Thanks Bluesfan. I was aware of the B12/folate thing but have never heard of folate being re-methylated and presumably recycled in the system. I will watch out for that kind of thing. I have some B12 in case but so far no problems.

I guess if you're somewhat deficient (at least in the brain) it may take a little time for these things to come forward.

B vitamins are quite complicated. The interplay, the conversion, FAD, the co-factors .. I think I study it for a week and still not quite fully understand it all.