Because thiamin has a central role in energy-yielding, and especially carbohydrate, metabolism, require-ments depend mainly on carbohydrate intake, and have been related to “non-fat calories.” In practice, requirements and reference intakes are calculated on the basis of total energy intake, assuming that the average diet provides 40% of energy from fat. For diets that are lower in fat, and hence higher in carbo-hydrate, thiamin requirements may be somewhat higher.
From depletion/repletion studies, an intake of at least 0.2 mg of thiamin/1000 kcal is required to prevent the development of deficiency signs and maintain normal urinary excretion, but an intake of 0.23 mg/1000 kcal is required for a normal transketo-lase activation coefficient .
Reference intakes are calculated on the basis of 100 μg/MJ (0.5 mg/1000 kcal) for adults consuming more than 2000 kcal/day, with a minimum require-ment for people with a low energy intake of 0.8– 1.0 mg/day to allow for metabolism of endogenous substrates.
The impairment of pyruvate dehydrogenase in thiamin deficiency results in a considerable increase in the plasma concentrations of lactate and pyruvate. This has been exploited as a means of assessing thiamin status, by measuring changes in the plasma concentrations of lactate and pyruvate after an oral dose of glucose and mild exercise. The test is not specific for thiamin deficiency since a variety of other conditions can also result in metabolic acidosis. Although it may be useful in depletion/repletion studies, it is little used nowadays in assessment of nutritional status.
Whole blood total thiamin below 150 nmol/l is considered to indicate deficiency. However, the changes observed in depletion studies are small. Even in patients with frank beriberi the total thiamin con-centration in erythrocytes is only 20% lower than normal, so whole blood thiamin is not a sensitive index of status.
Although there are several urinary metabolites of thiamin, a significant proportion is excreted either unchanged or as thiochrome, and therefore the urinary excretion of the vitamin (measured as thio-chrome) can provide information on nutritional status. Excretion decreases proportionally with intake in adequately nourished subjects, but at low intakes there is a threshold below which further reduction in intake has little effect on excretion.
The activation of apo-transketolase in erythrocyte lysate by thiamin diphosphate added in vitro has become the most widely used and accepted index of thiamin nutritional status. Apo-transketolase is unstable both in vivo and in vitro, so problems may arise in the interpretation of results, especially if samples have been stored for any appreciable time. An activation coefficient >1.25 is indicative of deficiency, and <1.15 is considered to reflect adequate thiamin status.
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