Absorption and Transport of Thiamin
The bioavailability of thiamin occurring naturally in foods is believed to be high. Foods containing the highest concentration of thiamin are listed below:
Yeasts
Pork
Sunflower seeds
Legumes
Occasionally, however, anti-thiamin factor may be present in the diet. For example, thiaminases present in the raw fish catalyze the cleavage of thiamin, thereby destroying its activity.
These thiaminases are thermolabile, however and cooking of fish rendered the enzymes inactive. Other anti-thiamin factors that are thermostable may be found in tea and certain fruits and vegetables such as blueberries, black currents, Brussels sprouts and red cabbage.
Absorption of thiamin can be both active and passive, depending upon the amount of the vitamin presented for absorption.At low physiologic concentrations, thiamin absorption is an active process. This Na+ dependent, carrier mediated absorption occurs primarily in the jejunum but can occur in other portions of the small intestine as well.
When intakes of thiamin are high, the absorption route is predominantly passive. The rate of thiamin absorption is always quite high except in the case of ethanol ingestion and/or folate deficiency.
Ethanol ingestion interferes with active transport of thiamin, and folate deficiency prevents the normal duplication of enterocytes, thereby decreasing absorption, both active and passive.
Within the mucosal cells, thiamin is converted into a phosphate ester, in which form it move into the plasma. To be active as a coenzyme, thiamin must be converted to its pyrophosphate (diphosphate) form.
Conversion to the active coenzyme form requires adenosine triphosphate (ATP) and thiamin pyrophosphokinase, an enzyme found in the liver and brain (and perhaps in other tissue as well).
Another form of thiamin (thiamin triphosphate, or ATP) is synthesized in the brain by action of a thiamin diphosphate (ADP) – ATP phosphoryl-transferase.
Absorption and Transport of Thiamin
