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DRI Dietary Reference Intakes: For Thiamin, Riboflavin, Niacin, Vitamin B6, Folate, Vitamin B12, Pantothenic Acid, Biotin, and Choline
(TPP). TPP, the coenzymatic form of thiamin, is involved in two main types of metabolic reactions: decarboxylation of α-ketoacids (e.g., pyruvate, α-ketoglutarate, and branched-chain keto acids) and transketolation (e.g., among hexose and pentose phosphates).
Physiology of Absorption, Metabolism, and Excretion
Following ingestion, absorption of thiamin occurs mainly in the jejunum, at lower concentrations as an active, carrier-mediated system involving phosphorylation and at higher concentrations by passive diffusion. Thiamin is transported in blood both in erythrocytes and plasma.
Only a small percentage of a high dose of thiamin is absorbed, and elevated serum values result in active urinary excretion of the vitamin (Davis et al., 1984). After an oral dose of thiamin, peak excretion occurs in about 2 hours, and excretion is nearly complete after 4 hours (Levy and Hewitt, 1971; Morrison and Campbell, 1960). In a study by Davis and colleagues (1984), a 10-mg oral dose of thiamin was given in water, and the mean serum thiamin peaked at 24 nmol/L (7.2 µg/L) —42 percent above baseline. Within 6 hours the serum thiamin concentration had returned to baseline, 17 nmol/L (5.2 µg/L). Prompt urinary excretion of thiamin was also reported by Najjar and Holt (1940) and McAlpine and Hills (1941).
With higher pharmacological levels, namely repetitive 250-mg amounts taken orally and 500 mg given intramuscularly, nearly 1 week was required for steady state plasma concentrations to be reached; a mean elimination half-life of 1.8 days was estimated (Royer-Morrot et al., 1992).
Total thiamin content of the adult human has been estimated to be approximately 30 mg, and the biological half-life of the vitamin is probably in the range of 9 to 18 days (Ariaey-Nejad et al., 1970).
Clinical Effects of Inadequate Intake
Early stages of thiamin deficiency may be accompanied by non-specific symptoms that may be overlooked or easily misinterpreted (Lonsdale and Shamberger, 1980). The clinical signs of deficiency include anorexia; weight loss; mental changes such as apathy, decrease in short-term memory, confusion, and irritability; muscle weakness; and cardiovascular effects such as an enlarged heart (Horwitt et al., 1948; Inouye and Katsura, 1965; Platt, 1967; Williams et al., 1942; Wilson, 1983). In wet beriberi, edema occurs; in dry