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Why Are Megadoses of Vitamins Considered Dangerous?

By Bonnie Singleton

Americans are sold on taking vitamin and other health supplements, to the tune of $23 billion annually, as reported in the “New York Times” in 2009. But more is not necessarily better, and if you take too much of certain types of vitamins, you could be placing your health at risk. Until there are definitive human clinical trials showing otherwise, using moderation with vitamin supplements is the safest course.


The United States Food and Drug Administration is responsible for establishing the Recommended Daily Intake, also known as the Recommended Daily Allowance, for all vitamins, minerals and other nutrients, with 100 percent being the maximum recommended dose per day. The threshold for megadose vitamin therapy is generally considered to be at least 10 times greater than the RDA, according to the “American Journal of Clinical Nutrition.” Most people get all the vitamins they need from foods, including the water-soluble B-complex and C vitamins, and the fat-soluble vitamins, which include A, E, D and K. Knowing this distinction is important because unlike water-soluble vitamins that dissolve in water and aren’t stored, fat-soluble vitamins are stored in your liver.


It’s difficult to consume a high enough dose of most water-soluble vitamins to cause dangerous side effects. However, very high doses of vitamin C can lead to diarrhea, bloating, cramps and an increased risk for kidney stones, and large amounts of the B-vitamin niacin can cause abnormal liver function, cramps, nausea and irritability. The fat-soluble vitamins are another matter: too much vitamin E often causes digestive tract disorders; too much vitamin A can cause growth retardation in children, an enlarged liver and spleen, hair loss, bone pain, skin problem and increased pressure in your skull; and overdoses of vitamin D can cause retardation, kidney damage and calcium loss.

Expert Insight

A study looking at vitamin use among the over 160,000 participants in the long-term Women’s Health Initiative, tried to find if there was a link between multivitamin use and risk of cancer and heart disease. The results, published in 2009 in the “Archives of Internal Medicine,” found convincing evidence that multivitamin use has almost no influence on the risk of common cancers, cardiovascular disease or total mortality in postmenopausal women. A separate study, published in “The New England Journal of Medicine” in 2003, showed that high concentration of vitamin A, even below the megadose threshold, led to a significant increase in the risk for high fractures and other broken bones.


Proponents of high-dose vitamin therapy have conducted their own research into how safe and effective such therapy can be. Doctors have been using mega-doses of vitamin D for years to treat tuberculosis, while many studies have tried to tie vitamin D to the prevention and treatment of autoimmune diseases, cancer and heart conditions, with mixed results. In the April 2002 issue of “The American Journal of Clinical Nutrition,” Bruce N. Ames reported a common thread between high-dose megavitamin therapies, particularly the eight B vitamins, and the effective treatment of genetic diseases. Until such claims are verified, however, consult your health care provider before taking megadoses of vitamins, and to be safe, don’t exceed the 100 percent RDA dose.

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