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Vitamin B12
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[What is Vitamin B12? | What foods provide Vitamin B12?]
[Recommended dietary allowance]
[When is a deficiency of B12 likely to occure?]
[Who may need Vitamin B12 supplements?]
[B12, homocysteine and heart disease]
[Too much B12 | Selected B12 food sources | References]

Vitamin B12 : What is it?
Vitamin B12, also called cobalamin, is important to good health. Vitamin B12 helps maintain healthy nerve cells and red blood cells, and Vitamin B12 is also needed to make DNA, the genetic material in all cells1-4. Vitamin B12 is bound to the protein in food. Hydrochloric acid in the stomach releases vitamin B12 from protein during digestion. Once released, B12 combines with a substance called intrinsic factor (IF) before it is absorbed into the bloodstream.

What foods provide vitamin B12?
Vitamin B12 is naturally found in animal foods including fish, milk and milk products, eggs, meat, and poultry. Fortified breakfast cereals are an excellent source of vitamin B12 and a particularly valuable source for vegetarians5, 6, 7. The table of selected food sources of vitamin B12 suggests dietary sources of vitamin B12.

What is the Recommended Dietary Allowance for vitamin B12 for adults?
The Recommended Dietary Allowance (RDA) is the average daily dietary intake level that is sufficient to meet the nutrient requirements of nearly all (97 to 98 percent) healthy individuals in each life-stage and gender group7. The 1998 RDAs for vitamin B12 (in micrograms) for adults7 are:






Ages 19+

2.4 mcg

2.4 mcg


All ages


2.6 mcg  

2.8 mcg

Results of two national surveys, the National Health and Nutrition Examination Survey (NHANES III-1988-91) (8) and the Continuing Survey of Food Intakes by Individuals (CSFII 1994-96) (7) found that most adult men and women consume recommended amounts of vitamin B12 (6-8).

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When is a deficiency of vitamin B12 likely to occur?
Diets of most adult Americans provide recommended intakes of vitamin B12, but deficiency may still occur as a result of an inability to absorb B12 from food. Vitamin B12 deficiency can also occur in individuals with dietary patterns that exclude animal or fortified foods (9). As a general rule, most individuals who develop a vitamin B12 deficiency have an underlying stomach or intestinal disorder that limits the absorption of vitamin B1210. Sometimes the only symptom of these intestinal disorders is anemia resulting from vitamin B12 deficiency.

Characteristic signs of vitamin B12 deficiency include fatigue, weakness, nausea, constipation, flatulence (gas), loss of appetite, and weight loss1, 3, 11. Vitamin B12 deficiency also can lead to neurological changes such as numbness and tingling in the hands and feet7, 12. Additional symptoms of vitamin B12 deficiency are difficulty in maintaining balance, depression, confusion, poor memory, and soreness of the mouth or tongue13. Some of these symptoms can also result from a variety of medical conditions other than vitamin B12 deficiency. It is important to have a physician evaluate these symptoms so that appropriate medical care can be given.

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Who may need a vitamin B12 supplement to prevent a deficiency?
Individuals with pernicious anemia
Pernicious anemia is a form of anemia that occurs when there is an absence of intrinsic factor, a substance normally present in the stomach. Vitamin B12 binds with intrinsic factor before it is absorbed and used by your body7,14,15. An absence of intrinsic factor prevents normal absorption of vitamin B12 and results in pernicious anemia.

Anyone with pernicious anemia usually needs intramuscular (IM) injections (shots) of vitamin B12. It is very important to remember that pernicious anemia is a chronic condition that should be monitored by a physician. Anyone with pernicious anemia has to take lifelong supplemental vitamin B12.

Individuals with gastrointestinal disorders
Individuals with stomach and small intestinal disorders may not absorb enough vitamin B12 from food to maintain healthy body stores16. Sprue and celiac disease are intestinal disorders caused by intolerance to protein in wheat and wheat products. Regional enteritis, localized inflammation of the stomach or small intestine, also results in generalized malabsorption of vitamin B127. Excess bacteria in the stomach and small intestine also can decrease vitamin B12 absorption.

