5-Methyltetrahydrofolic acid- 13 C 5 is the 13 C-labeled 5-Methyltetrahydrofolic acid. 5-Methyltetrahydrofolic acid (5-Methyl THF) is a biologically active form of folic acid. 5-Methyltetrahydrofolic acid is a methylated derivate of tetrahydrofolate. 5-Methyltetrahydrofolic acid is the predominant natural dietary folate and the principal form of folate in plasma and cerebrospinal fluid.