Thanks for the suggestion - I checked the MTHFR website, and couldn't find a specific explanation for "why". But, the first three things in Dr. Lynch's "protocol" for treating the MTHFR mutation are:

1. Avoid fortified foods with folic acid
2. Avoid supplements with folic acid
3. Avoid folic acid blocking drugs

Seems pretty clear that he doesn't feel folic acid is a good thing if you have the MTHFR mutation.