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
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).
Table 149-1 Vitamin B6 and
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Table 149-1 Vitamin B6 and
|Vitamin||Defect (Mechanisms)||Disorder and Clinical Condition||Diagnostic Tests||Effective Dose|
|Thiamine(Vitamin B1)||Defective intake||Total parenteral nutrition without B1 supplement1,2||High plasma lactate||2–4 mg/d (20 mg in emergency)|
|Breast-fed babies from mothers who eat B1-deficient
food; beriberi; Gayet-Wernicke encephalopathy||Urinary
alpha-keto acids (DNPH +)|
|Low transketolase in erythrocytes|
|Defective binding of coenzyme (TPP) to enzyme
or role of TPP in assisting protein folding: chaperone molecule
?||Maple syrup urine disease (branched-chain α-keto
acid dehydrogenase (see Section 156)||High leucine, isoleucine, valine, alloisoleucine||20–50 mg/d|
|Pyruvate dehydrogenase4||High lactate, pyruvate; L/P Nl||20–50 mg/d|
|Thiamine transporter THTR-15||Thiamine-responsive anemia with diabetes and deafness||Megaloblastic anemia||20–50 mg/d|
|No biochemical marker|
|Mitochondrial thiamine||Amish microcephaly||No biochemical marker||Not responsive|
|Pyrophosphate transporter6||DNA testing|
|Pyridoxine (VitaminB6)||Defective intake or malabsorption of B6 vitamers||Celiac disease and other malabsorption syndromes||Xanthurenic acid in urines||8 μg/kg|
|Dietary deficiency of vitamin B67||High threonine, glycine, and serine levels in plasma|
|Vitamin metabolism defects: defective metabolism
of pyridoxine, pyridoxal, pyridox(am)ine to its active metabolic
form, pyridoxal phosphate||Pyridox(am)ine phosphate oxidase defect...|
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