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RDAs and Megadoses
If you decide to take a vitamin/mineral supplement, the Food and Nutrition Board of the National Research Council recommends avoiding supplements that exceed 100 percent of the Recommended Daily Allowance. High doses of vitamins A, D, and E, and many minerals, can seriously harm your vital organs, and can also interfere with the absorption of other nutrients.
Almost all major health organizations agree, however, that the current RDAs are too low. The RDAs reflect the amount of each nutrient that's required to avoid deficiency; they do not indicate the amount of each nutrient that will promote optimal health. Researchers are currently working to establish new numbers for the essential nutrients based on recent findings that higher levels may offer myriad benefits.
Vitamin and mineral dosage is a complicated and controversial issue. Many "alternative medicine" practitioners, as well as some mainstream health experts, believe that so-called megadoses of certain vitamins, such as C, can do no harmand may do wonders to fight cancer, AIDS, and other diseases. For now, however, unless your health practitioner recommends high doses of nutrients to fight a disease or other illness, you are well advised to wait for conclusive evidence about the benefits and perils of megadoses.

Vitamin K
What it does: forms proteins that aid in blood clotting and other essential bodily functions. At least half the daily requirement is manufactured in the body.
What it may do: help prevent osteoporosis by aiding calcium retention.
RDA: 65 to 80 micrograms.
Where to find it: dairy products; vegetables, particularly green cabbage, Brussels sprouts, spinach, cauliflower, peas, and carrots; liver; and beans.

 
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