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.)