Vitamin B12


Vitamin B12 is a water-soluble, cobalt-containing member of the B complex of eight vitamins. Vitamin B12 is required for many bodily functions and must be obtained from specific foods or supplements, because the only organisms that can make it are bacteria, algae, yeast, molds, and other fungi. In recent years, vitamin B12 has become the focus of much interest, both because of claims that it promotes a myriad of health benefits and because of concerns about widespread vitamin B12 deficiencies. Vitamin B12 is also known as coenzyme B12, cobalamin, or cyanocobalamin.

Vitamin B12


Recommended dietary allowance

Children 0-6 mos.

0.4 μg (AI)

Children 7-12 mos.

0.5 μg (AI)

Children 1-3 yrs.

0.9 μg

Children 4-8 yrs.

1.2 μg

Children 9-13 yrs.

1.8 μg

Children 14-18 yrs.

2.4 μg

Adults 19≥ yrs.

2.4 μg

Pregnant women

2.6 μg

Breastfeeding women

2.8 μg


Vitamin B12 (μg)

Beef liver, cooked, 3 oz.


Mollusks or clams, cooked, 3 oz.


Alaskan king crab, 3 oz.


Salmon, 1/2 fillet


Cereal, fortified, 1 serving


Hamburger, fast food, double patty


Clam chowder, 1 cup


Rainbow trout, 3 oz.


Tuna, light, canned in water, 3 oz.


Ground beef, 80/20, 3 oz.


Lamb, 3 oz.


Milk, 2%, 1 cup


Cottage cheese, 2%, 1 cup


AI = Adequate intake

μg = microgram (mcg)

SOURCE: Office of Dietary Supplements. National Institutes of Health. “Vitamin B12: Fact Sheet for Health Professionals.” (accessed April 19, 2018).


Vitamin B12 is essential for metabolism and for the formation of red blood cells and the sheaths that surround nerve cells. It is also required for the synthesis of DNA, the genetic material that controls all bodily functions. Vitamin B12 is involved in the synthesis of many proteins in the body. In conjunction with vitamin B6 and folic acid (vitamin B9), vitamin B12 is involved in recycling the amino acid homocysteine.


B12 differs from other vitamins, including other B complex vitamins, in several ways. It is the only vitamin that is not made by any plant or animal, but only by microorganisms, including bacteria in the gastrointestinal tract of at least some humans and other animals and in the rumen of cows. However, it is not clear whether humans obtain any vitamin B12 from their own gut bacteria. Vitamin B12 is also the only vitamin to contain the metal cobalt (thus the name cobalamin), and it is the most structurally complex vitamin. Vitamin B12 is also absorbed by the body in an unusual way. First, hydrochloric acid and enzymes in the stomach separate B12 from proteins in food, although most multivitamins and vitamin B12 and B-complex dietary supplements, as well as fortified foods, contain a synthetic free form of vitamin B12 called cyanocobalamin. Regardless of whether B12 comes combined with protein in food or in a free form, it must combine with another protein in the duodenum called intrinsic factor (IF) before it can be absorbed by the body. The various forms of cobalamin are then converted in the body to methylcobalamin and 5-deoxyadenosylcobalamin. These are the forms of vitamin B12 that are active in human metabolism. As a water-soluble enzyme, vitamin B12 is both excreted in the urine and stored in the liver.

The role of vitamin B12 in health

Vitamin B12 is a coenzyme or cofactor, meaning that it combines with various enzymes in the body and is essential for the activity of those enzymes. This is the reason that B12 affects so many different aspects of physiology and metabolism. It is particularly important for the synthesis of new DNA, for the maturation of healthy red blood cells, and for the correct formation of the fatty myelin sheaths that surround nerves and are required for effective nerve transmission.

Of the many claims made for the benefits of vitamin B12, it is clear only that it prevents and treats diseases caused by vitamin B12 deficiency, including pernicious anemia, a red blood cell deficiency. When taken in combination with folate and vitamin B6, vitamin B12 is probably effective for treating a condition called hyperhomocysteinemia, which is related to heart disease. However, taking B12, B6, and folate supplements do not reduce the risk of heart disease. It is unclear whether vitamin B12, along with folate and vitamin B6, can help prevent dementia in the elderly, although patients with dementia often have high blood levels of homocysteine, which can be lowered by these B vitamins. Some research suggests that B12, again in conjunction with folate and B6, may help prevent the eye disease age-related macular degeneration. Most of the other health benefits that have been attributed to vitamin B12 are either doubtful or unproven. In particular, there is no evidence that B12 supplements increase energy or improve athletic performance except in people with a vitamin B12 deficiency.

