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Vitamin B12

What is vitamin B12?

Discovered in 1948, vitamin B12 is a water-soluble vitamin absorbed in the intestines and carried throughout the body in the bloodstream. Since it is not stored in body fat, after the body uses what it needs, any excess B12 is excreted via urine or sweat.

Why do we need it?

Vitamin B12 is essential for the production of red blood cells. It plays a role in the metabolization of proteins and fats and the synthesis of myelin, a fatty substance that encases nerve fibers. Vitamin B12 also displays some antioxidant properties. It works with folate to convert the amino acid homocysteine into methionine, a substance that helps prevent cells from becoming malignant.

Vitamin B12 injections have also been rumored to increase energy, although at present, there has been no scientific evidence to substantiate this claim.

How much vitamin B12 should I take?

According to the National Academy of Sciences, the recommended daily allowance (RDA) for vitamin B12 is as follows:

  • Adult men: 2-2.4 micrograms/day
  • Adult women: 2-2.4 micrograms/day
  • Children aged 7-10: 1.4 micrograms/day
  • Infants: 0.5 micrograms/day
  • Pregnant/lactating women: 2.6 micrograms/day

What are some good sources of vitamin B12?

The only natural dietary sources of vitamin B12 come from animal products such as meats, eggs, milk, cheese, yogurt and fish. Clams and oily fish such as tuna, cod and sardines are particularly high in B12. Most fortified cereals also contain high quantities of vitamin B12.

What can happen if I don't get enough vitamin B12?

While B12 deficiency is rare in young people, the elderly may have trouble absorbing natural vitamin B12 and require supplements. Symptoms of B12 deficiency can include memory loss, instability, disorientation, nerve damage, decreased reflexes and possible hearing loss. A lack of B12 has also been linked with increased levels of homocysteine, which in turn has been associated with heart disease, birth defects and Alzheimer's disease.

B12 deficiency may also be caused by a genetic defect, in which a protein known as gastric intrinsic factor is not present in the body. In such cases, a condition known as pernicious anemia can develop. The condition must be treated with B12 injections, or neurologic damage may occur.

What can happen if I take too much?

To date, no reports of toxicity have been associated with high intake levels of vitamin B12. Because it is water-soluble and is not stored in the body, the chances of enough B12 building up to toxic levels are extremely unlikely.

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• B vitamins may cut heart disease risk. Harvard Health News April 1998.
• B vitamins and the heart: what men can learn from women. Harvard Men's Health Watch June 1998.
Recommended Dietary Allowances, 10th ed. Washington, D.C.: National Academy Press, 1989.
• The effect of vitamin B12 deficiency on older veterans and its relationship to health. J Am Geriatr Soc October 1998.
Vitamin B12 Fact Sheet. American Health, 1999.

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