Vitamin B12 deficiency

Physiology · Gastrointestinal physiology

Vitamin B12 is mainly used in the body for red blood cell development and also maintenance of the nervous system. It is absorbed after binding to intrinsic factor (secreted from parietal cells in the stomach) and is actively absorbed in the terminal ileum. A small amount of vitamin B12 is passively absorbed without being bound to intrinsic factor.

Causes of vitamin B12 deficiency

pernicious anaemia
post gastrectomy
poor diet
disorders of terminal ileum (site of absorption): Crohn's, blind-loop etc
metformin (rare)

Features of vitamin B12 deficiency

macrocytic anaemia
sore tongue and mouth
neurological symptoms: e.g. Ataxia
neuropsychiatric symptoms: e.g. Mood disturbances

Management

if no neurological involvement 1 mg of IM hydroxocobalamin 3 times each week for 2 weeks, then once every 3 months
if a patient is also deficient in folic acid then it is important to treat the B12 deficiency first to avoid precipitating subacute combined degeneration of the cord