5-Methyltetrahydrofolic acid 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[1].
[1]. Wright AJ, et al. Comparison of (6 S)-5-methyltetrahydrofolic acid v. folic acid as the reference folate in longer-term human dietary intervention studies assessing the relative bioavailability of natural food folates: comparative changes in folate status following a 16-week placebo-controlled study in healthy adults. Br J Nutr. 2010 Mar;103(5):724-9.