Usual vitamin B12 requirements

Vitamin B12 can be stored in the liver for years, so the daily requirement is minimal: 2–3 micrograms (mcg). The Institute of Medicine (IOM) of the U.S. National Academy of Sciences has developed dietary reference intake (DRI) values for vitamins and minerals. DRIs consist of three sets of numbers. Recommended dietary allowance (RDA) defines the average daily amount of a nutrient needed to meet the health needs of 97%–98% of the population. Adequate intake (AI) is an estimate set when there is not enough information to determine an RDA. Tolerable upper intake level (UL) is the average maximum amount that can be taken daily without risking negative side effects. However, no UL levels have been set for vitamin B12 because no negative (toxic) side effects have been found, even when people have taken many hundreds of times the RDA for years. The vitamin B12 AIs and RDAs for healthy individuals are:

Because 10%–30% of older people may have trouble absorbing vitamin B12 bound to food, the IOM suggests that everyone over age 50 meet their RDA for vitamin B12 with fortified foods or supplements, which contain unbound or free B12.

Vitamin B12 sources

The best sources of vitamin B12 are clams and beef liver. Fish, meat, poultry, eggs, milk, and other dairy products are also good sources. One glass of milk (250 mL) contains about 50% of the B12 RDA. Furthermore, studies have shown that vitamin B12 in cow's milk is more readily available to the body than the most commonly used synthetic form of B12. Approximate B12 contents of some common foods include:

Almost all multivitamins, as well as B supplements, contain vitamin B12. Although B12 is available in sublingual forms that dissolve under the tongue, there is no evidence that these are absorbed better than tablets or pills. About 56% of a 1 mcg dose of vitamin B12 is absorbed by the body. More than 1–2 mcg exceeds the capacity of IF and so is not absorbed. Prescription vitamin B12 is available as a nasal gel or as an injection for treating vitamin B deficiency.

Vitamin B12 deficiency

There is little agreement as to the prevalence of vitamin B12 deficiency. Some sources estimate that 300,000 to 3 million Americans are deficient in vitamin B12. Other sources put the estimate as high as 15% of the population. It can take years to deplete the liver of stored vitamin B12, so symptoms are slow to appear, especially in adults. Most Americans who eat animal products obtain adequate vitamin B12 from their diet alone. Vegans who eat no animal products and breastfed infants of vegan mothers may not get sufficient vitamin B12. B12 deficiency can be determined with a simple blood test, although testing blood levels of methylmalonic acid, a protein breakdown product, is considered a better marker of vitamin B12 activity.

Conditions that interfere with any of the steps involved in absorbing vitamin B12 can lead to a deficiency. Older people are prone to mild B12 deficiency because their stomachs may not make enough hydrochloric acid to remove B12 from proteins in food. It is estimated that 10%–30% of adults over 50 have difficulty absorbing B12 from food. People who cannot produce IF are also at risk for vitamin B12 deficiency. The vitamin was originally discovered through its ability to cure pernicious anemia, which is an autoimmune disease that affects the lining of the stomach and eventually prevents patients from producing IF. Such people have difficulty absorbing vitamin B12 from either foods or supplements. Although very high oral B12 doses can sometimes treat pernicious anemia, as originally observed with the effect of eating liver, the anemia is usually treated with injected vitamin B12.

Other risk factors for vitamin B12 deficiency include:

Adequate intake (AI)—
The daily average intake level of a nutrient that is likely to be adequate for a healthy, moderately active individual, as determined by the U.S. Institute of Medicine.
Vitamin B12.
Cofactor; a small molecule, such as vitamin B12, that binds to an enzyme and catalyzes (stimulates) enzyme-mediated reactions.
The most common compound with vitamin B12 activity and the form usually included in vitamin supplements.
Dietary reference intake (DRI)—
A system of nutritional recommendations used by the Institute of Medicine and the U.S. Department of Agriculture.
Dietary supplement—
A product, such as a vitamin, that is consumed in addition to food with the expectation that it will improve or maintain health.
A protein that promotes a chemical reaction within the body; many proteins use vitamin B12 as a coenzyme.
Folic acid; vitamin B9.
Having added nutrients, such as vitamin B12, that do not occur naturally in that food.
An amino acid that is not usually in protein and that is turned over with the aid of vitamin B12; high blood levels of homocysteine have been associated with cardiovascular disease.
Intrinsic factor (IF)—
A protein produced in the stomach that binds to vitamin B12 and is required for its absorption by the body.
Megaloblastic anemia—
A blood disorder characterized by abnormal red blood cells and usually caused by a vitamin B12 or folate deficiency.
Pernicious anemia—
A low level of red blood cells caused by vitamin B12 deficiency.
Recommended dietary allowance (RDA)—
The approximate amount of a nutrient that should be ingested daily.
Tolerable upper intake level (UL)—
The highest daily intake level of a nutrient that is unlikely to cause adverse health effects in almost all individuals in the general population, as determined by the U.S. Institute of Medicine.
Vitamin B6
A vitamin that is present in many foods and is required for many enzyme reactions as well as for proper brain development and immune function.
Water-soluble vitamin—
A vitamin that dissolves in water and can be removed from the body in urine.


