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Thiamine has long been recognized as an essential component. Its minimal essential requirement is about 0.5 mg/1000 Kcal, which is usually covered by a normal, well-balanced diet. However, requirements are variable and increase in parallel with carbohydrate intake, during pregnancy, lactation, hypermetabolic states, and in infants. Thiamine acts under its phosphorylated form, thiamine pyrophosphate (TPP), which is the coenzyme of pyruvate decarboxylase in the pyruvate dehydrogenase complex, α-ketoglutarate dehydrogenase, and oxidative decarboxylation of the three-branched chain alpha-keto acids. It is also the coenzyme of the transketolase in the pentose phosphate pathway. Being placed at these highly regulated enzymatic steps, thiamine plays a crucial role in carbohydrate metabolism and in the metabolic switch from the fed to the fasting state. Acute thiamine deficiency states (as in total parenteral nutrition without thiamine supplement) are life-threatening emergencies and present as cardiac failure, Gayet-Wernicke encephalopathy, or lactic acidosis.1,2

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Metabolic markers are hyperlactatemia with hyperpyruvicemia and normal lactate-to-pyruvate ratio, slight elevation of branched-chain amino acids in plasma, presence of alpha-keto acids (ketoglutarate, pyruvate, branched-chain keto acids) in urines with a positive dinitrophenylhydrazine (DNPH) reaction, and low transketolase activity in red blood cells. However, these markers are rarely available in an emergency, and diagnosis relies on the primary care or emergency physician recognizing the disorder and administering the lifesaving therapeutic test of thiamine 5 mg/kg per day. There is no risk of adverse effects. Thiamine-dependent inborn errors of metabolism are very rare. They involve the binding of the coenzyme to the enzyme or specific cellular or mitochondrial transporters of thiamine and TPP, respectively (Table 149-1).

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Table 149-1 Vitamin B6 and B1 Disorders 

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