Hi, Rachel.
To convert folic acid to methylfolate, four sequential reactions are required. The first two require NADPH, which is based on vitamin B3, and it requires some additional metabolism to form NADPH, including burning of glucose via the pentose phosphate shunt on the glycolysis chain. Magnesium is also needed. These two reactions convert folic acid to tetrahydrofolate (THF).
Next, a reaction is needed to convert THF to methylene THF, and that is usually done using serine via the SHMT reaction. Then methylene THF must be converted to methylfolate via the MTHFR reaction, which requires vitamin B2 and NADPH.
I don't favor using folic acid to treat ME/CFS, because some people have genetic issues that slow these various reactions, and NADPH is found to be low in many PWMEs. It's preferable to use methylfolate, which avoids this reaction pathway.
Best regards,
Rich