Causes of B12 deficiency. 


This great diagram shows normal absorption and outlines most possible causes. At the top is a B12-deficient diet. But the rest involve some mechanism of not absorbing.


The vitamin B12 (Cbl) released from food protein by peptic action is bound to haptocorrin (HC) in the stomach and travels to the duodenum, where pancreatic proteases digest the HC, releasing Cbl to bind to intrinsic factor (IF). The IF-Cbl complex binds to a specific receptor in the distal ileum (the cubam receptor) and is internalized, eventually released from lysosomes, and transported into the blood. Both HC and transcobalamin (TC) bind Cbl in the circulation, although the latter is the cellular delivery protein. 


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