Breastfed infants of strict vegan mothers are at particular risk for vitamin B12 deficiency, as they have little or no B12 stored in their bodies at birth. Failure to get enough B12 during infancy and childhood can result in permanent damage to the nervous system. Vegan mothers should take supplemental vitamin B12, but breastfeeding women should not take more than 2.8 mcg per day because the safety of larger amounts is unknown.

There are a few other precautions concerning vitamin B12:


Many drugs used to treat gastroesophageal reflux disease (GERD) and peptic ulcer disease decrease the amount of hydrochloric acid secreted by the stomach. This may limit the amount of B12 available from food, but not from dietary supplements. These medications include the proton pump inhibitors omeprazole (Prilosec) and lansoprazole (Prevacid) and the histamine H2 receptor agonists cimetidine (Tagamet), famotidine (Pepcid), nizatidine (Axid), and ranitidine (Zantac). Antacid abuse may also limit the absorption of B12.


Other drug interactions include:


Vitamin B12 supplements taken by mouth or prescription injection are likely to be safe when used correctly. B12 can cause diarrhea, blood clots, itching, and allergic reactions in some people.

Parental concerns

Parents of vegetarian children must ensure that their children get sufficient vitamin B12 since it is essential for proper nervous system development. Nervous system damage from vitamin B12 deficiency in children is usually irreversible.

See also Cancer diet ; Folate ; Lacto-vegetarianism ; Menopause diet ; Ovolactovegetarianism ; Senior nutrition ; Veganism ; Vegetarianism ; Vitamins .



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Higdon, Jane, and Victoria J. Drake. An Evidence-Based Approach to Vitamins and Minerals: Health Benefits and Intake Recommendations. 2Nded. New York: Thieme, 2012.

Kroner, Zina. Vitamins and Minerals. Santa Barbara, CA: Greenwood, 2011.


Barnard, Neal D. “Getting Enough Vitamin B12.” Vegetarian Times, January 6, 2012. (accessed April 20, 2018).

Dangour, Alan D., et al. “A Randomised Controlled Trial Investigating the Effect of Vitamin B12 Supplementation on Neurological Function in Healthy Older People: The Older People and Enhanced Neurological Function (OPEN) Study Protocol.” Nutrition Journal 10 (March 2011): 22.

Lipman, Marvin M. “Vitamin B12: Panacea or Placebo?” Consumer Reports on Health 24, no. 2 (February 2012): 11.

Matte, J. Jacques, Frédéric Guay, and Christiane L. Girard. “Bioavailability of Vitamin B12 in Cows' Milk.” British Journal of Nutrition 107, no. 1 (January 14, 2012): 61–66.


Harvard T.H. Chan School of Public Health. “Vitamin B12 Deficiency: Causes and Symptoms.” The Nutrition Source, Department of Nutrition, Harvard University. (accessed April 20, 2018).

Office of Dietary Supplements. “Fact Sheet for Health Professionals: Vitamin B12.” National Institutes of Health. (accessed April 20, 2018).

U.S. Department of Agriculture, National Agricultural Library. “DRI Tables and Application Reports.” Food and Nutrition Information Center. (accessed March 15, 2018).


Food and Nutrition Information Center, National Agricultural Library, 10301 Baltimore Ave., Rm. 105, Beltsville, MD, 20705, (301) 504-5414, Fax: (301) 504-6409,, .

Linus Pauling Institute, Oregon State University, 307 Linus Pauling Science Center, Corvallis, OR, 97331, (541) 737-5075, Fax: (541) 737-5077,, .

Office of Dietary Supplements, National Institutes of Health, 6100 Executive Boulevard, Room 3B01, MSC 7517, Bethesda, MD, 20892-7517, (301) 435-2920, Fax: (301) 480-1845,, .

Tish Davidson, AM
Revised by Margaret Alic, PhD

  This information is not a tool for self-diagnosis or a substitute for professional care.