Absorption and metabolism of thiamin
Thiamin is absorbed in the duodenum and proximal jejunum, and then transferred to the portal circula-tion by an active transport process that is inhibited by alcohol. This may explain why alcoholics are especially susceptible to thiamin deficiency.
Tissues take up both free thiamin and thiamin monophosphate, then phosphorylate them further to yield thiamin diphosphate (the active coenzyme) and, in the nervous system, thiamin triphosphate.
Some free thiamin is excreted in the urine, increas-ing with diuresis, and a significant amount may also be lost in sweat. Most urinary excretion is as thio-chrome, the result of non-enzymic cyclization, as well as a variety of products of side-chain oxidation and ring cleavage.
There is little storage of thiamin in the body, and biochemical signs of deficiency can be observed within a few days of initiating a thiamin-free diet.