Vitamin B12 is classified as a water-soluble vitamin. Unlike other water-soluble vitamins, however, B12 is stored for years in the liver and surplus amounts the body doesn't use are not eliminated in the urine. B12 has the most complex and largest chemical composition of the vitamins, and is unique in containing a metal ion, states the Linus Pauling Institute.
Function and Absorption
Compounds containing vitamin B12 are necessary in the body for effective metabolism of nucleic acids, maintaining the fatty sheaths that surround nerve cells and forming red blood cells. A compound known as intrinsic factor is required for absorption of the vitamin. People without sufficient intrinsic factor will experience a B12 deficiency and a resulting decrease in healthy red blood cells. This condition is called pernicious anemia, and can result in a greater risk of stomach or gastrointestinal cancers.
Recommended Intake
The Institute of Medicine, Food and Nutrition Board's Recommended Dietary Allowance of B12 is 2.4 mcg per day, and most people consume adequate amounts of the vitamin through their diet. Vegetarians who do not eat any animal products should use supplemental versions of the vitamin to meet the RDA, notes the Linus Pauling Institute. Persons over the age of 50 should aim to get their daily B12 dose primarily through supplements or fortified foods, because of their increased risk of B12 malabsorption. Women who expect to become pregnant should do the same.
Toxicity
Large intakes of vitamin B12 are not correlated with any negative effects in healthy people, reports the Linus Pauling Institute. People taking B12 supplements by mouth or by intramuscular injection to treat pernicious anemia have taken doses as large as 1,000 mcg without adverse effects.
Sources
Vitamin B12 occurs naturally in meat, poultry and fish, and in smaller amounts in milk and milk products. While B12 is not found naturally in plant products, many fortified grain products like cereals contain the vitamin.
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