Vitamin A occurs in animal tissues as retinol. This vitamin does not occur in foods of vegetable origin, but provitamins for the formation of vitamin A do occur inabundance in many vegetable foods. These are the yellow and red carotenoid pigments, which, because their chemical structures are similar to that of vitamin A, can be changed into vitamin A in the liver.
Vitamin A Deficiency Causes “Night Blindness” and Abnormal Epithelial Cell Growth. One basic function of vitamin A isits use in the formation of the retinal pigments of the eye. Vitamin A is needed to form the visual pigments and, therefore, to prevent night blindness.
Vitamin A is also necessary for normal growth of most cells of the body and especially for normal growth and proliferation of the different types of epithelial cells. When vitamin A is lacking, the epithelial structures of the body tend to become stratified and keratinized. Vitamin A deficiency manifests itself by (1) scaliness of the skin and sometimes acne; (2) failure of growth of young animals, including cessation of skeletal growth; (3) failure of reproduction, associated especially with atrophy of the germinal epithelium of the testes and sometimes with interruption of the female sexual cycle; and (4) keratinization of the cornea, with resultant corneal opacity and blindness.
In vitamin A deficiency, the damaged epithelial struc-tures often become infected, for example, the conjunc-tivae of the eyes, the linings of the urinary tract, and the respiratory passages. Vitamin A has been called an “anti-infection” vitamin.