What is Vitamin A
Vitamin A is one of the fat soluble vitamins which required for general health and growth.Originally it was called substance fat-soluble A because of its presence in animal fats.
It was the first vitamin discovered after the scientists in the early of 1900s found that certain substances in animal fats and fish oils were important for the growth of cattle.
This vitamin must be obtained from animal and vegetable sources because it is not synthesized in human body.
What are the types of vitamins A?
Vitamin A can be divided into two main types according to the original dietary source of it.
1- Pre-formed vitamin A
Pre-formed vitamin A comes only from animal sources and the active forms of it are retinol, retinal and retinoic acid.
In animal tissue, pre-formed vitamin A present in an ester form especially retinyl palmitate. In small intestine, ester will be converted to alcohol active form of Vitamin A which is called retinol.
Retinol is the main storage form and the main storage site for vitamin A is the liver and to some extent, the retina.
Retinol can be converted to two other active forms which are the retinal and retinoid acid. Its conversion to retinal is reversible while its conversion to retinoic acid is irreversible. Retinoid acid has its effect in gene transcription.
2- Pro-vitamin A
Pro-vitamin A comes from vegetables sources and it is called carotenoids.
There are many forms of carotenoids but the most important forms are beta-carotene, alpha-carotene, gamma-carotene and beta-cryptoxanthin.
All these forms can be converted to retinol in human body. Beta carotene is the most important form because each molecule of it can convert to two molecules of retinol.