Paul, I diagnoised myself with a functional B12 deficiency.

My B12 was well within normal - 750, but my homocysteine was elevated, originally 13.5-ish. Supplementing with oral cyanocobalamin only resulted in a small decine into the mid 12's.

But starting mega doses of methycobalamin brought it down quickly into the normal range within a week. I just had more testing of this today and won't know the new levels for a few days.

I think that there are who have normal B12's above the median value, who are functionally deficient of the neurologically active form, methyl-B12 and have no clue, and their doctors have no clue either.