Fig 1. Cobalamin in the healthy human intestinal tract.

Ingested food containing cobalamin enters the stomach. IF is produced in the stomach and binds cobalamin in the small intestine. There, cobalamin–IF complexes are absorbed by host enterocytes in the terminal ileum (indicated by outward arrows in small intestine). In the colon, the microbiota take up unabsorbed cobalamin. Additionally, some members of the microbiota produce cobalamin, which can also be taken up by other microbiota members. Cobalamin that remains unabsorbed by the host and not taken up by the microbiota is excreted in stool (outward arrow in colon). IF, intrinsic factor.