Saturday, November 29, 2008

Big trouble: Vitamin megadoses

Can you get too much of a good thing? Darn right, you can. Some vitamins are toxic when taken in the very large amounts popularly known as megadoses. How much is a megadose? Nobody knows for sure. The general consensus, however, is that a megadose is several times the RDA, but the term is so vague that it’s in neither my medical dictionary nor the dictionary on my computer.
  • Megadoses of vitamin A (as retinol) may cause symptoms that make you think you have a brain tumor. Taken by a pregnant woman, megadoses of vitamin A may damage the fetus.
  • Megadoses of vitamin D may cause kidney stones and hard lumps of calcium in soft tissue (muscles and organs).
  • Megadoses of niacin (sometimes used to lower cholesterol levels) can damage liver tissue.
  • Megadoses of vitamin B6 can cause (temporary) damage to nerves in arms and legs, fingers, and toes.
But here’s an interesting fact: With one exception, the likeliest way to get a megadose of vitamins is to take supplements. It’s pretty much impossible for you to cram down enough food to overdose on vitamins D, E, K, C, and all the Bs. Did you notice the exception? Right: vitamin A. Liver and fish liver oils are concentrated sources of preformed vitamin A (retinol), the potentially toxic form of vitamin A. Liver contains so much retinol that early 20th century explorers to the South Pole made themselves sick on seal and whale liver. Cases of vitamin A toxicity also have been reported among children given daily servings of chicken liver. On the other hand, even very large doses of vitamin E, vitamin K, thiamin (vitamin B1), riboflavin (vitamin B2), folate, vitamin B12, biotin, and pantothenic acid appear safe for human beings.

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