Although well recognised, this is a much less common cause of macrocytosis than the foregoing conditions. It is usually caused by lack of intrinsic factor due to autoimmune atrophic changes and by gastrectomy. Anaemia does not develop for about three years after total gastrectomy. B12 deficiency may also be seen together with other deficiencies in some cases of malabsorption and Crohn's disease.
Vitamin B12 is found only in foods of animal origin and consequently very strict vegetarians may eventually develop deficiency. The clinical features are anaemia (macrocytic), weight loss and neurological symptoms especially a polyneuropathy. The serum Vitamin B12 is below the normal level.
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