As indicated by its position at the head of the vitamin alphabet, vitamin A was the first vitamin discovered. In the early 1900s, researchers recognized that a certain substance in animal fats and fish oils was necessary for the growth of young animals. Scientists originally called the substance fat-soluble A to signify its presence in animal fats. Later, they renamed it vitamin A.
The most clearly defined role of vitamin A is the part it plays in vision, especially the ability to see in the dark. Metabolites of the vitamin combine with certain proteins to make visual pigments that help the eye adjust from bright to dim light. This process, however, uses up a lot of vitamin A. If it's not replaced, night blindness can result.
Moreover, a deficiency of vitamin A dries out the transparent coating of the eye (the cornea) and the membrane over the whites of the eye (the conjunctiva). If not treated, this condition, called xerophthalmia, causes irreversible damage and blindness. Vitamin A deficiency is a major cause of blindness in the world.
Vitamin A is also important for normal growth and reproduction -- especially proper development of bones and teeth. Animal studies show that vitamin A is essential for normal sperm formation, for growth of a healthy fetus, and perhaps for the synthesis of steroid hormones.
Another important, but misunderstood, role of vitamin A involves preserving healthy skin -- inside and out. Taking extra vitamin A won't make your sagging skin suddenly beautiful, but a deficiency of it will cause major skin problems. Furthermore, an adequate vitamin A intake ensures healthy mucous membranes of the gastrointestinal and respiratory tracts. In this way, vitamin A helps the body resist infection.
Vitamin A is also being studied for its function in creating normal copies of each of our cells as they reproduce. Cells that do not differentiate into specific cell types have a greater chance of undergoing pre-cancerous changes than normally differentiated cells. Current studies suggest that adequate retinol levels can help prevent cancers of the liver, prostate and possibly the colon.
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