An abnormally low level of vitamin B12 (cobalamin) is a factor in many disorders.

The absorption of dietary vitamin B12 occurs in the small intestine and requires a secretion from the stomach known as intrinsic factor. If intrinsic factor is deficient, absorption of vitamin B12 is severely diminished. Vitamin B12 deficiency impairs the body’s ability to make blood, accelerates blood cell destruction, and damages the nervous system. The result is pernicious anemia (PA). In the classical definition, PA refers only to B12 deficiency anemia caused by a lack of intrinsic factor.

True pernicious anemia is probably an autoimmune disease. The immune system destroys cells in the stomach that secrete intrinsic factor. Many people with PA have both chronic inflammation of the stomach lining, called atrophic gastritis, and antibodies that fight their intrinsic factor-secreting cells.¹

The term pernicious anemia is sometimes used colloquially to refer to any anemia caused by vitamin B12 deficiency. Vitamin B12 deficiency can be due to malabsorption of dietary B12 despite normal levels of intrinsic factor. For example, celiac disease and Crohn’s disease may cause B12 malabsorption, which can lead to anemia. Less common causes of B12 deficiency include gastrointestinal surgery, pancreatic disease, intestinal parasites, and certain drugs. Pregnancy, hyperthyroidism, and advanced stages of cancer may increase the body’s requirement for B12, sometimes leading to a deficiency state.

Low stomach acid, known as hypochlorhydria, interferes with the absorption of B12 from food but not from supplements. Aging is associated with a decrease in the normal secretion of stomach acid. As a result, some older people with normal levels of intrinsic factor and with no clear cause for malabsorption will become vitamin B12-deficient unless they take at least a few micrograms per day of vitamin B12 from supplements.

Caution: Pernicious anemia is a serious medical condition. When fatigue, often the first symptom of PA, is present, a qualified healthcare practitioner should be consulted. Symptoms of PA can be caused by other conditions, none of which would respond to vitamin B12 supplementation. Moreover, if true vitamin B12 deficiency exists, the cause—lack of intrinsic factor, general malabsorption conditions, lack of stomach acidity, or dietary deficiency—must also be properly diagnosed by examination and blood tests before the appropriate treatment can be determined.