This potent molecule is the primary bioactive form of folate which is utilized by your body at the cellular level. It is also the form found in blood circulation which can be transported across cell membranes into tissues. Methylfolate rapidly improves endothelial function by increasing nitric oxide bioavailability.
By comparison, regular folic acid (vitamin B9) must undergo a slow enzymatic alteration in order to become bioactive, thus delaying peak onset. Only Methylfolate achieves peak plasma concentration rapidly, working to maximize blood vessel expansion where you need it most.
C. Antoniades et al., “5-Methyltetrahydrofolate rapidly improves endothelial function and decreases superoxide production in human vessels: Effects on vascular tetrahydrobiopterin availability and endothelial nitric oxide synthase coupling,” Circulation, 2006.
R. Van Etten, E. De Koning, M. Verhaar, C. Gaillard, and T. Rabelink, “Impaired NO-dependent vasodilation in patients with Type II (non-insulin-dependent) diabetes mellitus is restored by acute administration of folate,” Diabetologia, 2002.