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Vitamin B12 Deficiency

Effects of Inadequate Amounts of an Important Nutrient

Jun 26, 2009 Anthony Lee

What happens when an individual has a deficiency in vitamin B12?

Vitamin B12, also referred to as the cobalamins, is a vitamin found in meats and other animal-based products. It serves important functions in the formation of red blood cells, development of myelin nerve sheaths, and metabolism of nucleic acids. However, as with any nutrient, it is possible to have vitamin B12 deficiency. Such an abnormality can have various clinical consequences.

Causes

An understanding of the major causes of vitamin B12 deficiency requires an understanding of the its absorption. When vitamin B12 is ingested, acid in the stomach partially breaks it down. The stomach also produces a substance called intrinsic factor, which binds to vitamin B12. This enables absorption of vitamin B12 at the ileum, the last part of the small intestine.

One cause of vitamin B12 deficiency is inadequate oral intake. This can potentially occur in vegetarians, particularly those following the strictest vegan diets. Other people who are susceptible include chronic alcoholics and individuals attempting certain fad diets.

Otherwise, the major cause of vitamin B12 deficiency is malabsorption, which can involve one of the following portions of the gastrointestinal tract:

  • Stomach: Vitamin B12 deficiency can occur with pernicious anemia, an autoimmune condition that destroys the stomach cells producing intrinsic factor necessary for vitamin B12 absorption. Surgical removal of the stomach (gastrectomy) may also lead to vitamin B12 deficiency.
  • Intestine: The ileum cannot absorb vitamin B12 if it is surgically removed or suffers from Crohn's disease, overgrowth of intestinal bacteria (blind loop syndrome), or infestation with tapeworms that use vitamin B12.

Clinical Manifestations

In some cases, vitamin B12 deficiency results in the characteristic hematologic manifestation of megaloblastic anemia, which involves large immature red blood cells and hypersegmented neutrophils. In addition, patients with vitamin B12 deficiency suffer from various neurologic and psychiatric abnormalities, such as numbness and tingling (paresthesias), impaired sense of position (proprioception) or vibration in the lower extremities, muscle weakness, impaired memory, depression, and psychosis.

Diagnosis

The diagnosis of vitamin B12 deficiency is based on laboratory testing. A low serum vitamin B12 level confirms the diagnosis. However, if the diagnosis remains questionable because of a mildly low vitamin B12 level, then a physician can check serum levels of methylmalonic acid or homocysteine. These substances require vitamin B12 for metabolism, and elevated levels suggests vitamin B12 deficiency. Other tests should include a complete blood count to look for megaloblastic anemia, if present, and serum folate levels to ensure that megaloblastic anemia is not secondary to folate deficiency.

In certain cases, it may be important to determine the cause of vitamin B12 deficiency, particularly if pernicious anemia is a possibility. This can be done with the Schilling test. It involves the oral ingestion of radiolabeled vitamin B12, which is absorbed by the intestine and excreted in the urine. The test is performed as follows:

  • First Phase: The first round of oral radiolabeled vitamin B12 is given. If it is subsequently detected in the urine, it means that there is no problem with absorption from the intestine and that the cause of vitamin B12 deficiency is inadequate intake. Otherwise, it means there is a problem with the stomach or intestine
  • Second Phase: If there is a problem with the stomach or intestine, the second round of radiolabeled vitamin B12 is given along with intrinsic factor. If the B12 is later found in the urine, it means the cause of the deficiency is lack of intrinsic factor from the stomach. Otherwise, the cause is related to intestinal malabsorption.

Treatment

The treatment of vitamin B12 deficiency is replacement of the vitamin. This can be given orally for milder cases or as an intramuscular injection for severe cases. The duration of treatment depends on how long the underlying cause of vitamin B12 deficiency is present. In general, it takes approximately six weeks for hematologic manifestations of vitamin B12 deficiency to resolve. Neurologic and psychiatric manifestations may take a longer time to resolve or remain permanent for life.

References

The copyright of the article Vitamin B12 Deficiency in General Medicine is owned by Anthony Lee. Permission to republish Vitamin B12 Deficiency in print or online must be granted by the author in writing.
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