Optimize
← All biomarkers
B-VitaminsVitaminsAnemia

Vitamin B12

Vitamin B12 is a B vitamin your body needs for your nerves, DNA production, and blood formation.

What is Vitamin B12?

Vitamin B12 is a water-soluble vitamin your body needs to produce red blood cells, build DNA, and keep your nervous system working. It comes almost entirely from animal products — meat, fish, eggs, and dairy. B12 is unusual in that you need a stomach enzyme to absorb it. When that mechanism is disrupted — by age, acid-reducing medication, or stomach surgery — absorption falls even on a B12-rich diet. The standard blood test measures total B12, but that can mislead: methylmalonic acid (MMA) is a more sensitive measure of a genuine deficiency at the cellular level.

Why is Vitamin B12 relevant?

A B12 deficiency develops slowly, because your liver holds a large reserve. But once nerve damage sets in, it can be partly permanent before you notice it. Measuring early — even without symptoms — makes a real difference in at-risk groups. Those groups are clear: vegans and vegetarians (little dietary B12), older adults (reduced absorption), and people on long-term acid-reducing medication or metformin. B12 also links to homocysteine: a deficiency raises homocysteine, which has been associated with heart and vascular risk. So always read the two together.

Vitamin B12 high or low — what it means

Reference ranges vary between labs. A borderline result (roughly 150–300 pmol/L) always deserves additional assessment. When in doubt, add MMA and homocysteine: a raised MMA is strong evidence of a genuine cellular deficiency, even when your total B12 still looks normal. With symptoms such as tingling, unexplained fatigue, or memory problems — especially in a risk group — don't rely on total B12 alone. Note: after starting supplementation or injections, your serum B12 shoots up even if the cellular deficiency hasn't recovered yet. So re-test MMA or homocysteine after three to six months for an honest picture. Oral B12 works well for most people, provided the dose is high enough; with severe absorption problems, injections are more reliable.

Vitamin B12 reference ranges

DeficiencyBelow this almost always a B12 deficiency; start treatment< 148 pmol/L
Gray zoneDeficiency not excluded; add MMA and homocysteine if symptomatic148–250 pmol/L
NormalDeficiency unlikely with a normal total B12> 250 pmol/L
MMA (functional)Lab upper limit is around 350 nmol/L; a clearly raised MMA (> 450 nmol/L) fits a cellular-level deficiency, even with 'normal' B12> 450 nmol/L

Cut-offs vary by lab and method. The Dutch NHG uses 148 pmol/L (practically 150) as the lower limit and 148–250 pmol/L as the gray zone. For MMA the lab reference is around 0.35 µmol/L (350 nmol/L); values above ~450 nmol/L fit a functional deficiency. Values are the same for men and women and decline gradually with age.

Educational information only — not medical advice. Consult a healthcare professional for clinical decisions.

Read about our scientific approach →Read the guide: Energy →

Frequently asked questions

What is a normal vitamin B12 level?

In the Netherlands the rough guide is: below 148 pmol/L there is almost always a deficiency, between 148 and 250 pmol/L you are in the 'gray zone' where a deficiency cannot be excluded, and above 250 pmol/L a deficiency is unlikely. Exact cut-offs vary by lab. Values are the same for men and women and decline gradually with age.

What does a low vitamin B12 mean?

A value below 148 pmol/L almost always indicates a deficiency, which can cause fatigue, tingling, memory or concentration problems, and anemia. With a value in the gray zone (148–250 pmol/L) plus symptoms, methylmalonic acid (MMA) gives clarity: the lab upper limit is around 350 nmol/L, and a clearly raised MMA (above roughly 450 nmol/L) confirms a genuine cellular deficiency, even when your total B12 still looks normal.

When is a B12 level concerning?

A value below 148 pmol/L is concerning and warrants treatment, especially with symptoms such as tingling or unexplained fatigue. A value in the gray zone (148–250 pmol/L) in someone from an at-risk group — vegan, vegetarian, older adult, or user of acid-reducing medication or metformin — also deserves follow-up with MMA and homocysteine. Early detection helps prevent permanent nerve damage.

Is a high vitamin B12 harmful too?

For B12 what matters is almost entirely the low end: a deficiency is the problem, not a high value. A 'high' total B12 can be misleading, though — after starting supplementation or injections your serum B12 shoots up while the cellular deficiency hasn't recovered yet. So re-test MMA or homocysteine only after about three months (and at most six months) for an honest picture.

Why doesn't my B12 level match my symptoms?

The total-B12 blood test doesn't always reflect what's available inside your cells. Some people have a 'normal' B12 yet still a functional deficiency; that's why MMA and homocysteine are measured when in doubt. Note: after starting supplementation or injections your serum B12 shoots up while the cellular deficiency hasn't recovered yet — so re-test MMA or homocysteine only after about three months (and at most six months).

Related biomarkers

Folate (vitamin B9)
Folate is a B vitamin your body needs for DNA synthesis, cell division and blood production.
Homocysteine
Homocysteine is an amino acid your body clears with B-vitamins, rising when folate or B12 runs short.
Methylmalonic Acid (MMA)
Methylmalonic acid (MMA) is a metabolite that builds up when your cells run short on vitamin B12.
Erythrocytes (RBC Count)
Erythrocytes are your red blood cells, and the RBC count measures them per volume of blood to gauge oxygen transport.
Ferritin
Ferritin is the protein that stores iron in your cells, making it the most reliable blood test for your iron stores.
Hematocrit
Hematocrit is the percentage of your blood made up of red blood cells, your measure of oxygen-carrying capacity.

Vitamin B12 is one of the biomarkers in the Optimize blood test. Book a blood draw at any of 238+ partner labs in the Netherlands, or upload your existing results in the app.

See the full test