The early recognition of the significance of vitamin A as an essential constituent of the diet of man made the quantitative as well as the qualitative identification of food sources an important problem for investigators engaged in research on nutrition. Although vitamin A was one of the first, if not the first, of the vitamins to be discovered, accurate quantitative methods of analysis have been slow of development. This can be explained by the fact that, although its chemical identity had been established with reasonable certainty, synthesis of the vitamin and its separation in crystalline form was accomplished only recently.1 The situation has been further complicated by the necessity for adopting a device for expressing vitamin A potency, since obviously the values could not be expressed in terms of vitamin A. In some of the early studies attempts were made to find some product that might serve as a
MUNSELL HE. VITAMIN A: METHODS OF ASSAY AND SOURCES IN FOOD. JAMA. 1938;111(3):245–252. doi:10.1001/jama.1938.72790290007008
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