory and digestive tracts, and of other organs. A dryness and keratinization of the skin and an increase in pigmentation are observed.
Xerophthalmia is a very characteristic eye disease also-caused by vitamin A deficiency. In this disease, the eyes become haemorrhagic, encrusted and infected.
The earliest sign of vitamin A deficiency in man is nyctalopia (night blindness). In such patients vision is abnormally poor or fails completely when it is dark.
Recent investigation has shown that vitamin A is involved in the pigment of the eye.
Vitamin A toxicity or hypervitaminosis:
It has been observed that the excess of vitamin A or hypervitaminosis is harmful as it leads to anorexia, painful swellings over long bones, sparsity of hair, pruritic rash, nausea, weakness and dermatitis.
In rats, hyper vitaminosis A during pregnancy has a potent “teratogenic effect’, causing skeletal deformation of the foetus.