From The Harvard T.H. Chan School of Public Health

Several factors may cause vitamin B12 deficiency:

  • Avoiding animal products. People who do not eat any meat, fish, poultry, or dairy products are at risk of becoming deficient in vitamin B12, since B12 is only found naturally in animal products. That’s why vegans should make sure to include B12-fortified foods or a B12 supplement in their diets.
  • Lack of intrinsic factor. Vitamin B12 deficiency can also be caused by a lack of “intrinsic factor” (a substance secreted by stomach cells that binds to vitamin B12 and enables its absorption). This can lead to so-called “pernicious anemia,” and ultimately to serious neurological damage.
  • Inadequate stomach acid. A much more common cause of deficiency, especially in older people, is a lack of stomach acid, because stomach acid is required to liberate vitamin B12 from food. An estimated 10 to 30 percent of adults over the age of 50 have difficulty absorbing vitamin B12 from food. (1) People who regularly take medications that suppress stomach acid—such as proton-pump inhibitors, H2 blockers, or other antacids—may also have difficulty absorbing vitamin B12 from food. Even people who lack adequate stomach acid can typically absorb vitamin B12 from fortified foods or supplements, however, providing yet another reason to take a multivitamin.

Symptoms of B12 deficiency include memory loss, disorientation, hallucinations, and tingling in the arms and legs. Some people diagnosed with dementia or Alzheimer’s disease are actually suffering from the more reversible vitamin B12 deficiency.  Measuring vitamin B12 in the blood is actually not the best way to determine whether someone is deficient. Blood levels of methylmalonic acid, a protein breakdown product, are a much better marker that captures actual vitamin B12 activity.


1. Institute of Medicine. Dietary reference intakes for thiamin, riboflavin, niacin, vitamin B6, folate, vitamin B12, pantothenic acid, biotin, and choline. Washington, DC: National Academy Press1999.

Category: Key Issues

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