Vitamin B12 (cobalamin) is a water-soluble vitamin essential for DNA synthesis, red blood cell production and myelin sheath formation around nerves. Unlike other water-soluble vitamins, B12 is stored in the liver — stores last 3–5 years — which is why deficiency often develops slowly and remains silent for a long time before causing symptoms.
Who is at highest risk
- Vegans and strict vegetarians — B12 is found almost exclusively in animal-derived foods
- People over 60 — gastric acid production declines with age, reducing B12 absorption from food
- People with pernicious anaemia — an autoimmune condition where intrinsic factor (essential for B12 absorption) is destroyed
- Those taking long-term metformin — the drug reduces B12 absorption by around 30% over time
- Long-term proton pump inhibitor (PPI) users — acid suppression impairs B12 release from food
- People with coeliac disease or inflammatory bowel disease
- Those who have had gastric bypass surgery
Symptoms of deficiency
The neurological and haematological symptoms of B12 deficiency often develop in parallel but can occur independently. Neurological damage, once established, may be irreversible — which is why early identification matters.
- Fatigue and weakness — due to megaloblastic anaemia (large, dysfunctional red cells)
- Tingling or numbness in hands and feet — peripheral neuropathy from nerve demyelination
- Balance problems and difficulty walking — posterior column involvement
- Sore, inflamed tongue (glossitis)
- Mood changes — irritability, depression, psychosis in severe deficiency
- Memory problems and cognitive decline — particularly in older adults
- Breathlessness on exertion — if severe anaemia is present
What the blood test shows
A serum B12 level below 180 pmol/L (240 pg/mL) is generally considered deficient. However, serum B12 is an imperfect test — about 50% of people with early deficiency have borderline-normal levels. More sensitive markers include methylmalonic acid (MMA) and homocysteine, which rise before B12 falls below the reference range.
A full blood count showing macrocytosis (high MCV) and anaemia raises strong suspicion; intrinsic factor antibodies confirm pernicious anaemia as the cause.
"B12 deficiency can mimic Alzheimer's, multiple sclerosis and depression — and is often missed for years. A simple blood test changes that."
Dr. Lina Haddad, CMO, Looms
Treatment
For dietary deficiency (vegans and vegetarians), high-dose oral B12 (1,000 mcg/day) is effective — absorption via passive diffusion bypasses the need for intrinsic factor. For pernicious anaemia or malabsorption, intramuscular injections are required; these are typically given every 3 months lifelong.
Response is rapid for anaemia — haemoglobin starts to recover within 1–2 months. Neurological improvement is slower and depends on how long deficiency was present; some symptoms may be permanent.