When it is found in foods, the vitamin B12 is bound to a protein. Free form B12 is bound to saliva glycoproteins and after that it is released in the stomach (due to acidification and under the pepsin enzyme action). So impaired gastric acidification (achlorhydria or chronic use of antiacids) will also impair vitamin B12 absorption. The released B12 is then bound to R-proteins and released again in the upper small intestine under the action of the pancreatic enzyme called trypsin. Here will bind a stomach produced protein called intrisic factor, in ileum (lower part of the small intestine). The vitamin B12-Intrisic factor complex attaches to specific receptor cells (active absorption). Passive diffusion can also happen at high concentrations. Then, in the intestinal cell, B12 is bound to a specific binding protein called trans-cobalaminII and it is carried by the blood to the liver where it is converted in its active coenzyme forms. The liver is storing around 60% of all the B12 present in our body. A good part is excreted via bile and it is back to the liver after it is reabsorbed in the blood (the enterohepatic circulation). Semms very complicated, right? But i wanted to say all this to point out how many single points of failure this process has, and anyone on them steps not working right can lead to vitamin B12 deficiency. Many of them actually fail later in life, to the point where some researchers think that one of the ageing cause is the vitamin B12 deficiency. There is an easy solution to all these, and i will talk a bit more about this in the next posts, most probably.
These being said, i will stop here for the moment, to let you to assimilate all the information.
P.S. If you are Romanian, you can also publish your post link in the Publish0x Romanian Community Facebook group for others to read it.
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Disclaimer: This text is also re-published on my personal blogs, such as this one.