Vitamin B12 and folic acid: why the gap gets hidden
Futures Nutrition Editorial Team · 12 August 2026
Vitamin B12 and folic acid: why the gap gets hidden
The short answer: the two are named together because they cooperate in the same metabolic step — and because plenty of folic acid makes the most conspicuous trace of a vitamin B12 deficiency disappear again, while the neurological side carries on. This is precisely why folic acid has an upper limit at all: in 2023 the EFSA once again based it on the risk that neurological symptoms continue to develop under folic acid in someone with a cobalamin deficiency.
For shopping this does not mean "hands off folic acid". It means: anyone taking high-dose folic acid should know where they stand on vitamin B12. Both vitamins are cheap, both are widely available — only the order of the questions matters.
One enzyme, two vitamins
Folate arrives in the cell from the blood as 5-methyltetrahydrofolate (5-MTHF). There it is initially of no use at all: it is only taken up into the cellular folate pool once it has passed the enzyme methionine synthase — and this enzyme needs vitamin B12 as a cofactor. The EFSA describes the step in exactly this way in its assessment of the folic acid upper limit: the methyl group is removed and transferred to homocysteine, methionine is formed, and the tetrahydrofolate left behind becomes available again for purine and thymidine synthesis — that is, for building new DNA.
The German Federal Institute for Risk Assessment (BfR) states the same dependence from the other side: as the coenzyme methylcobalamin, vitamin B12 supports the remethylation of homocysteine to methionine, and the prerequisite for this is that vitamin B6 and folate are present in sufficient quantity alongside B12. If B12 is missing, homocysteine in the blood rises.
From this follows the peculiarity on which everything else turns: if vitamin B12 is missing, the folate in the cell is stuck in its methyl form. It is there, it is measurable in the blood — but it does not arrive where DNA is built. A B12 deficiency therefore produces in the bone marrow exactly the picture of a folate deficiency: disturbed cell division in a tissue that divides constantly, and from that an anaemia with conspicuously large red blood cells.
Why plenty of folic acid puts the blood count right again
If enough folic acid is added to this situation, the blocked route is bypassed: folate is available again for DNA synthesis, blood formation normalises, the full blood count looks unremarkable. The second B12-dependent metabolic pathway, however, is untouched by this — the one in which methylmalonic acid accumulates and whose failure explains the neurological abnormalities. This is the process the technical literature calls "masking": the most conspicuous and most easily measured clue disappears, the cause remains.
How seriously the expert bodies take this can be seen from the fact that the entire upper limit for folic acid rests on this single endpoint. The European Commission's former Scientific Committee on Food (SCF) derived in 2000, from case reports, a LOAEL — the lowest intake at which an adverse effect was observed — of 5,000 µg folic acid per day, divided by an uncertainty factor of 5 and thus arrived at 1,000 µg per day for adults. The EFSA panel reviewed this derivation in 2023, found no robust connection for cancer risks or cognitive function, and confirmed the old value: there were no new data that would describe the dose-response relationship any better.
Honesty requires adding that this data base is thin. It consists of old, uncontrolled case series in people with pernicious anaemia; the EFSA itself notes that the B12 storage status and pre-existing neurological changes cannot be factored out of them, and in the technical literature the historical evidence is expressly doubted. The upper limit nevertheless stands — not because the harm is well documented, but because it weighs heavily and the alternative would be to risk it for no good reason.
The figures side by side
| Folate / folic acid | Vitamin B12 | |
|---|---|---|
| Nutrient reference value (NRV) on the label | 200 µg | 2.5 µg |
| DGE reference value, adults | 300 µg folate equivalents | 4.0 µg (estimated value) |
| EFSA reference value, from age 15 | 330 µg folate equivalents | 4.0 µg |
| Pregnant women (DGE) | 550 µg folate equivalents | 4.5 µg |
| Upper limit (UL) for adults | 1,000 µg/day — synthetic intake only | none derived |
| BfR proposed maximum per daily portion | 200 µg (400 µg for products for risk reduction) | 25 µg |
| Common strength on the market | 400 to 1,000 µg | 1,000 µg |
Two rows of this deserve a second look.
The upper limit applies only to synthetic intake. There is none for folate from foods — the German Nutrition Society (DGE) states that a high intake of naturally occurring folates is, according to current knowledge, not harmful. The difference has a chemical reason: folic acid first has to be reduced in the body in two steps, and the enzyme responsible for this, dihydrofolate reductase, works only slowly in the human liver and very differently from person to person. At high intakes it is saturated, and unchanged folic acid appears in the bloodstream — a substance that does not occur there naturally.
