Vitamin B12 is a water-soluble vitamin that has a couple of interesting characteristics that separate it from other vitamins of this type. First, unlike most water-soluble vitamins, the liver actually stores vitamin B12. Second, a protein called intrinsic factor is necessary for the absorption of vitamin B12.
Function
Vitamin B12 is used by the body for synthesizing DNA, maintaining the nervous system and forming red blood cells. It is also important for metabolism and neurological function.
Recommendations
Babies up to 6 months old need 0.4 mcg per day of vitamin B12, and babies between 7 and 12 months old need 0.5 mcg per day. Children between the ages of 1 and 3 need 0.9 mcg per day, those between the ages of 4 and 8 years old need 1.2 mcg per day and those between the ages of 9 and 13 need 1.8 mcg per day. Children ages 14 and above and adults need 2.4 mcg per day, with those over 50 getting their vitamin B12 through supplements or fortified foods due to absorption issues. Women who are pregnant and nursing need 2.6 mcg per day and 2.8 mcg per day, respectively.
Sources
Vitamin B12 is found mainly in foods from animal sources, such as chicken, fish, dairy products, eggs, meat and seafood. However, many breakfast cereals are fortified with this vitamin, and some other foods are as well. You can take supplements of vitamin B12, B-complex vitamins or multivitamins if you do not eat enough of these foods to meet your needs.
Toxicity
No upper limit is set for vitamin B12 because there is no evidence of problems with high intakes of this vitamin for healthy people. However, those with Leber's disease, a hereditary condition involving the optic nerve, shouldn't take this supplement, nor should those undergoing angioplasty, according to the Mayo Clinic. Vitamin B12 can cause the optic nerve to atrophy quickly for those with Leber's disease, and it may increase the rate at which arteries become blocked again after angioplasty. Some people experience diarrhea, itching or rashes when they take vitamin B12 supplements. Pregnant women should stick to the recommended dietary allowances.
Significance
Because intrinsic factor is required for the absorption of vitamin B12, the percentage of this vitamin absorbed, when taken in large amounts, decreases as the amount goes up due to a lack of capacity based on amounts of intrinsic factor, the protein necessary for vitamin B12 absorption. This may be responsible for the lack of adverse effects at higher doses.
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