Site and extent of thiamin absorption in the rat.

The site and extent of absorption of thiamin hydrochloride was determined in rats using 51Cr-EDTA as a non-absorbed reference substance. The main site of absorption of thiamin was found to be the proximal 22 cm of the small intestine with relatively little absorption occurring distally, particularly at low dietary thiamine levels. Concentration of thiamin in the intestinal contents increased along the intestine reaching maximum levels in the distal small intestine. The results are compatible with two concurrent mechanisms of thiamin absorption with the active absorptive process being saturated at 0.5 to 1.0 muM.