Vitamin B1 Thiamin Absorption and Digestion
How the digestion and absorption of thiamin take place
The thiamin in plant foods occurs in a nonphosphorylated (free) form. In animal food, more than 90% of the thiamin occurs in a phosphorylated form, the primary example of which is thiamin diphosphate (TDP), otherwise known as thiamin pyrophosphate (TPP). The remaining phosphorylated thiamin in animal products exists in the form of thiamin monophosphate (TMP) and thiamin triphosphate (TTP).
The bioavailability of thiamin is based in part on the body’s need. The percent absorbed increases when thiamin intake is low and decreases when thiamin intake is high. In the first instance, the cells of the small intestine actively transport thiamin by a carrier-mediated process to the epithelial cells, which requires energy. Otherwise, absorption takes place at a slower rate by simple diffusion
Thiamin is distributed into the aqueous portions of the cell where it serves as an essential cofactor of various enzymes involved in metabolism. 90% of the thiamin circulated in the bloodstream is in the form of TPP that is transported by erythrocytes. However, a small amount of thiamin also appears in the blood as free thiamin and thiamin monophosphate (TMP) bound to albumin. Uptake of thiamin by the individual cells requires that it be in the form of TMP or free thiamin. Thiamin in the more complex form of TPP or TTP is hydrolyzed, as needed for this purpose.
Absorption of thiamin takes place primarily in the jejunum and ilium and to a lesser extent in the duodenum. While the absorption of the thiamin we ingest in our daily diet is ordinarily quite efficient, certain foods interfere with thiamin absorption and act as “anti-thiamin factors” or thiaminases. Some of the culprits are raw fish, coffee, tea, berries, Brussels sprouts, and cabbage. One reason is that they can cause thiamin to oxidize and become inactive. However, the antioxidants from a vitamin C-rich diet can help prevent that from happening.
On the other hand, a folate deficiency inhibits the absorption of thiamin because it interferes with the replication of the enterocytes. Another deterrent to thiamin absorption is alcohol.
Metabolism and Excretion
After thiamin is absorbed, it leaves the small intestine and is transported through the blood to the liver and other parts of the body However, only small amounts of thiamin are actually stored in the liver, as well as the muscles, kidneys, heart, and brain. The liver does not store appreciable amounts of excess thiamin like it does the fat-soluble vitamins. Any free or phosphorylated thiamin beyond what the tissues need or can be stored for later use is either excreted intact or catabolized and then excreted. The primary excretion route is the urine. However, some additional thiamin that is not being used may be lost in sweat as well. In the absence of adequate intake, this steady loss can add up quickly. That is why it is extremely important to include thiamin in the diet on a daily basis.