Medical brain notes 38

  The Vitamins Introduction In order to demonstrate that a compound is a vitamin, it is necessary to demonstrate both that deprivation of experimental subjects will lead to the development of a more or less specific clinical deficiency disease and abnormal metabolic signs, and that restoration of the missing compound will prevent or cure the deficiency disease and normalize metabolic abnormalities. It is not enough simply to demonstrate that a compound has a function in the body, since it may normally be synthesized in adequate amounts to meet requirements, or that a compound cures a disease, since this may simply reflect a pharmacological action and not indicate that the compound is a dietary essential. The vitamins, and their principal functions and deficiency signs, are shown in Table 8.1; the curious nomenclature is a consequence of the way in which they were discovered at the beginning of the twentieth century. Early studies showed that there was some-thing in milk that was es...

  The Vitamins Introduction In order to demonstrate that a compound is a vitamin, it is necessary to demonstrate both that deprivation of experimental subjects will lead to the development of a more or less specific clinical deficiency disease and abnormal metabolic signs, and that restoration of the missing compound will prevent or cure the deficiency disease and normalize metabolic abnormalities. It is not enough simply to demonstrate that a compound has a function in the body, since it may normally be synthesized in adequate amounts to meet requirements, or that a compound cures a disease, since this may simply reflect a pharmacological action and not indicate that the compound is a dietary essential. The vitamins, and their principal functions and deficiency signs, are shown in Table 8.1; the curious nomenclature is a consequence of the way in which they were discovered at the beginning of the twentieth century. Early studies showed that there was some-thing in milk that was es...

  Vitamin A Vitamin A was the first vitamin to be discovered, ini-tially as an essential dietary factor for growth. It has a role in vision, as the prosthetic group of the light-sensitive proteins in the retina, and a major role in the regulation of gene expression and tissue differentia-tion. Deficiency is a major public health problem in large areas of the world, and prevention of vitamin A deficiency is one of the three micronutrient priorities of the World Health Organization (WHO) (the other two are iron and iodine). Vitamers and international units Two groups of compounds, shown in Figure 8.1, have vitamin A activity: retinol, retinaldehyde, and retinoic acid (preformed vitamin A); and a variety of caro-tenes and related compounds (collectively known as carotenoids) that can be cleaved oxidatively to yield retinaldehyde, and hence retinol and retinoic acid. Those carotenoids that can be cleaved to yield retinal-dehyde are known as provitamin A carotenoids.   Preformed vi...