If I Already Take B-12, Could I Still Be Deficient?

The dietary absorption of vitamin B-12 is complex. It's not just a matter of swallowing a bunch of it (dietary or supplemental) and letting the guts do the work. There is something called "intrinsic factor" which is made in certain cells in the stomach that must be present in order for B-12 to be absorbed at a point in the very end of the small intestine (the ileum).  

The main sources of B-12 include meat, eggs, and dairy products. Acids in the stomach separate the B-12 from the protein source, at which point it must combine with intrinsic factor. The vitamin B-12/intrinsic factor complex travels through the intestine and is absorbed in the terminal ileum by cells with specific receptors for the complex. The absorbed complex is then transported via plasma and stored in the liver. The interruption of one or any combination of these steps places a person at risk of developing deficiency.

In most cases, vitamin B-12 deficiency is due to an inability of the intestine
to absorb the vitamin, which can happen in several ways.

1. As we age or become overly reliant on acid suppressing agents like antacids, our 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. Deficiency may result from an autoimmune disease that reduces the production or blocks the action of intrinsic factor, or from other diseases that result in intestinal malabsorption. The most frequent underlying cause of vitamin B-12 deficiency is pernicious anemia, which is associated with decreased production of intrinsic factor.
3. Also, abdominal surgery may cause B-12 deficiency in several ways. Gastrectomy, for example, 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. And surgical resection of the ileum eliminates the site of vitamin B-12 absorption.
4. Other causes of vitamin B-12 deficiency include pancreatic insufficiency, fish tapeworm infection—in which the parasite competes for B-12—and severe Crohn's disease, which results in reduced absorption of B-12 in the intestine.

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