Please note that folic acid is not a natural form of folate and while we can convert it into a useable form, the process is not without problems. Folic acid is processed by the enzyme dihydrohydrofolate reductase (DHFR) in a two step reaction. First folic acid is converted to dihydrofolate (DHF), which is the natural substrate for the enzyme DHFR. Then, in the normal reaction for the enzyme, DHF is conveted to tetrahydrofolate (THF) which can then feed into the folate cycle. As this paper shows, DHFR processes folic acid about 1,300 times as slowly as its natural substrate DHF. Furthermore, there is an approximately 5 fold variation in activity of the enzyme among individuals. So everyone processes folic acid very slowly and some people do it extremely slowly. This means that even modest doses of folic acid can readily accumulate. As the paper shows, folic acid at even modest levels saturates the DHFR enzyme and inhibits its activity. This means that in the presence of folic acid, the natural substrate DHF accumulates. DHF in turn inhibits several other folate cycle enzymes, including MTHFR (which provides folate for B12 dependant methionine synthase) and thymidylate synthase. Thus accumulating folic acid can interfere in the wider folate cycle. The five-fold variation in natural DHFR activity means that the extent of interference will vary among individuals and of course, the higher the folic acid dose, the more likely interference will occur. So it seems wise to avoid folic acid and stick to natural folate supplements (methylfolate and/or folinic). The linked paper canvases reasons for the variable response to B12 among patients. One they don't consider is the capacity of folic acid to interfere in the folate cycle and hence in B12 dependant enzymes.