For vitamin B12 no upper limit exists. The BfR records that no adverse effects have so far been established from which a NOAEL or LOAEL could be derived. The proposal of 25 µg per daily portion is therefore not a hazard threshold but a calculation from consumption data.
What this means for your own tin
A tablet with 1,000 µg folic acid sits exactly on the upper limit for adults and clearly above what the BfR would like to see as the legal maximum for food supplements. That is neither a grey area nor a reproach against the product — it is information you should have when buying. This applies to folic acid 1,000 µg just as it does to every other high-dose preparation in this group.
Three practical conclusions follow from this:
- The order matters. If a blood count shows conspicuously large red blood cells, the B12 question belongs settled before high-dose folic acid is added permanently.
- Homocysteine alone does not separate the two. The value rises with a deficiency of B12 just as it does with a deficiency of folate — and additionally with a poor B6 supply. Only methylmalonic acid rises with a B12 deficiency alone.
- Anyone supplementing both anyway does not have the problem. A B12 preparation alongside the folic acid removes the basis for masking. The strengths and forms stand side by side in the vitamin B12 category and in the folic acid category.
Trying to conceive: the exception nobody relativises
For all that is said above about the upper limit: for women who want to become pregnant or could become pregnant, a separate and very clear recommendation applies. The DGE and the BfR independently name 400 µg folic acid daily in addition to a folate-rich diet, in order to reduce the risk of neural tube defects in the child. It should be started at the latest four weeks before the beginning of pregnancy and continued until the end of the first third.
The BfR expressly describes this supplementation as the most suitable measure for that purpose and exempts products at this dosage from its own 200 µg proposal — they should then, however, be clearly labelled as such. Incidentally, these 400 µg are still below the upper limit of 1,000 µg, which applies unchanged to pregnant and breastfeeding women.
The authorised claim for this reads: Folate contributes to maternal tissue growth during pregnancy. (Regulation (EU) No 432/2012)
On the plate the two part company
The reason the combination comes up so often in advice also lies in the food itself. Good natural folate sources are, according to the BfR, green vegetables, pulses, wholegrain products and citrus fruits — plus egg yolk and liver. Vitamin B12, by contrast, comes from microorganisms and occurs in relevant amounts only in animal foods: liver, red muscle meat, fish, eggs, dairy products. Conventional plant foods do not contain it.
Two vitamins that cooperate in the same metabolic step therefore come from two different corners of the range. Anyone who cuts out the animal side loses B12 and keeps folate — the one way of eating in which masking can build up as a constellation at all. Which portions deliver how much is set out in Vitamin B12 foods: what one portion delivers.
On top of this comes a German peculiarity that appears in no nutrient table: table salt fortified with folic acid is available in Germany, and the BfR expressly counts it in when deriving maximum levels — at 1 to 2 g of added salt per person per day it can contribute noticeably to folic acid intake. Anyone estimating their own intake should therefore count the salt shaker too.
Among the authorised claims are these: Folate contributes to normal blood formation. Vitamin B12 contributes to normal red blood cell formation. Vitamin B12 contributes to normal functioning of the nervous system. (Regulation (EU) No 432/2012). Food supplements are not a substitute for a balanced and varied diet and a healthy lifestyle.
Frequently asked questions
May I take folic acid and vitamin B12 together? Yes — the combination is precisely the route that rules out masking. The problem is not the two side by side, but high-dose folic acid alone in someone whose B12 supply is unclear. Many combination preparations contain both for exactly this reason.
From what amount of folic acid does one even speak of masking? The old case reports from which the upper limit was derived concerned 5,000 µg per day — five times today's upper limit and more than twelve times the amount recommended for reducing the risk of neural tube defects. The safety margin of a factor of 5 is built in because the data between 1,000 and 5,000 µg are not sufficient for an assessment.
Is folic acid the same as folate? No. Folate is the umbrella term for the naturally occurring compounds, folic acid the synthetic form (pteroylmonoglutamic acid) in preparations and fortified foods. It is absorbed better, which is why the reference values are calculated in folate equivalents — 1 µg folate equivalent corresponds to 1 µg food folate or 0.5 µg folic acid on an empty stomach. The upper limit applies only to the synthetic side.
Why are both often measured when homocysteine is elevated? Because the value does not say which of the vitamins involved is missing. It hangs on the remethylation of homocysteine to methionine, and B12, folate and vitamin B6 are jointly involved in that. Only a second, specific marker — methylmalonic acid — narrows down the B12 share.
