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To prevent destruction by stomach acid, Vitamin B12 must usually be bound by haptocorrin (R-factor, transcobalamin-1). It would make sense for haptocorrin secretion to be in the ballpark of the maximum amount typically required for typical foods to make good use of the B12 contained therein.

In contrast, supplements can contain over a hundred times more B12 than commonly found in food. I find it hard to believe that the body would regularly secrete sufficient haptocorrin to bind these amounts. On the product pages of the orally-taken high-dosage supplements that I have looked at, I haven’t seen any indication that the B12 had been pre-bound to haptocorrin.

Does this mean that most of the B12 in supplements will be destroyed right-away by stomach acid, for a mere lack of haptocorrin to protect it?

(I am aware that the amount of intrinsic factor (IF) produced might be just as insufficient. This does not matter as much, since it only takes over past the stomach, within the small intestine, where pH is less extreme, and since uptake of B12 not bound to IF is possible through passive diffusion, to some degree. – This, however, can only happen if the B12 has not been disintegrated beforehand.)

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I haven't found any source that would claim that a normal level of haptocorrin in saliva is a limiting factor for the absorption of vitamin B12 from supplements.

On the other hand, haptocorrin deficiency can limit the absorption of vitamin B12 even from food thus leading to vitamin B12 deficiency (Clinical Chemistry, 2003).

It is the intrinsic factor that is a bottleneck for the absorption of large oral doses of vitamin B12 (Office of Dietary Supplements):

Approximately 56% of a 1 mcg oral dose of vitamin B12 is absorbed.

Existing evidence does not suggest any differences among forms with respect to absorption or bioavailability. However the body’s ability to absorb vitamin B12 from dietary supplements is largely limited by the capacity of intrinsic factor. For example, only about 10 mcg of a 500 mcg oral supplement is actually absorbed in healthy people.

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