VITAMIN A

The fat soluble vitamin A, as such is present only in foods of animal origin.
Its provitamins carotenes are found in plants.
He prescribed to the patients ox liver (in honey), which is now known to contain high quantity of vitamin A
The term retinoids is often used to include the natural and synthetic forms of vitamin A.
Retinol, retinal and retinoic acid are regarded as vitamers of vitamin A.

Retinol (vitamin A alcohol) : It is a primary alcohol containing E-ionone ring. The side chain has two isoprenoid units, four double bonds and one hydroxyl group. Retinol is present in animal tissues as retinyl ester with long chain fatty acids.
Retinal (vitamin A aldehyde) : This is an aldehyde form obtained by the oxidation of retinol. Retinal and retinol are interconvertible. Previously, the name retinine was used for retinal.
Retinoic acid (vitamin A acid) : This is produced by the oxidation of retinal. However, retinoic acid cannot give rise to the formation of retinal or retinol.
E-Carotene (provitamin A) : This is found in plant foods. It is cleaved in the intestine to produce two moles of retinal. In humans, this conversion is inefficient, hence E-carotene possesses about one-sixth vitamin A activity compared to that of retinol.

Dietary retinyl esters are hydrolysed by pancreatic or intestinal brush border hydrolases in the intestine, releasing retinol and free fatty acids.
Carotenes are hydrolysed by E-carotene 15-15c-dioxygenase of intestinal cells to release 2 moles of retinal which is reduced to retinol.
In the intestinal mucosal cells, retinol is reesterified to long chain fatty acids, incorporated into chylomicrons and transferred to the lymph.
The retinol esters of chylomicrons are taken up by the liver and stored
As and when needed, vitamin A is released from the liver as free retinol.
It is believed that zinc plays an important role in retinol mobilization.
Retinol is transported in the circulation by the plasma retinol binding protein in association with pre-albumin.
The retinol-RBP complex binds to specific receptors on the cell membrane of peripheral tissue and enters the cells.
It is here that retinol exerts its function in a manner analogous to that of a steroid hormone

Vitamin A is necessary for a variety of functions such as vision, proper growth and differentiation, reproduction and maintenance of epithelial cells.
Vitamin A and vision : The biochemical function of vitamin A in the process of vision was first elucidated by George Wald (Nobel Prize 1968).
The events occur in a cyclic process known as Rhodopsin cycle or Wald’s visual cycle.

Retinol and retinoic acid function almost like steroid hormones. They regulate the protein synthesis and thus are involved in the cell growth and differentiation.
Vitamin A is essential to maintain healthy epithelial tissue. This is due to the fact that retinol and retinoic acid are required to prevent keratin synthesis (responsible for horny surface).
Retinyl phosphate synthesized from retinol is necessary for the synthesis of certain glycoproteins, and mucopolysaccharides which are required for growth and mucus secretion.
Retinol is necessary for normal reproduction. It acts like a hormone and regulates gene expression.
Vitamin A is considered to be essential for the maintenance of proper immune system to fight against various infections.
Cholesterol synthesis requires vitamin A. Mevalonate, an intermediate in the cholesterol biosynthesis, is diverted for the synthesis of coenzyme Q in vitamin A deficiency. It is pertinent to note that the discovery of coenzyme Q was originally made in vitamin A deficient animals.
Carotenoids (most important E-carotene) function as antioxidants and reduce the risk of cancers initiated by free radicals and strong oxidants. E-Carotene is found to be beneficial to prevent heart attacks. This is also attributed to the antioxidant property.

Recommended dietary allowance (RDA)

The daily requirement of vitamin A is expressed as retinol equivalents (RE) rather than International Units (IU).

1 retinol equivalent	=1 Pg retinol
	=6 Pg E-carotene
	=12 Pg other carotenoids
	=3.33 IU of vitamin A activity from retinol
	=10 IU of vitamin A activity from E-carotene

The RDA of vitamin A for adults is around 1,000 retinol equivalents (3,500 IU) for man and 800 retinol equivalents (2,500 IU) for woman

One International Unit (IU) equals to 0.3 mg of retinol.

The requirement increases in pregnant women and lactating mothers.

Animal sources contain (preformed) vitamin A.
The best sources are liver, kidney, egg yolk, milk, cheese, butter.
Fish (cod or shark) liver oils are very rich in vitamin A.
Vegetable sources contain the provitamin A-carotenes.
Yellow and dark green vegetables and fruits are good sources of carotenes e.g. carrots, spinach, pumpkins, mango, papaya etc