http://forums.phoenixrising.me/index.php?threads/mthfr-folate-usage-and-blocking.28229/#post-430132
I adopted the "blocking" idea from folic acid research dating back to the 60s. I don't know if that is the actual cause. All I can tell you for sure is that inflammation and histamine responses increase tremendously when I eat too much folate containing vegetable matter, folic acid and folinic acid. I don't deprive myself of any vegetables, just eat somewhat fewer servings or smaller servings. Tonight and the past few days I have been enjoyiong fresh asparagus. NAC and other glutathione promoting items cause the most severe results with large visible amounts of b12 being flushed out in my urine within hours, with sudden onset of obvious methyltrap (a completely different mechanism than folic acid or folinic acid or veggies) inflammation starting within hours, acne, IBS, angular cheilitis starting in a day or two and allergic responses, asthma, widespread inflammatory pain worsening day by day and within a couple of weeks, MCS is getting started and not responsive to anything except discontinuing NAC/glutathione promoters, folic acid (takes days to have an effect, in theory time for unconverted folic acid to build up to the "blockage" point, the hypothesis of many researchers and clears in about 24 hours allowing more normal effectiveness of Metafolin) and folinic acid (which builds up quicker, blocks in 24 hours and takes a couple of days to clear). The difference between folic and folinic acid could be entirely because of serum clearance half life difference. To break the methyltrap took some sizeable doses of AdoCbl and MeCbl, causing 4 distinct startup responses all over again, the only time besides initial startup for each at both levels (4 in all). It may not be tied to any polymorphisms at all because 50% can convert to a biological maximum of 800-1000mcg daily, 30% less than that but some conversion and 20% no conversion at all. Those probably represent some polymorphisms however, 50% can still have a buildup if they take more than minimum doses. So which 50% is "normal"? It looks like all can be affected on the folic acid. I have no idea what percentage are represented in the folinic acid equivalent groups or veggie folate groups.
You may be asking a question to which you will get answers of all kinds. It may not be a question with a nice neat single answer. I would be inclined to bet on that option as these differences have been known since the 60s, long before the polymorphisms were even discoverable. I would be interested in seeing the results.