Reversal by bile acid on the inhibition of alpha-tocopherol absorption by retinoic acid.

This study explores several possible mechanisms by which dietary retinoic acid may cause the previously described reduced intestinal absorption of alpha-tocopherol. Measurement of fecal excretion of rats showed that dietary retinoic acid caused twice as much alpha-tocopherol to be excreted as when retinol was the source of vitamin A. Excretion was the same for free and esterified alpha-tocopherol, thus, the retinoic acid effect originally observed was not due to impaired hydrolysis of the ester. There was no effect of retinoic acid on triglyceride absorption. Collection and analysis of bile from rats fed either form of vitamin A showed no difference in bile volume or bile acid composition. The addition of 0.2% taurocholic acid to the diet, however, reversed the effect of retinoic acid on tocopherol absorption. In vitro studies of mixed micelles containing 3H-alpha-tocopherol and retinoic acid or retinol showed no difference in size due to the form of vitamin A in the micelles.