Vitamin B-12 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 B-12. Second, a protein called intrinsic factor is necessary for the absorption of vitamin B-12. Most people can stomach quite a lot of this vitamin without problems.
Your body uses vitamin B-12 for synthesizing DNA, maintaining the nervous system and forming red blood cells. It is also important for metabolism and neurological function.
Most adults need 2.4 micrograms per day. Women who are pregnant and nursing need 2.6 micrograms per day and 2.8 micrograms per day, respectively.
Vitamin B-12 mainly comes from animal sources: chicken, fish, dairy products, eggs, meat and seafood. Many breakfast cereals are also fortified with it. You can take supplements of vitamin B-12, B-complex vitamins or multivitamins if you don't eat enough of these foods to meet your needs. Vegans in particular need to be careful about getting enough B-12.
There's no official maximum dosage of vitamin B-12 because there's no evidence of problems with high intakes of this vitamin for healthy people. If you have Leber's disease, vitamin B-12 can cause your optic nerve to atrophy quickly. It may also increase the rate at which your arteries become blocked again after angioplasty. Some people experience diarrhea, itching or rashes when they take vitamin B-12 supplements. Pregnant women should stick to the recommended dietary allowances.
If you're over 50, you may need to get your daily dosage through supplements or fortified foods because the vitamin can become more difficult to absorb as you age. Taking B-12 in large amounts doesn't always help you absorb more -- intrinsic factor, the stomach protein that binds with vitamin B-12 so you can absorb it, gets overloaded after a certain point. This may explain the lack of adverse effects at higher doses.
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