The body requires vitamin B-12 for synthesis of genetic materials such as DNA and RNA, neurological functions, and healthy red blood cell formation. It is a water-soluble vitamin available naturally in foods, such as meat, shellfish, chicken, milk, and eggs, and as a supplement. Recommended intake is between 2.4 to 2.8 micrograms per day, depending on gender. An upper limit has not been set as there is little potential for toxicity.
Toxicity
Unlike most water-soluble vitamins, the liver is capable of storing vitamin B-12 for years, suggesting B-12 could build up in the body. However, there is no evidence that high intake of B-12 either from food, supplement or injections leads to toxicity. Vitamin B-12 levels as high as 1000 micrograms have been given with no toxic side effects.
Absorption
Vitamin B-12 requires stomach acid to release it from protein, and intrinsic factor, a protein secreted by stomach cells, for absorption in the blood. It can also be absorbed by passive diffusion, but this method is very inefficient. When high doses of vitamin B-12 are given orally, little is actually absorbed. This may be the reason for no toxic side effects.
Side Effects
While no major toxicity has been reported, MayoClinic.com reports possible side effects including diarrhea, itching, a rash and rosacea fulminans. Symptoms can continue for up to four months after the supplement is stopped. High doses used to treat vitamin B-12 deficiency can mask folate deficiency, lead to peripheral vascular thrombosis, and potentially, hypokalemia and gout in some individuals. Care should be taken when treating deficiencies. Vitamin B-12 is also contraindicated in individuals with Leber's disease.
Angioplasty Interaction
Vitamin B-12 complex, which includes folic acid, vitamin B6 and B-12, can lead to re-narrowing of the blood vessels in patients who have undergone angioplasty. MayoClinic.com reports that this combination given intravenously followed by oral administration taken daily can lead to restenosis. Supplements should not be taken without speaking to your health care provider first.
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