The warning sign disappears and the cause does not
B12 deficiency shows up first as anaemia, which is what sends someone to a doctor. Folic acid corrects that anaemia on its own while the neurological damage carries on.
What the study found
Folic acid can fix the anaemia while the nerve damage continues
Vitamin B12 deficiency shows up first as anaemia, which is what usually sends someone to a doctor. Folic acid corrects that anaemia on its own, while methylmalonic acid stays high and the neurological damage carries on unchecked. The warning sign goes away and the cause does not.
The caveat · 1940s cases took over 5 mg a day
The clinical cases that established this used folic acid above 5mg a day in the 1940s and 1950s, roughly ten times a modern supplement and far above what fortified flour delivers, so it does not transfer straight to a 400 microgram tablet. The authors of the review say themselves that the modern evidence is correlative or from uncontrolled observation, and that their proposed mechanism has not been tested. They are also the group that proposed it.