Vitamin A, (2E,4E,6E,8E)-3,7-Dimethyl-9-(2,6,6-trimethyl-1-cyclohexen-1-yl)-2,4,6,8-nonatetraen-1-ol to give its full chemical name or just plain Retinol is a vitamin needed by the retina of the eye.
Image kindly reproduced according to the licence at: http://en.wikipedia.org/wiki/File:All-trans-Retinol2.svg
It is found in two forms in food, either retinol or alpha-carotene, beta-carotene or gamma-carotene. Herbivores and omnivores have an enzyme which can convert these carotenes into retinal.
Vitamin A was discovered in 1906 and is considered to be a fat soluble vitamin.
Image kindly reproduced according to the licence at: http://en.wikipedia.org/wiki/File:Retinol_3Dstructure.png
As well as needed by the eye, it is also needed in gene transcription, the immune system, bone metabolism and the skin.
“Vitamin A deficiency is estimated to affect approximately one third of children under the age of five around the world. It is estimated to claim the lives of 670,000 children under five annually.”1
A deficiency of vitamin A in the diet will need to impaired vision and night blindness.
1 Quote taken directly from the article on vitamin A at http://en.wikipedia.org/wiki/Vitamin_A