Shortly before the late Dr. Lafayette B. Mendel wrote an article on vitamin A for the previous vitamin symposium in The Journal, the perplexing question as to why the yellow-red plant pigment carotene exhibits vitamin A activity although the familiar vitamin A of liver oils is essentially a colorless substance had been answered by the discovery1 that carotene is convertible in the body to vitamin A. In fact the chemical basis for such a relationship had just been established by the brilliant researches of Karrer and his associates, who had determined the chemical constitution both of plant carotene2 and also of vitamin A3 from fish liver oil.
The first complete structural formula for carotene (fig. 1) has since turned out to be that of β-carotene, which is by far the most important and widely distributed of the known coloring matters which have vitamin A activity.
PALMER LS. THE CHEMISTRY OF VITAMIN A AND SUBSTANCES HAVING A VITAMIN A EFFECT. JAMA. 1938;110(21):1748-1751. doi:10.1001/jama.1938.62790210005009