 After digestion, food folates are converted in the small intestine to 5-methltetrahydrofolic acid, a biologically active vitamer of vitamin B9.Folic acid is absorbed and transported in the bloodstream to the liver, where it is converted to tetrahydrofolate, a second biologically active vitamer, by dihydrofolate reductase.The liver has a limited capacity to metabolize folic acid into tetrahydrofolate.