Folic acid and vitamin B12: the overlooked gap
Futures Nutrition Editorial Team · 12 August 2026
Folic acid and vitamin B12: the overlooked gap
The short answer: folic acid does not create a vitamin B12 deficiency. It can, however, make its most conspicuous sign — the anaemia with enlarged red blood cells — disappear again, while the neurological side of the deficiency carries on. The German Federal Institute for Risk Assessment (BfR) puts it like this: at folic acid intakes above 1,000 µg, neurological symptoms of a possibly existing vitamin B12 deficiency can be masked.
The practical question is therefore not "folic acid, yes or no". It is: do I belong to the groups whose B12 supply is uncertain anyway — and if so, does that get settled before permanently high-dose folic acid is added on top? For the vast majority the answer is unspectacular, and anyone who takes both together does not have the problem in the first place.
Why it is these two of all things that hang together
Vitamin B12 works as the coenzyme methylcobalamin in the conversion of homocysteine into methionine. The BfR names one prerequisite for this that goes to the heart of the matter: alongside B12, vitamin B6 and folate must also be present in sufficient quantity. If B12 is missing, the folate in the cell stays stuck in its methyl form and is not available for building new DNA.
In the bone marrow a B12 deficiency therefore looks like a folate deficiency — the same disturbed cell division, the same anaemia. If plenty of folic acid is added, the blocked step is bypassed, blood formation normalises and the finding is unremarkable. The second, B12-dependent metabolic pathway is untouched by this: without B12, methylmalonic acid accumulates, and the neurological abnormalities hang on that. The full derivation, including the figures behind the upper limit, is set out in Vitamin B12 and folic acid: why the gap gets hidden.
Important for placing this correctly: here folic acid is not the cause, it is the fog. It fixes part of the consequences and makes the diagnosis harder — it does not create the deficiency.
From what amount the question becomes practical
| Intake of synthetic folic acid | Where it usually comes from | Classification |
|---|---|---|
| 200 µg | basic preparation, 100 % NRV | BfR proposed maximum per daily portion (opinion 009/2024) |
| 400 µg | standard strength when trying to conceive | expressly recommended; exempted from the 200 µg proposal |
| 800 µg | preparations for a late start in pregnancy | below the upper limit |
| 1,000 µg | high-dose single preparations | exactly on the upper limit (UL) for adults |
| 200 µg on top | 2 g of fortified table salt | rarely counted in |
| 5,000 µg | the old case reports on masking | starting point of the derivation, safety factor 5 |
The tolerable total intake of 1,000 µg per day applies to adults including pregnant and breastfeeding women and refers exclusively to synthetic folic acid from preparations and fortified foods. For folate from ordinary foods there is no upper limit — a high intake of natural folates is, in the assessment of the German Nutrition Society (DGE), not harmful. How the reference values of 200, 300 and 400 µg relate to one another is set out in Folic acid daily requirement: 200, 300 or 400 µg?.
What counts is the sum, not the individual tablet. Anyone taking a 1,000 µg preparation, eating fortified breakfast cereals on top of it and cooking with folic-acid-fortified salt is arithmetically above the upper limit without ever having swallowed a second tablet.
Who should settle the B12 question first
In its questions and answers on vitamin B12 (as at 18 October 2023) the BfR names three groups with an increased risk of an inadequate supply: people who eat vegan, older people, and people with certain stomach or bowel diseases such as atrophic gastritis, Crohn's disease or ulcerative colitis. Added to these are medicines that inhibit absorption in the gut — proton pump inhibitors, H2 receptor blockers, cholestyramine, various antibiotics and metformin.
| Constellation | Why B12 can run short here |
|---|---|
| Vegan diet | B12 occurs in relevant amounts only in animal foods; the BfR recommends securing the supply through preparations and having it checked medically on a regular basis |
| Older age | atrophic gastritis is more common, less stomach acid, less B12 released from dietary protein |
| Atrophic gastritis, Crohn's disease, ulcerative colitis | disturbed release or absorption in the gastrointestinal tract |
| Stomach or bowel surgery | missing intrinsic factor or missing absorption stretch in the ileum |
| Acid blockers (PPIs, H2 blockers) | less stomach acid, the same problem as with gastritis |
| Metformin | the British medicines regulator MHRA classified lowered B12 levels as a common side effect in 2022 and recommends monitoring in patients at risk |
Anyone who appears in none of these rows — who eats a mixed diet, has none of the named diseases and takes none of the named medicines permanently — has no cause for concern at usual folic acid amounts. Anyone who meets two rows at once, however — say 70 years old and on an acid blocker for years — should not leave the B12 question to chance, quite independently of the folic acid.
Which values still say something under folic acid
This is the most practically useful part, because it often gets muddled. Not every laboratory value loses its meaning under folic acid:
- The full blood count becomes unreliable. That is precisely the masking: the mean cell volume of the red blood cells can normalise under plenty of folic acid even though the B12 deficiency persists.
