Vitamin B12
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Vitamin B12

Vitamin B12, also called cobalamin, is a water-soluble vitamin. Water-soluble vitamins are stored in the body in very limited amounts and are excreted through the urine. Therefore, it is a good idea to have them in your daily diet. Vitamin B12, the most complex of the vitamins, contains the metal ion, cobalt, in its structure.

Functions

Vitamin B12’s functions include the following:

  • Assisting in the production of neurotransmitters, which are chemicals that regulate sleep, pain, and mood
  • Playing a role in carbohydrate, protein, and fat metabolism
  • Enabling proper DNA replication
  • Helping catalyze the reaction that produces succinyl-CoA, a chemical required for the synthesis of hemoglobin
  • Making red blood cells and preventing anemia
  • Aiding in the conversion of homocysteine to methionine—Elevated blood levels of homocysteine have been associated with an increased risk of heart attack.

Recommended Intake:

Age Group (in years) Recommended Dietary Allowance
(mcg)
FemalesMales
1-30.90.9
4-81.21.2
9-131.81.8
14-182.42.4
19+2.42.4
Pregnancy: 14+2.6n/a
Lactation: 14+2.8n/a

Vitamin B12 Deficiency

Most diets provide adequate B12; deficiency is often a result of absorption problems. In the stomach's acidic environment and through the action of the enzyme pepsin, vitamin B12 is released from food. People who do not have enough stomach acid (such as those taking strong acid-neutralizing medications) may not be able to separate B12 from food. People with this problem, however, absorb B12 supplements without difficulty.

Two proteins are also important for the vitamin's absorption and transport: intrinsic factor (IF) and R proteins. A B12 deficiency can result if there are any problems with pepsin, IF, or R proteins. Reduced secretion may occur in mid- to late-life. Severely reduced levels of IF lead to a condition called pernicious anemia. People with pernicious anemia have trouble absorbing B12 supplements and may need either very high doses or injections.

Symptoms of vitamin B12 deficiency include the following:

  • Pernicious or megaloblastic anemia
  • Numbness and tingling of the arms or legs
  • Difficulty walking
  • Fatigue
  • Sore tongue
  • Loss of appetite
  • Constipation
  • Memory loss
  • Disorientation
  • Moodiness
  • Damage to the myelin sheath, the protective covering around nerves
  • Dementia

Vitamin B12 Toxicity

Vitamin B12 has a very low potential for toxicity. The tolerable upper intake level (UL) for vitamin B12 from dietary sources and supplements combined has not been determined. This does not mean that there is no potential for adverse effects resulting from high intakes. Because data is limited, caution should be used when supplementing.

No symptoms of vitamin B12 toxicity have been reported.

Major Food Sources

FoodServing size Vitamin B12 content
(mcg)
Clams, steamed3 oz84
Beef liver, cooked3 oz60
Mussels, steamed3 oz20.4
Fortified breakfast cereal¾ cup6
Rainbow trout, cooked3 oz5.3
Salmon, cooked3 oz4.9
Beef, cooked3 oz2.1
Milk1 cup0.9
Yogurt1 cup0.9
Egg1 large0.5
Brie cheese1 oz0.5
American cheese1 oz0.4
Chicken, roasted3 oz0.3

Health Implications

Populations at Risk for Vitamin B12 Deficiency

The following populations may be at risk for vitamin B12 deficiency and may require a supplement:

  • People with pernicious anemia—This condition occurs when there is an absence of intrinsic factor. A person who has pernicious anemia will need to be monitored by a physician and take lifelong supplemental vitamin B12.
  • Older adults—The incidence of B12 deficiency among the elderly may be as high as 15%. A multivitamin may not contain enough B12 to compensate; talk with your healthcare provider about what your specific needs are for supplementation.
  • People with gastrointestinal (GI) disorders—GI disorders, like celiac disease, Crohn's disease, or GI surgery, can lead to malabsorption of B12.
  • Strict vegetarians—B12 is not present in any meaningful amounts in plant foods. (Certain blue-green algae do contain B12, but in a form that is not usable). Total vegetarians who consume no animal products may need to supplement with B12. Fortified cereal can be a good source of the vitamin for a vegetarian.
  • People who consume excessive amounts of alcohol—Alcoholics tend to have diets lacking in several essential nutrients, including B12.
  • People who take strong acid-neutralizing drugs—Those who take medications like Prilosec cannot absorb B12 well from food and need to take supplements.

Folate Supplementation May Mask a B12 Deficiency

Megaloblastic anemia can occur as a result of either a folate deficiency or a vitamin B12 deficiency. Supplementing with folate can correct this anemia. But, it will not correct the B12 deficiency. Permanent nerve damage can result if a B12 deficiency is left untreated. If you have megaloblastic anemia, talk with your doctor about assessing your B12 status as well as your folate status.

Homocysteine, Cardiovascular Disease, and Vitamin B12

Homocysteine is an amino acid. While most amino acids found in the body are building blocks of protein or muscle, homocysteine is not a component of protein. It is formed as an intermediate step in the production of another amino acid, methionine. The conversion of homocysteine to methionine requires a number of vitamins, including folate, B6, and B12. A deficiency of any one of these vitamins can cause the level of homocysteine in the blood to rise; this in turn might increase risk of heart disease and strokes, although the evidence for such a connection is far from conclusive.

Tips For Increasing Your Vitamin B12 Intake:

Here are tips to help increase your intake of vitamin B12:

  • Rub a little olive oil, squeeze a bit of fresh lemon, and crack some black pepper and salt on a fresh piece of salmon. Grill on the barbeque or broil it in the oven.
  • Have a bowl of fortified, high-fiber breakfast cereal in the morning.
  • Mix canned tuna with some olive oil, white beans, and salt and pepper. Enjoy with some whole wheat crackers.
  • For an afternoon snack, try a cup of yogurt. Jazz it up with some sliced fresh fruit or crunchy granola.
  • Skewer large shrimp with mushrooms, tomatoes, onion, and zucchini. Brush on a marinade and toss on the barbeque.
  • If you take a multivitamin/mineral supplement, make sure that it contains B12.

RESOURCES

American Dietetic Association
http://www.eatright.org

Nutrition.gov
http://nutrition.gov/

References:

Advanced Nutrition and Human Metabolism. Wadsworth Thomson Learning; 2000.

Andres E, Federici L, Affenberger S, et al. B12 deficiency: a look beyond pernicious anemia [review]. J Fam Pract. 2007;56:537-542.

National Institutes of Health. Facts About Dietary Supplements: Vitamin B12. Clinical Nutrition Service, Warren Grant Magnuson Clinical Center, National Institutes of Health Office of Dietary Supplements; August 7, 2001.

Wierzbicki AS. Homocysteine and cardiovascular disease: a review of the evidence. Diab Vasc Dis Res. 2007;4:143-150.



Last reviewed August 2007 by Marcin Chwistek, MD

Please be aware that this information is provided to supplement the care provided by your physician. It is neither intended nor implied to be a substitute for professional medical advice. CALL YOUR HEALTHCARE PROVIDER IMMEDIATELY IF YOU THINK YOU MAY HAVE A MEDICAL EMERGENCY. Always seek the advice of your physician or other qualified health provider prior to starting any new treatment or with any questions you may have regarding a medical condition.


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