Greg will not like me reproducing this here, but I am simply too lazy to edit it so that it does not appear as a direct quote. Honestly he should be here doing it himself occasionally, but I think he tends not to want to put stuff 'out there' for fear of wasting time in contradictory conversations.
"Going through lots and lots of data, the common thread to many people's problems is low or inactive vitamin B2/riboflavin. Riboflavin is converted to FMN and FAD, which are essential co-factors in many, many enzyme reactions. The relevance for methylation is huge, thus MTHFR, MTRR and Thymidine synthase, all require FAD, so just to get proper folate cycling and methylation need FAD. We know that methylation is a key factor for CFS, etc.
Next there is the use of betaine (TMG) for use in the methylation cycle. The breakdown of BMG and MMG (which come after betaine is stripped of one methyl group) requires FAD, so you cna't completely use betaine if your FAD levels are low.
Then we have the relevance to the synthesis and break-down of neurotransmitters. Critical in this is the formation of BH4, which is dependent upon an active folate cycle. BH4 is required for synthesis of DOPA, serotonin, and also for incorporation into NOS. NOS also requires FAD/FMN and iron. The break-down of neurotransmitters Dopamine, Epinephrine, nor-epinephrine, serotonin and HISTAMINE (in reference to your second link) uses MAO (which the majority of CFS/ME people are +/+ in), which has FAD as an essential co-factor. The intolerance to histamine that is often seen in CFS sufferers is most likely an FAD deficiency (from B2).
FAD is also required in the synthesis of heme, which is an essential part of NOS, peroxidase, myeloperoxidase and SUOX, and FAD is also required directly for SOD."
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boohealth I attempted to understand it myself for sometime, like with the above correspondence but just ended up concluding what I have said here, and just refining it by myself in practise; plus working on other aspects of my rehab I found just as important and realise now play a role in the chemistry as well. But B2 and the other right co-factors, prior to tackling B12 would be the ideal approach IMO.