Vitamin B-12 is considered one of the safer vitamins from a toxicity perspective. People are much more likely to have a deficiency than to overdose. But in cases of extremely high doses, such troubling symptoms as numbness and tingling, heart palpitations, anxiety attacks, insomnia and skin rashes may occur.
Function of B12
Vitamin B-12 is one of the water-soluble vitamins, meaning it is not stored in the body. It can be consumed in food, or taken as a prescription medicine or dietary supplement. Since B-12 contains cobalt, B-12 compounds are called "cobalamins." The human body requires methylcobalamin and 5-deoxyadenosylcobalamin for neurological functions, red blood cell formation and DNA synthesis. In food, B-12 is bound to protein. Stomach acid frees the B-12 from the food. The form of B-12 in supplements is already free. Only a small part of the B-12 dose is successfully absorbed when taken orally, which may explain why it is usually non-toxic. Absorption rate decreases with the quantity.
Dosage
According to the National Institutes of Health, the recommended daily dosage of B-12 for people over 14 is 2.4 mcg; pregnant women should take 2.6 mcg. Babies 0 to 6 months should get .4 mcg and then .5 mcg until their first birthdays. Children 1 to 3 years old should take .9 mcg, 4 to 8 year olds should get 1.2 mcg and 9 to 13 year olds should take 1.8 mcg. Multivitamins usually contain B-12 far beyond the recommended daily allowance, providing between 2,000 and 5,000 mcg, according to the Cellular Nutrition website.
Toxic Symptoms
B12 is considered safe enough that the Food and Drug Administration has set no upper limit on its dosage. In cases of megadoses, toxicity can occur. Symptoms to watch out for are numbness and tingling in the hands and feet. If this happens to you, immediately discontinue your B12 usage.
B12 Toxicity and Cancer
Vitamin B12 indiscriminately increases cell division. This action may exert a negative influence on cancer cells. One study in the June 1984 issue of "American Journal of Medicine" reports that a patient developed leukemia after receiving megadoses of B-12 to treat her anemia. However, this is an isolated case. A study conducted in Umeå University Hospital in Sweden and published in February 2005 in "International Journal of Cancer"suggests that high levels of B-12 and folate may increase the risk of prostate cancer. However, the results weren't conclusive. The researchers characterized the findings as "novel" and called for further studies.
Toxicity from Injection
Vitamin B-12 is often injected as a therapy for pernicious anemia. Injecting the vitamin leads to a higher absorption rate than taking it orally. According to the Cellular Nutrition website, some cases of hyperthyroid and atrophy of the optic nerve have been linked to B-12 injections.
Deficiency
Vitamin B-12 deficiency is much more common than toxicity. According to the Oregon State University Information Center, 10 to 15 percent of people over 60 do not absorb enough B-12. Since the foods richest in B-12 are of animal origin, vegans and vegetarians are at risk of B-12 deficiency. Pernicious anemia is one condition caused by not getting enough B-12.
References
- Linus Pauling Insitute: Micronutrient Information Center
- Cellular Nutrition: B Vitamins
- Vitabase: B12 Overdose
- "International Journal of Cancer"; Plasma folate, vitamin B12, and homocysteine and prostate cancer risk: a prospective study; Hultdin, Van Guelpen, et. al; February 2005
- National Institutes of Health: Office of Dietary Supplements
- "American Journal of Medicine"; Unusual case of acute leukemia: Coexisting acute leukemia and pernicious anemia; Vogelsang, Spivak; June 1984
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