D I Thurnham, University of Ulster, Coleraine, UK
© 2005 Elsevier Ltd. All rights reserved.
Thiamin is a water-soluble vitamin and the structure comprises a pyrimidine and a thiazole ring linked by a methylene bridge (Figure 1). In its metabolically active forms, the hydroxyl group on the thiazole moiety is replaced by one, two, or three phosphate groups to form three phosphorylated coenzymes. A well-nourished human adult body contains approximately 30 mg of thiamin—approximately 80-90% as thiamin diphosphate (TDP), 10% as thiamin triphosphate (TTP), and a small amount of thiamin monophosphate (TMP) and thiamin. Like most water-soluble vitamins, there is no definable store in the body; the only reserves are thiamin coenzymes that are present in most cells in combination with appropriate thiamin-requiring enzymes. The predominant need for thiamin is linked to energy production but there is increasing evidence that thiamin is also needed for additional neurological functions. Thiamin is found in the aleuron layer of cereal grains as well as in animal food products such as liver. Man's desire for high-extraction cereal products in situations in which the diets contained little more than the cereal was a main contributory factor to the scourge of beriberi throughout much of Southeast Asia at the end of nineteenth and beginning of the twentieth century. Thiamin is relatively unstable and destroyed by poor cooking habits, and it is susceptible to degradation in foods that are not stored properly. Thiamin turnover is also quite rapid, and the absence of stores means that a continuous supply of thiamin is required. So thiamin status can be fairly rapidly impaired by factors affecting intake (e.g., vomiting and alcohol abuse) or excessive excretion (e.g., induced by diuretics). Thus, thiamin deficiency is sometimes a problem in pregnancy, in alcohol abuse, and in the elderly. Seasonal outbreaks can also occur in poor developing countries when energy output is high and cereals may have been stored for many months and food supplies are restricted.
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