Vitamin A is best known for its role in vision, but it is also crucial for proper growth, reproduction, immunity and cell differentiation.
The term vitamin A is generally used to refer to a group of compounds that possess the biological activity of all-trans retinol.
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.
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.
The initial symptoms of vitamin A deficiency are night blindness and keratinization of hair follicles.
Continued deficiency leads to damage to eye tissue and irreversible blindness.
The body uses three active forms of vitamin A, known collectively as the retinoids. These compounds are retinol, the alcohol form of vitamin A; retinal, the aldehyde form of vitamin A; and retinoic acid, the acid of vitamin A.
Retinol is the active form of vitamin A and has a yellow color. Carotene is the precursor of retinal and is a bright orange.
The US recommended Daily Allowance (RDA) of vitamin A for adults is 5000 IU (1000 retinol equivalents).
Some carotenoids (found in deep-yellow and dark green vegetables) can be converted to vitamin A during digestion.
In the US diet, approximately half of the vitamin A activity is derived from B-carotene and other carotenoids.
Vitamin A is found primarily in selected foods if animal origin, especially liver; dairy products (cheese, milk, and butter); eggs, fish; such tuna, sardines and herring, fish oil such as cod liver.
Fat soluble vitamin: Vitamin A
