What this means in practice
Nothing changes for someone taking 400 micrograms of folic acid before and during a pregnancy. What changes is who gets B12 checked rather than assumed: people over about 60, vegans and vegetarians, people taking metformin, people with gut conditions that block absorption, and anyone with unexplained numbness, tingling or memory trouble. The practical point is that normal-sized red blood cells no longer rule a B12 deficiency out.
What people say
Folic acid masks B12 deficiency, so fortified flour and folic acid supplements are quietly causing nerve damage in people who will never find out they are deficient.
What the evidence supports
The masking itself is established and was seen directly: in the 1940s and 1950s, folic acid above 5 mg a day was used to treat the anaemia of pernicious anaemia, the anaemia lifted, and the neurological damage carried on. Folic acid corrects the anaemia while methylmalonic acid stays elevated. At today's doses the evidence is both weaker and different in kind. People with low B12 and high folate score lower on cognitive tests and show higher homocysteine and methylmalonic acid than people with low B12 and normal folate, and among American seniors in the low-B12 group, detectable unmetabolized folic acid went with lower cognitive scores and a smaller red cell size. All of that is association measured at one point in time, and the depletion mechanism proposed to explain it has not been tested.
How it is supposed to work
Both vitamins are needed to build red blood cells, so enough folic acid can keep them forming normally even when B12 is missing. The oversized red cells that would have prompted a blood test never appear, while the separate job B12 does in the nervous system goes on failing unnoticed. A newer proposal adds that high folic acid depletes the carrier protein that delivers B12 into cells, which would make the deficiency worse rather than merely invisible.
Which way it runs
This runs one way. Folic acid can hide the blood sign of a vitamin B12 deficiency; vitamin B12 hides nothing about folate. Treating it as a mutual interaction gets the risk backwards, because the person at risk is the one short of B12, not the one short of folate.
What is still missing
The convincing evidence comes from a dose about ten times a modern tablet, given as a treatment to people who already had the deficiency. The evidence about fortified flour and ordinary supplements is correlational, its mechanism is still a hypothesis, and the authors reviewing it say so. That is why the concern has changed who gets tested for B12 without changing any country's fortification policy.
Sources
- Excess Folic Acid and Vitamin B12 Deficiency: Clinical Implications?Miller JW, Smith A, Troen AM, Mason JB, Jacques PF, Selhub J, 2024
- Circulating unmetabolized folic acid and 5-methyltetrahydrofolate in relation to anemia, macrocytosis, and cognitive test performance in American seniorsMorris MS, Jacques PF, Rosenberg IH, Selhub J, 2010
- Folic Acid (StatPearls)Merrell BJ and Patel P, 2026 (last updated 19 June 2026)