Folic acid is the man-made form of folate. Folate is a B-vitamin naturally found in some foods and our bodies need it to form healthy cells. For it to do its job, the most common form of folate in supplements and fortified foods must be converted to its active form, methylfolate, to be used by the body. But, not everyone is born equal, as far as methylation is concerned. In fact, as much as 70% (in the US) inherit a defective or partially defective gene that lacks the enzyme needed to convert folate, this is the methylenetetrahydrofolate reductase (MTHFR) gene. The problem lies in the fact that almost half of all people have genetic variations that reduce the ability of the MTHFR enzyme to make this conversion.
For people with MTHFR gene variations, supplementing with the already activated form of folate (5-MTHF) is far more effective than introducing the pre-converted form to the body through typical folate supplements. L-Methylfolate (5-MTHF) supplements provide the active form of folate naturally present in the body and available for biological action.
Summary of Dietary, Nutritional, and Medical Treatments for Autism Article from Arizona State University