Are there any vitamins or minerals that are harmful if taken in large quantities?

The American supplement market is a multibillion dollar industry.  People often feel that taking supplements will help them meet their nutritional needs which they miss due to poor dietary intake. Others believe that more of one vitamin or mineral is better and will not harm them.  This is a fallacy as some vitamins and minerals are toxic at high levels. 

Vitamin A is a good example of the dangers of high vitamin intake. A fat-soluble vitamin, in large quantities vitamin A can cause yellowing of the skin, hair loss, headaches, dry skin, joint pain and birth defects. Vitamin B6, which is water-soluble, can also be toxic at high doses.  People suffering from carpal tunnel syndrome or premenstrual syndrome often take large doses of vitamin B6 to lessen the side effects.  Taking more than 150 milligrams a day can cause irreversible peripheral nerve damage, chest pain and muscle weakness. 

Mega doses of vitamins or minerals may interfere with the absorption and metabolism of other nutrients in the body.  For example, large amounts of iron can decrease the body’s absorption of zinc, copper and calcium. High levels of vitamin D can cause a vitamin K deficiency and a reduction in calcium absorption.  Large doses of folic acid may interfere with zinc absorption. 

It is best to follow the Recommended Daily Intakes (RDI’s) for vitamin and minerals and eat a well balanced diet with a variety of fruits, vegetables, complex carbohydrates and protein each day to meet your dietary needs. 

Because of the lack of regulations in the supplement industry it is a “buyer beware” market.  Before taking large doses of any supplement, research the product for safety and efficacy and inform your medical doctor that you are taking the supplement. In some instances, supplements interact with medications and affect medical conditions. For further information on buying supplements see the following article from TheDietChannel: How to Protect Yourself When Purchasing Supplements.

Dena McDowell, MS, RD Contributing Expert

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