Vitamin B12 (Cobalamin)

What is vitamin B12?

Vitamin B12 (cobalamin) is a water-soluble vitamin found almost exclusively in animal-derived foods — meat, fish, eggs, dairy, and to a lesser extent some fortified plant foods. It plays fundamental roles in DNA synthesis, the formation and maintenance of the myelin sheath that insulates nerve fibres, and the production of healthy red blood cells. Without adequate B12, these processes break down.

B12 absorption is a complex process that requires the presence of a glycoprotein called intrinsic factor, produced by specialised cells (parietal cells) in the stomach lining. Intrinsic factor binds dietary B12 and carries it to the terminal ileum, where it is absorbed. Disruption at any point in this pathway — impaired gastric acid production (increasingly common with age and with proton pump inhibitors), lack of intrinsic factor (pernicious anaemia), or ileal disease (Crohn's disease, resection) — leads to deficiency regardless of dietary intake.

Importantly, the body stores approximately 3–5 years' worth of B12 in the liver, which is why deficiency develops slowly and may not become apparent for years after a change in diet or absorption begins. This delayed presentation means deficiency is often diagnosed late, by which time neurological damage may already have occurred.

The blood test measures serum vitamin B12, the total circulating amount. In borderline cases, clinicians may request additional tests — methylmalonic acid (MMA) and homocysteine — which are more sensitive functional markers of B12 deficiency at the cellular level and rise before the serum level falls dramatically.

Normal reference range

• Normal serum B12: 150–700 pmol/L
• Deficiency: below 150 pmol/L
• Grey zone (early depletion possible): 150–200 pmol/L

Important: Reference ranges vary between laboratories and depend on individual factors including age, sex, pregnancy status, and medication. Always interpret your result in the context of a clinician review. The Vesey reviews every result before release.

What low vitamin B12 may indicate

B12 deficiency has diverse causes. Identifying the correct cause determines the appropriate treatment (oral supplements vs. intramuscular injection):

• Pernicious anaemia — autoimmune destruction of parietal cells reduces intrinsic factor production; requires lifelong B12 injections
• Vegan or strict vegetarian diet — B12 is absent from plant foods; supplementation is essential
• Metformin use — the diabetes drug reduces B12 absorption by approximately 30%; NICE recommends B12 monitoring in long-term metformin users
• Gastric surgery — gastrectomy or bariatric surgery removes the site of intrinsic factor production
• Proton pump inhibitor (PPI) use — long-term use reduces gastric acid needed for B12 release from food
• Older age (over 60) — gastric acid production naturally declines; atrophic gastritis is common
• Ileal disease or resection — Crohn's disease affecting the terminal ileum, or surgical resection, removes the absorption site

Symptoms of B12 deficiency include fatigue, tingling or numbness in the hands or feet, balance problems, cognitive difficulties, depression, and megaloblastic anaemia (large, dysfunctional red blood cells causing breathlessness and pallor).

What high vitamin B12 may indicate

Elevated B12 is rarely clinically significant in most people and is commonly due to supplementation. However, very high B12 without obvious supplementation warrants investigation, as it may indicate:

• Liver disease — damaged liver cells release stored B12 into the circulation
• Myeloproliferative disorders — conditions such as polycythaemia vera, chronic myeloid leukaemia, or myelofibrosis are associated with markedly elevated serum B12 due to increased carrier protein production
• Solid tumours — certain malignancies can cause B12 elevation through increased production of B12-binding proteins

Get tested at The Vesey, Sutton Coldfield

Vitamin B12 is measured within the following panels at The Vesey Private Hospital:

• Standard Screen Plus B12 — £189 (comprehensive screen with vitamin B12)
• Nutritional Health panel — B12, folate, vitamin D, ferritin, and more
• Vitamins & Minerals panel — comprehensive micronutrient screen

Results are reviewed by a clinician before secure release, typically within 24 hours. Appointments available Monday–Saturday in Sutton Coldfield.

Frequently asked questions

What is a normal vitamin B12 level?

Normal serum B12 is 150–700 pmol/L. Below 150 pmol/L indicates deficiency. A grey zone of 150–200 pmol/L may indicate early depletion and warrants further assessment with functional B12 markers such as methylmalonic acid or homocysteine.

What are the symptoms of vitamin B12 deficiency?

Symptoms include fatigue, weakness, breathlessness from anaemia, tingling or numbness in the hands and feet, balance problems, cognitive impairment, depression, and a sore red tongue (glossitis). Neurological damage can be irreversible if deficiency is untreated for a prolonged period.

Who is most at risk of vitamin B12 deficiency?

Key risk groups include vegans and vegetarians, people aged over 60, those taking metformin (long-term), people who have had gastric surgery, and those with pernicious anaemia or other malabsorption conditions. Anyone in these groups should have B12 monitored regularly.

What is pernicious anaemia?

Pernicious anaemia is an autoimmune condition where the stomach cannot produce intrinsic factor, essential for B12 absorption. Dietary B12 cannot be absorbed from the gut. It is treated with regular B12 injections (every 3 months in the UK) rather than oral supplements, which are ineffective without intrinsic factor.

Can oral B12 supplements correct deficiency?

For most people with dietary deficiency, high-dose oral B12 supplements (1000 µg daily) can correct levels over several months — even some absorption occurs by passive diffusion without intrinsic factor at very high doses. However, for pernicious anaemia or post-gastric surgery, intramuscular B12 injections are required. Your clinician will determine the appropriate route based on the cause of deficiency.