When the B-12 vitamin enters your body in the stomach (through food or oral pills), acids in the stomach separate the B-12 from its protein source. It then must combine with intrinsic factor cells in the stomach. This B-12/intrinsic factor complex travels to your intestine, where it is absorbed in the terminal ileum. The absorbed complex is then transported via blood plasma and stored in the liver.

The interruption of any of these steps affects your body’s ability to absorb Vitamin B-12. Here is a list of common interruptions to Vitamin B-12 absorption:

1. As you age (over 45) or become overly reliant on acid suppressing agents like antacids, your ability to produce gastric acids in the stomach decreases, meaning that the B-12 is less likely to be released from its food source.
2. An auto immune or other disease reduces the production or blocks the action of intrinsic factor, resulting in intestinal malabsorption.
3. People with pernicious anemia have decreased production of intrinsic factor.
4. Abdominal surgery reduces Vitamin B-12 absorption.
   • Gastrectomy eliminates the site of intrinsic factor production.
   • Blind loop syndrome results in competition for vitamin B-12 by bacterial overgrowth in the lumen of the small intestine.
   • Surgical resection of the ileum eliminates the site of vitamin B-12 absorption.
5. Pancreatic insufficiency such as fish tapeworm infection and severe Crohn's disease affect absorption.