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
Vitamins are defined as a group of complex organic compounds present in minute amounts in natural foodstuff that are essential to normal metabolism and lack of which in the diet causes deficiency diseases. Vitamins are required in trace amounts (micrograms to milligrams per day) in the diet for health, growth and reproduction.
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