Vitamin A is produced from beta carotene in the liver. The 2D carbon chain is shown below:
The 3D structure is shown below (it can be manipulated by moving the mouse over the image with the left button selected):
Besides being important for normal vision, vitamin A plays a vital role in gene expression, reproduction, embryonic development, growth, and immune function. The Recommended Daily Amount (RDA) of vitamin A for men is 900µg and 700µg for women. The most common symptom of vitamin A deficiency is vision impairment, in particular night blindness, in severe cases total blindness can occur, this generally happens to 500,000 children a year, most of which live in developing countries. According to the World Health Organisation (WHO) many more children die from measles, due to a lowered immune response, caused by vitamin A deficiency. The safe Upper Limit (UL) for vitamin A consumption is 3000µg. Overcomsumption of vitamin A can lead to liver abnormalities in adults, increased risk of physical birth defects and bulging of the skull where the bones have not yet formed in infants.
Vitamin A is the precursor to 11-cis-retinal, which is a prosthetic group which makes up part of the photosensitive molecule rhodopsin. Rhodopsin is present in the rod cells of the retina, making it sensitive to light.
There are many sources of vitamin A, in the form of beta carotene in red, yellow and orange fruit and vegetables. Green leafy vegetables are also a good source. Vitamin A itself can be found in most animal products such as meat and milk, but especially liver.
Fascinating Fact: Eskimos, who hunt for polar bears, don't eat the liver because it is so rich in vitamin A, it's toxic.