signs of vitamin b12 deficiencies

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THE MERCK MANUAL MEDICAL LIBRARY: The Merck Manual of Medical Information--Home Edition

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In This Topic

Vitamin B12

Vitamin B12 Deficiency

Causes

Symptoms

Diagnosis

Prevention and Treatment

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Section

Disorders of Nutrition and Metabolism

Subject

Vitamins

Topics

Introduction· Folate· Niacin· Riboflavin· Thiamin· Vitamin A·Vitamin B12·
Vitamin B6· Vitamin C· Vitamin D· Vitamin E· Vitamin K

Vitamin B12(Cobalamins)

 

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Pronunciations

cobalamin

dementia

deoxyribonucleic acid

endoscopy

gastrointestinal

ileum

metformin

neuropathy

ribonucleic acid

Vitamin B12 (cobalamins), with folate, is necessary for the formation
and maturation of red blood cells and the synthesis of DNA
(deoxyribonucleic acid), which is the genetic material of cells.
Vitamin B12 is also necessary for normal nerve function. Unlike most
other vitamins, B12 is stored in substantial amounts, mainly in the
liver, until it is needed by the body. Usually, the body's stores of
this vitamin would 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.

Vitamin B12 Deficiency

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

A severe deficiency may cause tingling or loss of sensation in the
 hands and feet, muscle weakness, loss of reflexes, difficulty
 walking, confusion, and dementia.

The diagnosis is based on blood tests.

When high doses of vitamin B12 supplements are taken, most
 symptoms resolve.

Symptoms due to nerve damage, such as neuropathy or dementia in
 older people, may persist.

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. The anemia is
characterized by abnormally large red blood cells (macrocytes) and
white blood cells with abnormal nuclei. 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, particularly in people older than 60.

Causes

Vitamin B12 deficiency can result when people do not consume enough
vitamin B12 or when 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.

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

Overgrowth of bacteria in part of the small intestine

Malabsorption disorders

Inflammatory bowel disease

Fish tapeworm infection

Surgery that removes the part of the small intestine where vitamin
 B12 is absorbed

Drugs such as antacids and metformin Some Trade Names
 GLUCOPHAGE
 (used to treat diabetes)

Lack of intrinsic factor

Decreased stomach acidity (common among older people)

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. Intrinsic factor
may be lacking because the part of the stomach where it 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. The vitamin B12
found in vitamin supplements, however, can continue to be well
absorbed even in people with decreased stomach acid.

Inadequate storage: Liver disorders may interfere with the storage of
vitamin B12.

Symptoms

Anemia due to vitamin B12 deficiency develops gradually, allowing the
body to adapt somewhat. Consequently, symptoms may be mild even when
anemia is severe. Symptoms of anemia are paleness, weakness, and
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, and
sensation in the legs, feet, and hands is lost. 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. Some people become
confused, irritable, and mildly depressed. Advanced vitamin B12
deficiency may lead to delirium, paranoia, and impaired mental
function, including dementia.

Diagnosis

Usually, vitamin B12 deficiency is suspected when routine blood tests
detect large red blood cells. If the deficiency is suspected, the
level of vitamin B12 in the blood is measured. Usually, doctors also
measure the blood level of folate to rule out folate deficiency, which
can also result in large red blood cells.

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. These tests
(including the Schilling test) usually focus on intrinsic factor.
Endoscopy (use of a flexible viewing tube to directly examine a body
cavity) may be done to check for destruction of stomach cells that
produce intrinsic factor.

Prevention and Treatment

Giving infants of vegan mothers vitamin B12 supplements from birth
helps prevent the deficiency.

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 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 it
can be corrected. Anemia usually resolves in about 6 weeks. But severe
symptoms due to nerve damage—for example, dementia in older people—may
not resolve.

Last full review/revision August 2007 by Larry E. Johnson, MD, PhD

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