Among the more striking recent advances in nutrition are those concerned with the physiologic significance of the less well known constituents of plant and animal tissues. These substances are likely to be overlooked entirely when a dietary component is evaluated for nutritional purposes on the basis of the conventional food analysis alone. Thus the various carotenes and kryptoxanthine, which appear as natural pigments in plants, are now known to be precursors of vitamin A; the traces of copper, manganese and zinc occurring in plant and animal tissues and previously receiving little if any attention have been demonstrated to be indispensable for nutritive well being in the body; the small amounts of the nitrogenous base choline, which is rather widespread in food substances but which heretofore has been looked on as a pharmacologic agent with limited application, is now known to exert a profound influence in certain phases of the metabolism
CITRIC ACID IN METABOLISM. JAMA. 1936;106(20):1737–1738. doi:10.1001/jama.1936.02770200043019
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