Summary
Vitamin B12 is an essential water-soluble vitamin that the human body cannot produce itself — it must come entirely from diet or supplements. It is critical for making red blood cells, maintaining the myelin sheath around nerve fibres, and synthesising DNA. Deficiency causes a distinctive anaemia and can lead to irreversible neurological damage if untreated.
B12 deficiency is surprisingly common, particularly in vegetarians, vegans, older adults, and people taking metformin or acid-reducing medications. Symptoms develop gradually and are often attributed to other causes — fatigue, tingling hands and feet, and mood changes can all precede any change in blood count.nnThe Total B12 test measures all B12 in the bloodstream, including both active and inactive (bound to a non-functional carrier) fractions. For greater precision, Active B12 (holotranscobalamin) can be measured alongside — a better indicator of tissue-available B12.
What It Is
Vitamin B12 (cobalamin) is found almost exclusively in animal-derived foods — meat, fish, eggs, and dairy. In the digestive system, it binds to intrinsic factor (a protein produced by stomach cells) to be absorbed in the terminal ileum. This complex absorption mechanism means that stomach or bowel disease, or a lack of intrinsic factor, can cause B12 deficiency regardless of dietary intake.nnOnce absorbed, B12 circulates bound to two carrier proteins: transcobalamin (active — delivers B12 to cells) and haptocorrin (inactive — a storage carrier). Total B12 measures both fractions together, making it a useful first-line screen, though active B12 is more specific for tissue availability.
Functions
Red blood cell formation
Essential cofactor in the maturation of red blood cells in bone marrow — deficiency causes megaloblastic (large cell) anaemia.
Neurological function and myelin synthesis
Required to maintain the myelin sheath around nerve fibres — deficiency causes peripheral neuropathy and, if severe, subacute combined degeneration of the spinal cord.
DNA synthesis
Works with folate to synthesise DNA — critical in all rapidly dividing cells including red blood cells, gut epithelium, and immune cells.
Homocysteine regulation
Converts homocysteine to methionine. Deficiency raises homocysteine, an independent cardiovascular risk factor associated with atherosclerosis and stroke.
Reference Ranges
Serum Total Vitamin B12
Measured in pmol/L| Status | Range (pmol/L) | What it means |
|---|---|---|
| Deficient | < 148 | Clear deficiency — high risk of megaloblastic anaemia and neurological complications. Treatment required. |
| Borderline | 148–250 | Sub-optimal — consider Active B12 testing and assessment of dietary intake and absorption factors. |
| Adequate | 250–700 | Sufficient B12 status for normal physiological function. |
| High | > 700 | Elevated — usually from supplementation. Very high levels (>1500 pmol/L) without supplement use warrant clinical review. |
Total B12 ranges have significant limitations — a normal result does not exclude functional deficiency. Active B12 and/or methylmalonic acid should be measured if clinical suspicion remains. Interpret alongside symptoms.
Symptoms of Imbalance
B12 deficiency symptoms develop gradually and can affect the blood, nervous system, and mental health.
- Fatigue and weakness out of proportion to activity
- Pale or slightly yellow skin tone
- Tingling, numbness, or burning sensations in hands and feet
- Difficulty walking, balance problems, or coordination difficulties
- Cognitive slowing, memory problems, and confusion
- Low mood and depression
- Glossitis (smooth, sore, inflamed tongue)
- Elevated B12 from supplements is generally harmless and not associated with symptoms
- Unexplained very high B12 without supplementation warrants investigation (liver disease, leukaemia, polycythaemia vera)
Causes of Imbalance
- Vegan or strict vegetarian diet (B12 found only in animal products)
- Pernicious anaemia — autoimmune destruction of intrinsic factor (the most common cause of severe deficiency)
- Older age (reduced gastric acid and intrinsic factor production)
- Metformin use (impairs B12 absorption in the ileum)
- Proton pump inhibitors and antacids (reduce gastric acid needed for B12 release from food)
- Coeliac disease or Crohn's disease affecting terminal ileum absorption
- Gastric bypass or sleeve gastrectomy surgery
- B12 supplementation or B12-containing multivitamins (most common cause, harmless)
- Liver disease (releases stored B12)
- Myeloproliferative disorders (CML, polycythaemia vera)
- Solid tumours (rare)
FAQs
Even with adequate dietary intake, B12 can be low if absorption is impaired. The most common cause in adults over 50 is reduced gastric acid production, which prevents B12 from being released from food. Pernicious anaemia (autoimmune loss of intrinsic factor) is another important cause. Metformin and proton pump inhibitors also impair absorption. Testing helps identify whether the issue is dietary intake or absorption.
Total B12 measures all B12 in the blood, including both the active fraction (holotranscobalamin — delivered to cells) and an inactive storage fraction (haptocorrin-bound). About 70–80% of circulating B12 is haptocorrin-bound and cannot be used by cells. Active B12 measures only the deliverable fraction, making it a more sensitive early marker of functional deficiency.
Yes, oral high-dose B12 supplements (1000 mcg/day) are effective for most people with mild to moderate deficiency from dietary inadequacy. However, in pernicious anaemia (intrinsic factor deficiency), the gut cannot absorb oral B12 adequately, and B12 injections (intramuscular hydroxocobalamin) are required. The cause of deficiency determines the treatment.
Elevated B12 from supplements is generally harmless. B12 is water-soluble and excess is excreted. However, unexpectedly high B12 in someone not taking supplements warrants investigation, as it can be a marker of liver disease, myeloproliferative conditions, or certain cancers — conditions where B12 is released from storage.
With adequate supplementation, B12 levels typically begin to rise within 4–6 weeks. Neurological symptoms may take longer to improve — weeks to months — and in severe cases of prolonged deficiency, some neurological changes may be permanent. Re-testing at 3 months confirms whether levels have normalised.
References
- Stabler SP. Vitamin B12 deficiency. N Engl J Med. 2013;368(2):149–160. View source
- Hunt A, et al. Vitamin B12 deficiency. BMJ. 2014;349:g5226. View source
- Pawlak R, et al. How prevalent is vitamin B12 deficiency among vegetarians? Nutr Rev. 2013;71(2):110–117. View source