Vitamin B12 (Cobalamin)
B12 is a cobalt-containing water-soluble vitamin that the body cannot synthesise. It is required for DNA synthesis, red blood cell formation, and myelin maintenance in nerves. Deficiency is common — up to 20% of adults over 60 have marginal or low B12 — and neurological damage can be irreversible if left untreated.
Daily requirements
- Adults: 2.4 µg/day (IOM RDA).
- Pregnancy: 2.6 µg/day.
- Lactation: 2.8 µg/day.
- No Tolerable Upper Intake Level — B12 has no known toxicity from oral intake.
Deficiency causes
Dietary deficiency is rare in omnivores but common in strict vegans (B12 is exclusively animal-derived in nature). Absorption problems are more common than dietary shortage:
- Pernicious anaemia — autoimmune destruction of gastric parietal cells; loss of intrinsic factor.
- Atrophic gastritis — age-related reduction in stomach acid; affects ~10–30% of adults ≥ 60.
- Proton pump inhibitors and H₂ blockers — chronic acid suppression impairs cleavage of B12 from food proteins.
- Metformin — long-term use lowers serum B12 by ~10–30%.
- Bariatric surgery — bypass of gastric acid production.
- Crohn's, coeliac disease, ileal resection — reduced ileal absorption.
Symptoms of deficiency
- Fatigue and weakness (macrocytic anaemia)
- Peripheral neuropathy — numbness, tingling, burning in hands and feet
- Cognitive changes — brain fog, memory impairment, confusion
- Balance problems and gait instability (posterior column dysfunction)
- Glossitis (smooth, red, painful tongue)
- Depression and psychiatric symptoms in severe cases
Food sources
Clams (up to 84 µg per 3 oz), beef liver (70 µg per 3 oz), fortified nutritional yeast (10 µg per 2 tbsp), salmon, tuna, beef, milk, eggs. Vegans must supplement or reliably consume fortified foods.
Supplementation
- Cyanocobalamin — most common and stable form. 500–1,000 µg oral daily is standard for maintenance.
- Methylcobalamin — active form, favoured by some clinicians for neuropathy. Cost is higher; efficacy is similar for most people.
- Sublingual and injectable — used in pernicious anaemia and severe deficiency (1,000 µg IM weekly × 4, then monthly).
- Vegans: 1,000 µg/week or 10 µg/day covers requirements safely.
Testing
Serum B12 is the first-line test. Normal range 200–900 pg/mL. Values below 200 pg/mL indicate deficiency. Between 200–400 pg/mL is a grey zone; check methylmalonic acid (MMA) and homocysteine — both are elevated in true cellular deficiency and confirm the diagnosis.