The first nutritional deficiency disease to be identified and studied was night blindness. Ancient cultures recognized this and utilized cooked liver as both a topical and systemic cure for this malady. Retinol was first identified in 1920 and, being the first vitamin, was named vitamin A under the alphabetical nomenclature system. Its high concentration in the liver is due to the fat-soluble nature of this polyene biochemical, although because of the storage mechanism, excessive doses of vitamin A can be very toxic. The most critical biochemical to vision is retinal, an aldehyde that can be synthesized in vivo from retinol by NAD oxidation. Retinal exists in several isomeric states that are modulated by photon irridation and are very important in the conversion of light waves into vision, immune defenses, maintenance of body linings and skin, bone and body growth, normal cell development and reproduction. In addition to being crucial for eye health, Vitamin A is needed by all epithelial tissue; that is the tissue which has an external surface, and an internal lining; such as nose, eyes, mouth, genitalia, and so on. The skin and all of the protective linings of these areas serve as barriers to infection by bacteria and to damage from other sources. Vitamin A works at the genetic level to promote the process of cell differentiation, which allows each type of cell to mature so that it is capable of performing a particular function to help bar infections from taking hold. Another emerging area of research concerns the roles of Vitamin A in the regulation of the genes that produce proteins involved in immunity. Without sufficient Vitamin A , these complex genetic interactions produce an altered response to infection that weakens the body's defenses against disease.  Vitamin A also assist in bone growth. Normal children's bones grow longer, and the children grow taller, by remodeling each old bone into a new bigger version. To do so, the body dismantles the old bone structures and replaces them with new, larger bone parts. Growth cannot take place just by adding on to the original small bone; Vitamin A is needed in the crucial dismantling steps. In some children, failure to grow is one of the first signs of Vitamin A deficiency.  The most common source of vitamin A is liver, but many plants contain beta-carotene, which is metabolically converted to vitamin A. Symptoms associated with a deficiency of vitamin A are night blindness, changes in the eyes, poor bone growth, weak tooth enamel, slow growth, and dry skin.

Recommended Dietary Allowances: Men = 5000 IU (or 3 mg beta carotene); Women = 4000 IU (or 2.4 mg beta carotene)