Vitamin - A

Vitamin - A

Vitamin A is found only in foods of animal origin. It is present in almost all species of fish, birds and mammals. The yellow plant pigments α, β and γ carotenes and cryptoxanthin are precursors of vitamin A. The body has the ability to convert these carotenoid compounds present in the diet into vitamin A.

The chemical structure of β - carotene is such that it oxidizes to form two molecules of vitamin A, the other provitamins form only one. b-carotene is more efficiently converted to vitamin A than α - or γ - carotene or cryptoxanthin.

There are two forms of vitamin A: Vitamin A₁ which occurs in the liver of marine water fish and Vitamin A₂ found in the liver of fresh water fish. The vitamin A which contain alcoholic group in the side chain is called as retinol (Fig. 8.1) and which contain aldehyde group is known as retinal. Though the two vitamins differ slightly in their chemical structures their physiological functions are the same.

Functions

Vitamin A is essential

·           for the growth and metabolism of all body cells

·           for the formation of rhodopsin (visual purple) a complex substance formed from retinol and protein. Rhodopsin, a pigment found in retina is necessary for vision in dim light.

·           for the maintenance of healthy skin, particularly mucous membrane of the cornea and the lining of respiratory tract.

Sources

The liver of any animal is a rich source of vitamin A. Fish liver oil is an excellent source. Whole milk, egg yolk, dark green leafy vegetables and deep yellow vegetables and fruits are rich in carotenes, which can be converted into vitamin A by the intestinal wall.

Requirements

Vitamin A requirement is based on the intake to maintain the normal blood level. Adults placed on a vitamin A free diet are found to show no change in the level for several weeks.The capacity of the body to store vitamin A provides for an effective emergency supply. Recommended amount of Vitamin A for different age group is as follows:

Infants - 1500 IU / day

Children - 2000-3000 IU / day

Adults Pregnant and - 5000 IU / day

lactating women - 6000-8000 IU / day (IU = International units)

Absorption and storage

Vitamin A and carotene are absorbed from the small intestine into the lymph system. The maximum absorption is reached 3 to 5 hours after consumption. The rate of absorption of vitamin A is more rapid than that of carotene. In the human being about 95% of the vitamin A stored in the body is found in the liver with small amount in the lungs, adipose tissue and kidneys.

Deficiency

The earliest sign of vitamin A deficiency is concerned with vision. Initially there is a loss of sensitivity to green light, followed by impairment to adapt to dim light. This condition leads to night blindness. More prolonged or severe deficiency leads to the ulceration of cornea and this condition is known as xerophthalmia or keratomalacia.