There are three active forms of vitamin A in the body known as retinoids – retinol (an alcohol), retinal (an aldehyde), and retinoic acid (an acid). Carotenoid (plant pigments) can also be converted to vitamin A by the liver.
Vitamin A (retinol) functions in reproduction, growth, the maintenance of skin and mucous membranes and the visual process.
Epithelial cells (those cells present in the lining of body cavities and in the skin and glands) require vitamin A.
Vitamin A is normally transported in the blood linked to a specific protein, retinol binding protein (RBP). Specific proteins on cell surfaces and within cells are also involved with intracellular transport of the vitamin.
The role of vitamin A in maintenance of eye health and the visual cycle is quite well understood. Retinoic acid is required for normal differentiation of the cornea, conjunctival membranes and photoreceptor rod and cone cells of the retina.
Vitamin A is fat soluble and is primarily stored in the liver, where RBP is synthesized. In a well nourished person, vitamin A stores are generally sufficient to last many months on a vitamins A-deficient diet before signs of deficiency appear.
What are the roles of vitamin A in human body?