Thanks - glad I was able to explain it well enough. The whole thing is extremely complex...I've been working on trying to understand it for years now, and still feel like I've barely scratched the surface...
Absorption of B12 is very complicated. I don't seem to absorb it at all orally, apparently, so have found getting shots to be much better.
Methylcobolamin may absorb better for some individuals. I find I get side-effects from cyanocobolamin, so I do much better with Methyl-B12.
For B12 to be absorbed from food, first there has to be enough stomach acid and protease enzyme present to break the B12 apart from the protein (B12 food sources include meat, milk, eggs, and fortified cereals; there are no plant sources, though some species of fungus/yeast may also be a source).
After the B12 is in "free-form," it is then combined with Intrinsic Factor (also produced in the lining of the stomach). This B12-IF molecule then gets absorbed through the gut wall in the lower part of the small intestine (the lower ileum). Certain enzymes break apart the B12 from the IF for transport through the gut wall, and then certain other carrier molecules bring the B12 to the liver.
B12 supplements only get to skip the stomach acid/protease step...but must go through the rest of the steps for absorption.
Things that can impair oral B12 absorption include:
*Low stomach acid
*Poor enzyme production (pancreatic insufficiency)
*Inflammation in the stomach (preventing Intrinsic Factor secretion)
*Inflammation in the small intestine (lower ileum)
*Surgical removal of the lower ileum (such as in those who have had surgery for Crohn's Disease or other bowel disorders)
*Weight loss surgery can sometimes cause problems for B12 absorption
*Deficiency of any of the carrier molecules needed to bring the B12 through the gut wall or carry the B12 to the liver for further processing or storage
*A diet high in sulfites - B12 binds directly to the sulfites
*Poor dietary intake of foods rich in B12 - this is an issue for Vegetarians/Vegans especially
* Small Bowel Bacterial Dysbiosis - the bacteria may use the B12 instead of letting it pass by to the lower small intestine for absorption
*Alcohol (chronic high consumption)
*PPI medication (reduces stomach acid)
*Diseases that cause malabsorption (Celiac, Crohn's, etc.)
Conditions that increase one's need for B12:
MS (B12 is needed for repair of the mylen sheath)
Lyme (I've read that the bacteria uses up B12)
Shots may be needed for the following scenarios:
Surgical removal of the lower small intestine (Crohn's, etc.)
Weight loss surgery
Pernicious Anemia (autoimmune inflammation that specifically targets Intrinsic Factor producing cells in the stomach lining)
B12 Dependence (a poorly understood condition in which a person seems to either have difficulty maintaining sufficient blood levels of B12 despite adequate oral supplementation)
Additional info can be found at the following websites:http://ods.od.nih.gov/factsheets/vitaminb12/
I hope this info is helpful...take care,