Vitamin B12 (cobalamin)

Vitamin B12 (cobalamins), with folate, is necessary for the formation and maturation of red blood cells and the synthesis of DNA. 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 vitamin B12 would take about 3 to 5 years to exhaust.
Vitamin B12 occurs in foods that come from animals, including eggs. Normally, vitamin B12 is readily absorbed in the last part of the small intestine. However, to be absorbed, the vitamin must combine with intrinsic factor, a protein produced in the stomach.
Because vitamin B12 is necessary for the formation of mature blood cells, deficiency of this vitamin can result in anaemia. The anaemia is characterized by abnormally large red blood cells (macrocytes) and white blood cells with abnormal nuclei. Vitamin B12 deficiency can cause nerve damage (neuropathy) even when no anaemia develops, particularly in people older than 60.
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.
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.
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
Surgery that removes the part of the small intestine where vitamin B12 is absorbed
Drugs such as antacids and metformin (used to treat diabetes)
Lack of intrinsic factor
Decreased stomach acidity (common among older people)
Intrinsic factor may be lacking because of an autoimmune reaction, or 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 anaemia called pernicious anaemia.
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 supplements, however, can continue to be well absorbed even in people with decreased stomach acid.
Liver disorders may interfere with the storage of vitamin B12.
Anaemia caused by vitamin B12 deficiency develops gradually, allowing the body to adapt somewhat. Consequently, symptoms may be mild even when anaemia is severe. 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. Advanced vitamin B12 deficiency may lead to various mental symptoms.
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.