Vitamin B12 deficiency

Vitamin B12 (cobalamin) is essential for DNA synthesis and neurological function.

It is found in meat and dairy products, so most people (except vegans) should have enough in their diet.

Absorption of B12 requires intrinsic factor (which is released from parietal cells in the stomach) to bind to it to facilitate its absorption as a complex uniquely in the terminal ileum.

Deficiency of B12 may result in:
  • Megaloblastic anaemia
  • Pancytopaenia
  • Peripheral neuropathy
  • Dementia/depression/psycohsis
  • Subacute combined degeneration of the spinal cord

Causes of B12 deficiency can be split into:
  • Insufficient intake – vegans, alcoholics, anorexics
  • Lack of absorption
    • Lack of intrinsic factor
      • Pernicious anaemia - autoimmune condition in which autoantibodies to parietal cells cause B12 deficiency.  Female>males.
      • Gastrectomy
    • fish tapeworm causing disassociation of B12 from IF
    • proton pump inhibitors – decreased acidity stops release of B12 from food
    • Lack of terminal ileum
      • Crohns disease
      • Resection
      • Whipples disease
    • Imerslund-Grasbeck syndrome – rare autosomal recessive condition in which there is selective B12 malabsorption

Diagnosis is usually clinical combined with blood tests showing low B12. Folic acid must always be measured too and this should be replaced first if found to be low. If the result is inconclusive serum methylmalonic and homocysteine levels may be measured; if either are elevated this also suggests B12 deficiency (these are substrates for the two enzymatic reactions B12 usually catalyses)

If B12 deficiency is diagnosed the Schilling test may be performed to determine if the deficiency is due to lack of intrinsic factor. This involves giving radioactively labelled B12 orally and unlabelled B12 IM. The amount of labelled B12 in 24hrs of urine is then recorded. This procedure is then repeated with IF given at the same time – if the amount excreted increases, this suggests lack of IF is the cause of the B12 deficiency. The Schilling test has fallen from fashion, replaced by blood tests for IF antibodies (pathognomic for pernicious anaemia – specificity 100%, sensitivity 70%) or parietal cell antibodies (low specificity)

Treatment of B12 deficiency is to treat the cause/give B12.

Small print gem: The body has approximately 5 years worth of B12 stored, 'just in case.'

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