Causes of vitamin B12 malabsorption

Vitamin B12 deficiency results from a failure of intestinal
absorption or subsequent transport to the tissues;
it is rarely, if ever, caused by a lack of B12 in the diet.
Disorders of vitamin B12 absorption and transport
have been discussed by Kapadia & Donaldson (1985)
and just a few examples of malabsorption are mentioned
here.
• Elderly people are prone to atrophic gastritis, a
condition in which the gastric oxyntic mucosa
atrophies to such an extent that virtually no hydrochloric
acid or intrinsic factor is secreted.
• In patients with diverticula, strictures and fi stulas
of the small intestine, stagnant regions of the lumen
may become contaminated with colonic bacteria
which can take up much of the dietary vitamin B12
passing by. Bacteria can take up vitamin B12 bound
to intrinsic factor, although not as avidly as they can
take up the free vitamin. Intrinsic factor and bacteria
have a similar affi nity for B12, so it is possible
that bacterial uptake could take place following the
vitamin’s release from haptocorrin but before its
transfer to intrinsic factor.
• The fish tapeworm Diphyllobothrium latum competes
with the host for B12, making it less available
for absorption.
• A common inherited disorder is an auto-immune reaction
with formation of antibodies against intrinsic
factor. Such cases involve two types of antibody: type
I prevents intrinsic factor from binding to cobalamin
and type II blocks the binding of the intrinsic factor–
cobalamin complex to the ileal receptor.