Is Carotene a Provitamin A, or has it Biological Activity per se?

Abstract

IT is generally acknowledged that the carotenes are 'provitamins', but there is no clear definition of the term 'provitamin' in the literature ; the general view seems to be that it is a compound which is not a vitamin itself but after minor changes is transformed into one. The organism may be incapable of carrying out this transformation (D-provitamins), or it may be able to do so (A-provitamins). The term' provitamin ' was first used by Harris1, while Moore^2,3 called carotene a precursor of vitamin A. The opinion of these and all subsequent authors seems to be based only upon Moore's experiments on storage of vitamin A. As in the rat the minimal doses necessary to cause storage are about ten times those giving optimal biological effect^4,5, the deposition and the biological action take place at widely separated dose-levels. Further, the former takes place in the liver^6,4,5, while the latter takes place all over the organism (surface epithelium, nerve cells, retina) where it cannot be followed by chemical analyses. Thus these two processes are not identical, and one cannot conclude that the carotenes do not possess biological A-action per se because they are stored in the form of vitamin A.