Vitamin B12 (Cobalamin)
Water-soluble vitamin for the nervous system, DNA synthesis, and blood formation
Also known as
Definition
Vitamin B12 (cobalamin) is an essential water-soluble vitamin found exclusively in animal foods. It is indispensable for myelination of nerve fibers, DNA synthesis, and the formation of red blood cells. Vegans, older adults, and individuals with stomach problems (intrinsic factor deficiency) are particularly at risk. Deficiency develops insidiously over months to years, as body stores are large.
| Parameter | Value |
|---|---|
| Unit | pmol/l (pg/ml) |
| Reference Range | 148–664 pmol/l (200–900 pg/ml) |
| Optimal Range (lab2go) | > 300 pmol/l (lab2go Optimum Zone; > 400 pmol/l with neurological symptoms or elevated homocysteine) |
Conversion: 1 pg/ml ≈ 0.738 pmol/l. US labs often use pg/ml (normal value 200–900 pg/ml).
↓ What a low value means
A low B12 level indicates inadequate intake (vegan/vegetarian), absorption disorder (intrinsic factor deficiency, atrophic gastritis, metformin use), or increased demand. Consequences: megaloblastic anemia, peripheral neuropathy, cognitive impairment, elevated homocysteine and methylmalonic acid.
↑ What a high value means
Very high B12 values (> 1000 pmol/l) without supplementation may indicate liver disease, myeloproliferative disorders (e.g., polycythemia vera), or solid tumors. With known supplementation, elevated values are generally harmless.
✓ How to optimize this marker
Animal sources: liver, shellfish, meat, eggs, dairy. With vegan diet: supplement with methylcobalamin or cyanocobalamin 500–1000 µg daily or 2000 µg weekly (NIH recommendation). With absorption disorders: intramuscular injections or high-dose oral supplementation (1000–2000 µg/day, since ~1% is passively absorbed).
When to test
Annually with vegan/vegetarian diet, metformin use, long-term PPI therapy, or from age 60. Immediately with neurological symptoms. Holotranscobalamin (active B12) is a more sensitive early marker than total B12.
Frequently asked questions
Which B12 value is truly optimal and not just 'sufficient'? +
The laboratory cutoff (> 148 pmol/l) merely prevents clinical deficiency symptoms. For optimal neurological function and low homocysteine, lab2go recommends values > 300 pmol/l. Studies show that cognitive decline can begin at 'normal' values below 250 pmol/l.
Which form of B12 is best for supplementation? +
Methylcobalamin is the active form and is especially recommended for individuals with MTHFR polymorphisms. Cyanocobalamin is less expensive, well-researched, and equally effective for most people. Adenosylcobalamin complements for mitochondrial function.
How long does it take to correct a B12 deficiency? +
With high-dose oral supplementation (1000 µg/day), blood values normalize within 4–8 weeks. Neurological symptoms may take months to resolve. The body can draw on B12 stored in the liver and muscles for ~3–5 years before deficiency becomes clinically visible.
Is it true that metformin depletes B12? +
Yes. Metformin inhibits vitamin B12 absorption in the ileum through calcium-dependent mechanisms. Studies show that up to 30% of metformin users develop a B12 decline. Annual B12 monitoring is recommended for all metformin users.
Sources
Last Reviewed: May 28, 2026 · sina
This information is for orientation only and does not replace medical advice. Reference ranges can vary by laboratory, method and country.