Vitamin B12

What is Vitamin B12?

Vitamin B12 is needed for healthy nerves and red blood cells. Your body doesn't make it itself; it comes from your diet, mainly animal foods, and a small store in your liver. A healthy value supports your energy, your concentration and a smoothly working nervous system.

Clinical frame

Vitamin B12 is a cofactor of two enzymes: methionine synthase, which converts homocysteine and frees folate for DNA synthesis, and methylmalonyl-CoA mutase. A deficiency thus blocks the production of red blood cells (megaloblastic, macrocytic anaemia) and damages the myelin of the spinal cord and nerves ^[1].

B12 comes only from animal foods, with a liver store that lasts for years. Deficiencies arise through malabsorption (pernicious anaemia, atrophic gastritis, bariatric surgery, coeliac disease), medication such as metformin and proton pump inhibitors, and a vegan diet. Prevalence rises sharply with age ^{[1] [2]}.

Serum B12 has a grey zone (about 180 to 350 pg/mL) in which methylmalonic acid, and secondarily homocysteine, confirm a functional deficiency. Neurological damage occurs in almost a third of patients without anaemia or macrocytosis and can become irreversible. A normal blood count therefore does not rule out a deficiency ^{[2] [3]}.

What moves this value?

Few animal foods, growing older or reduced absorption in the gut can lower the value.

Food rich in B12 and, on a plant-based diet, a supplement keep the store topped up. Discuss the right form and amount with your doctor.

What this connects

• Homocysteine

References

1. Green, R. et al. Vitamin B12 deficiency. Nat Rev Dis Primers 3, 17040 (2017). doi.org/10.1038/nrdp.2017.40
2. Stabler, S. P. Clinical practice. Vitamin B12 deficiency. N Engl J Med 368, 149–160 (2013). doi.org/10.1056/NEJMcp1113996
3. Lindenbaum, J. et al. Neuropsychiatric disorders caused by cobalamin deficiency in the absence of anemia or macrocytosis. N Engl J Med 318, 1720–1728 (1988). doi.org/10.1056/NEJM198806303182604