Utilization of intravenously infused thiamin hydrochloride in healthy adult males

The utilization of intravenously administered thiamin hydrochloride in humans was assessed. One hundred mg (0.297 mmol) of thiamin hydrochloride was given by continuous infusion to 10 healthy males. The concentration versus time curves of thiamin and its phosphate esters in blood plasma, erythrocytes and urine were analyzed by high-performance liquid chromatography over a period of 240 hours. The phosphorylated forms of thiamin were assessed as total phosphates. By the end of infusion, non-phosphorylated thiamin reached its maximal plasma concentration (1106.5 nmol/l). Elimination of free thiamin followed a tri-exponential function with disposition rate constants of 4.56, 0.56, and 0.01 per h. Low concentrations of thiamin phosphates (Conc(max) = 71.5 nmol/l) were observed in blood plasma (t(max) = 6.3 h). Free thiamin but no phosphorylated metabolites were recovered in urine (0.213 mmol). In spite of rapid renal thiamin excretion, a large proportion of the infused vitamin was accumulated and metabolized to thiamin phosphates in erythrocytes (Conc(max) = 631.8 nmol/l; t(max) = 6.5 h). Rate constants of absorption (k(a) = 0.224 per h) and disposition (k(el) = 0.074 per h) were calculated for erythrocytes.