Vitamin B12 (also known as Cobalamin) is a member of Vitamin B
Complex. It is exclusively synthesized by bacteria and is found
primarily in meat, eggs and dairy products. Fermented soy products,
seaweeds, and algae (such as Spirulina) have all been suggested to
contain significant B12; however, the consensus is that any B12
present in plant foods is likely to be unavailable and these foods
should not be relied upon as safe sources. Many vegan foods are
supplemented with B12. B12 is necessary for the synthesis of red blood
cells, the maintenance of the nervous system, and growth and
development in children. Deficiency can cause anemia.
Deficiency is more
commonly linked to a failure to effectively absorb B12 through the
intestine rather than a dietary deficiency. Absorption of B12 requires
the secretion of a glycoprotein, known as intrinsic factor, from the
cells lining the stomach. The B12-intrinsic factor complex is then
absorbed in the small intestine in the presence of calcium. Certain
people are unable to produce intrinsic factor and the subsequent
anemia is treated with injections of B12.
Vitamin B12 can be
stored in small amounts by the body. Total body store is 2-5 mg in
adults. Around 80% of this is stored in the liver.
Vitamin B12 is excreted
in bile and is effectively reabsorbed. This is known as enterohepatic
circulation. The amount of B12 excreted in bile can vary from 1 to 10
micrograms a day. People on diets low in B12, including vegans and
some vegetarians, may be obtaining more B12 from re-absorption than
from dietary sources. Re-absorption is the reason it can take over 20
years for deficiency disease to develop in people changing to diets
absent in B12. In comparison, if B12 deficiency is due to a failure in
absorption it can take only 3 years for deficiency disease to occur.
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