Vitamin B12

Other name(s):

cobalamin, cyanocobalamin, hydroxocobalamin

General description

Vitamin B12 is a water-soluble vitamin that is essential for cell division and is found only in animal-based foods. Like the other B vitamins, B12 plays an important role in energy production.

Vitamin B12 functions are closely associated with those of folic acid (vitamin B9). Vitamin B12 and folic acid are necessary for the synthesis of purines and pyrimidines, which are the building blocks of DNA. Generation of DNA is necessary every time a cell divides.

Vitamin B12 deficiency, like folic acid deficiency, results in megaloblastic anemia (abnormally large red blood cells and immature, abnormal white cells). This anemia is also referred to as pernicious anemia and occurs when the body is unable to absorb dietary vitamin B12 due to the lack of a specific protein (intrinsic factor) in the stomach. This condition is often hereditary.

Vitamin B12 also helps transform homocysteine (an amino acid) into methionine (also an amino acid) in the body, thus preventing the accumulation of homocysteine. A high level of homocysteine increases the risk of heart attack (myocardial infarction). Vitamin B12 maintains the myelin insulation sheath of the nerves. Thus, lack of B12 can cause nerve damage (neuropathy), which results in numbness and abnormal skin sensations.

Vitamin B12 is also a cofactor in carbohydrate metabolism.

Medically valid uses

Vitamin B12 is used to treat pernicious anemia caused by lack of an intrinsic factor. Supplements are also used for vitamin B12 deficiencies resulting from a vegan (strict vegetarian) diet, poor eating habits, thyrotoxicosis (toxicity caused by an overactive thyroid gland), bleeding, cancer, liver or kidney disease, and infestation by the fish tapeworm (Diphyllobothrium latum). Vitamin B12 is used to treat certain inherited or genetic disorders (such as Immerslund-Graesbeck syndrome, homocystinuria, and cobalamin C, D, and F disease) that affect metabolic functions in the body.

Unsubstantiated claims

Please note that this section reports on claims that have NOT yet been substantiated through scientific studies.

Vitamin B12 is claimed to increase energy, impart a sense of well being, improve immune functioning and memory, and maintain fertility. Again, these claims are not supported in scientific research.

Recommended intake

As indicated below, vitamin B12 is measured in micrograms (mcg). It is available as a tablet in strengths of 25 to 250 mcg. It is also available as a nasal gel or sublingual pills. The RDA is the Recommended Dietary Allowance.



Infants (0 to 6 months)

0.3 mcg

Infants (6 months to 1 year)

0.5 mcg

Children (1 to 3 years)

0.7 mcg

Children (4 to 6 years)

1.0 mcg

Children (7 to 10 years)

1.4 mcg

Adults (11+ years)

2.0 mcg

Pregnant women

2.2 mcg

Breast-feeding women

2.6 mcg

Food source

Nutrient content per 100 grams


85.9 mcg


32.2 mcg


18.8 mcg

Herring/ mackerel

9.44 mcg


8.3 mcg


5.6 mcg


1.8 mcg

Cheese, Swiss

0.8 mcg


0.77 mcg

No vitamin B12 is found in vegetables or fruits. Bacteria in the large intestine produce about 10 to 15 mcg a day of vitamin B12. The small intestine, being above the large intestine, is not in a position to absorb vitamin B12, so approximately the same amount is lost in the stool.

Vitamin B12 is stable at room temperature and therefore does not need to be refrigerated. It is not destroyed by cooking. Vitamin B12 resists breakdown even at boiling point of water for several hours.

A diet deficient in animal proteins, milk, or dairy products may increase the need for vitamin B12. In particular, vegan diets may require supplemental vitamin B-12. Breastfed infants of vegan-vegetarians also need supplements.

Conditions leading to malabsorption of vitamin B12, such as gluten-induced enteropathy, celiac disease, sprue, gastrectomy, or fish tapeworm infestation (Diphyllobothrium latum), may also require supplements of this vitamin.

Heavy alcohol use or alcoholism and conditions such as liver disease, hypothyroidism, and thalassemia increase the need for B12.

Because vitamin B12 absorption in the intestinal tract has been shown to decrease with aging, those over 60 may need vitamin B12 supplements.

Women who are pregnant or breastfeeding may need to take vitamin supplements, but must consult a physician before doing so.

Vitamin B12 deficiencies are usually caused by inadequate consumption in the diet or by reduced secretion or lack of intrinsic factor, a stomach secretion that helps the body absorb vitamin B12.

A deficiency of vitamin B12 in the diet or lack of absorption leads to a condition known as pernicious anemia. The bone marrow is markedly changed, with the appearance of megaloblasts (large, very immature cells) present in the marrow. The white blood cells appear abnormal, with excess numbers of lobes on the nucleus. Degeneration also appears in the spinal-cord neurons. As a result, severe and irreversible neurological damage may occur.

General symptoms of pernicious anemia include:

  • Weakness and fatigue or vertigo

  • Light-headedness

  • Dizziness

  • Ringing in the ears (tinnitus)

  • Rapid heart rate (tachycardia)

  • Yellowish color to the skin (Icterus or jaundice)

Gastrointestinal symptoms of pernicious anemia may include:

  • Sore tongue, or a smooth "beefy" red tongue

  • Loss of appetite with accompanying weight loss (anorexia)

  • Diarrhea, occasionally present

Nervous system symptoms of pernicious anemia may include:

  • Numbness in the extremities

  • Tingling

  • Heightened sensitivity or unusual sensations (paresthesia) in the extremities

  • Muscle weakness

  • Unstable gait (ataxia)

  • Changes in mental function including irritability, memory loss, dementia, and psychosis

Side effects, toxicity, and interactions

There are no known side effects or significant food or drug interactions associated with vitamin B12.

Do not take vitamin B12 if you are sensitive to vitamin B12 or cobalt. Only in very rare occasions do allergic reactions occur with the injectable form of vitamin B12.

Additional Information

Vitamin B12 is absorbed in the lower part of the small intestine. In the bloodstream it is highly bound to plasma proteins called transcobalamins. The half-life is approximately six days.

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