Vitamin B12 [Absorption and Deficiency]

How Vitamin B12 is absorbed in our body?

Have to eat meats or dairy products to get B12.  The first thing B12 does is binds to R factor in saliva.  R factor protects B12 from destruction    by acid in the stomach.  Intrinsic factor (IF) made by parietal cells in the body fundus; they also make acid.  IF is not destroyed by acid, therefore does not need anything to protect it.  So the B12/R factor complex goes into the duodenum, where there is IF waiting for it.  R factor must be cleaved off, which is done with enzymes from the functioning pancreas. Then, intrinsic factor and B12 bind to each other and take a long trip.  Do not go to duodenum (where Iron is absorbed), do not go to ligamentum of trietz in the jejunum (where folate is absorbed); so they go all the way to the terminal ileum, where there are receptors for IF, and it is reabsorbed.  This is the same place bile salts are reabsorbed, and the same place the Crohn’s disease hits which is the terminal ileum.  Therefore, it is fair to say that with Crohn’s disease, you also have bile salt reabsorption problems and B12 def. 

Main common cause of Vitamin B12 deficiency

Pernicious anemia - this is an autoimmune disease with destruction of the parietal cells; autoantibody attack the parietal cells and there are autoanitbodies against IF and destroys the parietal cells which are located in the body and fundus.  Everything gets destroyed leading to an atrophic gastritis of the body and fundus.  No parietal cells = no acid = achyloridria, and no IF.  Achyloridria is a major predisposing factor for gastric adenocarcinoma