- Homocysteine loses its power to discriminate. The value rises with a deficiency of B12, with a deficiency of folate and with a poor B6 supply — and it falls under folic acid regardless of which of the three vitamins was missing. As a distinguishing feature it is then no longer any use.
- Methylmalonic acid remains informative. It arises in a metabolic pathway in which folate plays no part; it rises with a B12 deficiency and is not pushed down by folic acid. That is why it is the marker that still answers even while folic acid is being taken.
- Holo-transcobalamin measures status, not function. It shows the share of B12 that actually reaches the cells, and it too is untouched by folic acid.
Which marker shows what, in which order the values change and which cut-offs go with them is set out in detail in Spotting a vitamin B12 deficiency: serum, holo-TC, MMA. The DGE recommends there the combination of a status marker and a functional marker — and it is precisely this combination that taking folic acid does not defeat.
What this means for choosing on the shelf
The German working group Folsäure & Gesundheit (Folic Acid & Health), whose members include the BfR, the DGE and the Robert Koch Institute, names in its practice guide the simplest way out of the question: the risk of masking can be countered by giving vitamin B12 and folic acid in combination. For vitamin B12 no upper limit has been derived — the BfR records that neither a NOAEL nor a LOAEL can be derived from the available data.
In practice this means: anyone taking folic acid 1000 µg permanently should not leave the B12 side open — either through a laboratory value or by having B12 run alongside. A high-dose B12 preparation works even when absorption is impaired, because besides the active route via intrinsic factor there is a passive route that takes up about 1 % of the dose: 1,000 µg thus become around 10 µg, and that is above the daily loss of 2 to 6 µg that the EFSA assumes. The details are set out in Taking vitamin B12: timing, stomach acid, metformin.
The strengths and forms stand side by side in the categories folic acid and vitamin B12. Food supplements are not a substitute for a balanced and varied diet and a healthy lifestyle.
Trying to conceive: this changes nothing about the 400 µg
The recommendation for women who want to become pregnant or could become pregnant is untouched by all of this: 400 µg folic acid daily in addition to a folate-rich diet, beginning at the latest four weeks before the start of pregnancy and continuing until the end of the first third. The DGE and the BfR name this amount independently of one another, and it lies clearly below the upper limit. The timing and the periods are set out in Folic acid in pregnancy: 400 µg, from when.
One addition belongs here, because two recommendations overlap: with a vegan diet the B12 supply comes from a preparation anyway — before and during a pregnancy it is therefore not an extra topic but part of the same planning.
The folic acid nobody ordered
In Germany, breakfast cereals, dairy products, soft drinks and table salt, among others, are fortified with folic acid. The order of magnitude is surprising: 2 g of fortified salt — half a teaspoon — deliver 200 µg folic acid, equivalent to 340 µg folate equivalents and thus more than the entire daily reference value for adults. The BfR therefore considers such products unsuitable for improving the folate supply in a targeted way: they lead to uncontrolled increases in intake.
For the B12 question this is the quiet part of the calculation. Anyone wanting to check how much synthetic folic acid actually adds up should count the cereals and the salt shaker too — the difference between folic acid and natural folate is not a detail here but the actual point: Folic acid or folate: where the difference lies.
Under Regulation (EU) No 432/2012, the following claims, among others, are authorised for folate and vitamin B12:
Folate contributes to normal blood formation.
Folate contributes to the reduction of tiredness and fatigue.
Vitamin B12 contributes to normal functioning of the nervous system.
Frequently asked questions
I have been taking 400 µg folic acid for years — do I need to worry? By the BfR's wording, the masking risk relates to intakes above 1,000 µg per day. 400 µg is clearly below that and is exactly the amount recommended when trying to conceive. The question becomes relevant when fortified foods and further preparations come on top of the tablet — or when one of the risk constellations for a B12 deficiency applies.
Is a normal blood count enough to give the all-clear? Not if high-dose folic acid is being taken permanently — that is the core of the masking. An unremarkable blood count then does not rule out a B12 deficiency. Methylmalonic acid and holo-transcobalamin remain informative.
Are combination preparations with folic acid and B12 the better choice? They take away the basis for masking, because both vitamins arrive at the same time. Whether a combination preparation or two single preparations make more sense depends on the amounts: many combination products contain B12 in low dosages that need not be sufficient when absorption is impaired. Anyone who absorbs less well for one of the reasons named is more reliably served by a high-dose single B12 tablet.
Does folic acid cause a vitamin B12 deficiency? No. It is involved in no step that consumes B12 or hinders its absorption. What it changes is visibility: it corrects blood formation, which would otherwise be the first thing to stand out. The deficiency arises independently of it — through the diet, through the gastrointestinal tract or through medicines.
