The gut gate, and why injection bypasses it

B12 in food is prised off protein by stomach acid, bound to intrinsic factor made by the stomach's parietal cells, and taken up by a specific receptor in the ileum, then handed to transcobalamin in the blood. Thinning stomach lining with age, acid-lowering medicine, metformin, gut surgery or an autoimmune attack on intrinsic factor each break that chain. An injection under the skin or into muscle puts cobalamin straight into tissue and blood and needs none of it. That is why a shot works when swallowing a tablet does not.