Vitamin A
Vitamin A (retinol) functions in reproduction, growth, the maintenance of skin and mucous membranes and the visual process.
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 US recommended Daily Allowance (RDA) of vitamin A for adults is 5000 IU (1000 retinol equivalents).
Rich dietary sources of retinol (preformed vitamin A) include dairy products, eggs and organ meats.
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
