Intestinal folic acid absorption and the acid microclimate. The effects of compounds relevant to folate malabsorption.

The effect s of various substances and circumstances on folic acid absorption and surface pH were investigated in vitro in rat proximal jejunum. Prior consumption of alcohol and libitum, phenytoin and in vitro methotrexate had no effect on folate absorption. In contrast, ethanol in vitro, low sodium ion containing buffers in vitro and oral methotrexate given prior to experiment reduced absorption. Folate absorption did not correlate with water movement since methotrexate decreased folate but enhanced fluid absorption, ruling out direct coupling. Surface pH was elevated by the low sodium ion containing buffers, ethanol in vitro and oral methotrexate, correlating inversely with folate absorption. The parallelisms between absorption and changes in surface pH support the microclimate hypothesis of folic acid absorption where the rate of transport is largely determined by the surface pH of the proximal jejunum.