Vitamin A consists of several active compounds, including alpha-carotene, beta carotene, retinal, retinol, retinoic acid and the carotenoids. Natural sources of vitamin A are egg yolks, fish-liver oils, liver, cream, butter, green leafy and yellow vegetables, pineapples, prunes, cantaloupes, oranges and limes. The richest sources include cod-liver oil, butter, butterfat in milk and egg yolk. Vitamin A can also be synthetically produced.
Early in the 20th century, Englishman Frederick Gowland Hopkins discovered a growth-stimulating substance inn milk. Later, a German researcher named Stepp identified a substance he had labeled âminimal qualitative factorsâ as a lipid. In 1913, E. V. McCollum and Marquerite David demonstrated an essential growth factor for rats in butter and egg yolk, which they called âfat soluble A.â
Simultaneously, Osborn and Mendel, working in New Haven, Connecticut, found a similar fat soluble substance in cod-liver oil and butter.
Moore, an English scientist, demonstrated that beta-carotene obtained as a colored substance from plants was transformed in the human body to a colorless form of vitamin A, which was then stored in the liver. It was not until 1930 that Karrer and his group in Switzerland determined the chemical structures of vitamin A and beta-carotene. In 1935, Wald defined a biochemical in the retina that he termed retinene, later renamed retinal.