Absorption of folic acid from the small intestine of the rat.

The absorption of folic acid from segments of the small intestine of the rat has been measured in situ with tritium-labelled folic acid. The fraction absorbed was independent of concentration below 10(-6) M but was depressed to half at 4x10(-5) M. Direct measurements of fluxes showed that the mucosal --> serosal flux was about 14 times the serosal --> mucosal flux, and therefore that uptake of folic acid is an active process. In the ileum but not in the duodeno-jejunum, absorption was depressed by the presence of electrolyte. There was little difference in absorptive capacity between jejunum and ileum nor was there any significant change in animals suffering from a dietary deficiency of folic acid.