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Vitamin B12 Deficiency

(Pernicious Anemia)


Larry E. Johnson

, MD, PhD, University of Arkansas for Medical Sciences

Reviewed/Revised Nov 2022

Vitamin B12 deficiency can occur in vegans who do not take supplements or as a result of an absorption disorder.

  • Anemia develops, causing paleness, weakness, fatigue, and, if severe, shortness of breath and dizziness.

  • A severe vitamin B12 deficiency may damage nerves, causing tingling or loss of sensation in the hands and feet, muscle weakness, loss of reflexes, difficulty walking, confusion, and dementia.

  • The diagnosis of vitamin B12 deficiency is based on blood tests.

  • When high doses of vitamin B12 supplements are taken, symptoms due to anemia tend to resolve.

  • Symptoms due to nerve damage and dementia in older people may persist.

Vitamin B12 (cobalamins), with folate, is necessary for the formation and maturation of red blood cells Formation of Blood Cells Red blood cells, most white blood cells, and platelets are produced in the bone marrow. However, 2 types of white blood cells—T cells and B cells ( lymphocytes)—are also produced in the lymph... read more and the synthesis of DNA (deoxyribonucleic acid), which is the genetic material of cells. Vitamin B12 is also necessary for normal nerve function. Good sources of vitamin B12 include meats (especially beef, pork, liver, and other organ meats), eggs, fortified cereals, milk, clams, oysters, salmon, and tuna. (See also Overview of Vitamins Overview of Vitamins Vitamins are a vital part of a healthy diet. The recommended dietary allowance (RDA)—the amount most healthy people need each day to remain healthy—has been determined for most vitamins. A safe... read more .)

Unlike most other vitamins, B12 is stored in substantial amounts, mainly in the liver, until it is needed by the body. If a person stops consuming the vitamin, the body’s stores of this vitamin usually take about 3 to 5 years to exhaust.

People should not take high doses of vitamin B12 as a cure-all, but otherwise the vitamin does not appear to be toxic; consuming excess amounts of B12 is not recommended.

Vitamin B12 occurs in foods that come from animals. Normally, vitamin B12 is readily absorbed in the last part of the small intestine (ileum), which leads to the large intestine. However, to be absorbed, the vitamin must combine with intrinsic factor, a protein produced in the stomach. Without intrinsic factor, vitamin B12 moves through the intestine and is excreted in stool.

Because vitamin B12 is necessary for the formation of mature blood cells, deficiency of this vitamin can result in anemia Vitamin Deficiency Anemia Vitamin deficiency anemia results from low or depleted levels of vitamin B12 or folate (folic acid). People may be weak, short of breath, and pale. Nerves may malfunction. Blood tests can detect... read more . The anemia is characterized by abnormally large red blood cells (macrocytes) and abnormal white blood cells. Anemia may not develop until 3 to 5 years after the deficiency begins because a large amount of vitamin B12 is stored in the liver.

Vitamin B12 deficiency can cause nerve damage (neuropathy) even when no anemia develops.

Causes of Vitamin B12 Deficiency

Vitamin B12 deficiency can result when the following occur:

  • People do not consume enough vitamin B12.

  • The body does not absorb or store enough of the vitamin.

Inadequate consumption

Vitamin B12 deficiency develops in people who do not consume any animal products (vegans) unless they take supplements. If a vegan mother breastfeeds her infant, the infant is at risk of vitamin B12 deficiency. Deficiency due to inadequate consumption is unlikely in other people.

Inadequate absorption

The most common cause of vitamin B12 deficiency is inadequate absorption. The following can cause absorption to be inadequate:

Intrinsic factor may be lacking because abnormal antibodies, produced by an overactive immune system, attack and destroy the stomach cells that produce intrinsic factor—an autoimmune reaction called autoimmune metaplastic atrophic gastritis. Intrinsic factor may also be lacking because the part of the stomach where intrinsic factor is produced was surgically removed. Vitamin B12 deficiency due to lack of intrinsic factor causes a type of anemia called pernicious anemia.

Among older people, absorption may be inadequate because stomach acidity is decreased. Decreased stomach acidity reduces the body’s ability to remove vitamin B12 from the protein in meat. However, the vitamin B12 found in vitamin supplements can continue to be well absorbed even in people with decreased stomach acid.

Inadequate storage

Symptoms of Vitamin B12 Deficiency

Symptoms of anemia are

  • Paleness

  • Weakness

  • Fatigue

If severe, anemia causes shortness of breath, dizziness, and a rapid heart rate.

Occasionally, the spleen and liver enlarge.

Younger adults who have pernicious anemia (due to lack of intrinsic factor) are more likely to develop stomach and other gastrointestinal cancers.

In people with nerve damage, the legs are affected earlier and more often than the arms. Tingling is felt in the feet and hands, or sensation in the legs, feet, and hands is lost. Their arms and legs may feel weak. People become less able to tell where their arms and legs are (position sense) and to feel vibrations. Mild to moderate muscle weakness develops, and reflexes may be lost. Walking becomes difficult.

Diagnosis of Vitamin B12 Deficiency

  • Blood tests

  • Sometimes endoscopy

Usually, vitamin B12 deficiency is suspected when routine blood tests detect large red blood cells. Doctors sometimes suspect it when people have typical symptoms of nerve damage, such as tingling or loss of sensation. If the deficiency is suspected, the level of vitamin B12 in the blood is measured.

If vitamin B12 deficiency is confirmed in an older person, no other tests are done because the cause, such as low stomach acidity, is usually not serious. In a younger person, other tests, including other blood tests, may be done to determine the cause.

Endoscopy Endoscopy Endoscopy is an examination of internal structures using a flexible viewing tube (endoscope). In addition to examinations, doctors can use endoscopy to do biopsies and give treatment. Endoscopes... read more (use of a flexible viewing tube to directly examine internal structures) may be done to check for destruction of stomach cells that produce intrinsic factor.

Prevention of Vitamin B12 Deficiency

For infants of vegan mothers, starting vitamin B12 supplements immediately after birth helps prevent vitamin B12 deficiency.

Treatment of Vitamin B12 Deficiency

  • Vitamin B12 supplements

  • For people with nerve damage, vitamin B12 given by injection

Older people with vitamin B12 deficiency benefit from taking vitamin B12 supplements because the deficiency usually results from difficulty absorbing the vitamin from meat. They can absorb the vitamin more easily from supplements than from meat.

Treatment of vitamin B12 deficiency or pernicious anemia consists of high doses of vitamin B12 supplements. If people have the deficiency but no symptoms, the vitamin may be taken by mouth. Blood tests are done periodically to make sure the vitamin B12 level returns to and remains normal.

People who have very low levels of vitamin B12 or symptoms due to nerve damage are usually given vitamin B12 by injection into a muscle. Injections, which may be self-administered, are given daily or weekly for several weeks until the vitamin B12 level returns to normal. Then injections are given once a month indefinitely, unless the disorder causing the deficiency can be corrected.

Anemia usually resolves in about 6 weeks. But if severe symptoms due to nerve damage last for months or years, they may become permanent. In most older people with vitamin B12 deficiency and dementia, mental function does not improve after treatment.

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