Surgical procedures of the gastrointestinal tract such as surgery to remove all or part of the stomach often result in a loss of cells that secrete stomach acid and intrinsic factor7, 17, 18. Surgical removal of the distal ileum, a section of the intestines, also can result in the inability to absorb vitamin B12. Anyone who has had either of these surgeries usually requires lifelong supplemental vitamin B12 to prevent a deficiency.

Older Adults
Vitamin B12 must be separated from protein in food before it can bind with intrinsic factor and be absorbed by your body. Bacterial overgrowth in the stomach and/or atrophic gastritis, an inflammation of the stomach, contribute to vitamin B12 deficiency in adults by limiting secretions of stomach acid needed to separate vitamin B12 from protein in food10, 20-24. Adults 50 years of age and older with these conditions are able to absorb the vitamin B12 in fortified foods and dietary supplements. Health care professionals may advise adults over the age of 50 to get their vitamin B12 from a dietary supplement or from foods fortified with vitamin B12 because 10 to 30 percent of older people may be unable to absorb vitamin B12 in food7, 19.

Vegetarians who do not eat meats, fish, eggs, milk or milk products, or vitamin B12 fortified foods consume no vitamin B12 and are at high risk of developing a deficiency of vitamin B129, 25. When adults adopt a vegetarian diet, vitamin B12 deficiency symptoms can be slow to appear because it usually takes years to deplete normal body stores of vitamin B12. However, severe symptoms of vitamin B12 deficiency, most often featuring poor neurological development, can show up quickly in children and breast-fed infants of women who follow a strict vegetarian diet26.

Fortified cereals are one of the few plant food sources of vitamin B12, and are an important dietary source of vitamin B12 for vegetarians who consume no eggs, milk or milk products. Vegetarian adults who do not consume plant foods fortified with vitamin B12 need to consider taking a vitamin B12-containing supplement. Vegetarian mothers should consult with a pediatrician regarding appropriate vitamin B12 supplementation for their infants and children.

Caution: Folic acid may mask signs of vitamin B12 deficiency. Folic acid can correct the anemia that is caused by vitamin B12 deficiency. Unfortunately, folic acid will not correct the underlying vitamin B12 deficiency1, 27, 28. Permanent nerve damage can occur if vitamin B12 deficiency is not treated. Folic acid intake from food and supplements should not exceed 1,000 micrograms (mcg) daily because large amounts of folic acid can hide the damaging effects of vitamin B12 deficiency7. Adults older than 50 years are advised to consult with their physician about the advisability of taking folic acid without also taking a vitamin B12 supplement.

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What is the relationship between vitamin B12, homocysteine, and heart disease?
A deficiency of vitamin B12, folate, or vitamin B6 may increase your blood level of homocysteine, an amino acid normally found in your blood. There is evidence that an elevated blood level of homocysteine is an independent risk factor for heart disease and stroke29-38. The evidence suggests that high levels of homocysteine may damage coronary arteries34 or make it easier for blood clotting cells called platelets to clump together and form a clot. However, there is currently no evidence available to suggest that lowering homocysteine level with vitamins will actually reduce your risk of heart disease. Clinical intervention trials are needed to determine whether supplementation with vitamin B12, folic acid, or vitamin B6 can help protect you against developing coronary heart disease.

What is the health risk of too much vitamin B12?
Vitamin B12 has a very low potential for toxicity. The Institute of Medicine states that "no adverse effects have been associated with excess vitamin B12 intake from food and supplements in healthy individuals7." The Institute recommends that adults over 50 years of age get most of their vitamin B12 from supplements or fortified food because of the high incidence of impaired absorption of vitamin B12 from unfortified foods in this population7.

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