It used to be that we took vitamins to prevent deficiency diseases. But nowadays, scientific studies are showing how some vitamins and minerals may actually prevent diseases as big and deadly as osteoporosis, neural tube defects, heart disease and cancer. In our zeal to take advantage of those benefits, many of us take supplements in megadoses, hoping that if a little is good, more is better. But that's seldom the case. The fact is some nutrients are potentially toxic when ingested in sufficiently large amounts. In addition, high dosage vitamin and mineral supplements can interfere with the normal metabolism of other nutrients and with the therapeutic effects of certain drugs.
The following vitamins and minerals are ones that you should be particularly cautious about when taken in supplemental form. When choosing these supplements, stick to those that don't exceed 100% of the RDA.
- Vitamin A: When taken for a sustained period of time, vitamin A is toxic at doses as low as 50,000 IU (15,000ug of retinol) a day (10 times the RDA). Vitamin A is stored in fat, particularly in the liver, where it can cause jaundice and liver damage. There are many other toxic manifestations including: headache, nausea and vomiting, blurred and double vision, hair loss, bone pain and dry mucus membranes. On the other hand, carotenoids, which your body turns into vitamin A, are not known to be toxic even when ingested in large amounts for weeks to years, but they may turn your skin orange.
- Vitamin D: This vitamin is potentially toxic, especially to children. Too much can lead to the deposition of calcium in soft tissue, causing irreversible renal and cardiovascular damage. Sometimes as little as 5 times the RDA can be toxic. During the warmer months, sun stimulated production of vitamin D, plus the consumption of two glasses of milk a day is adequate for kids and adults. Use a dietary supplement only if you don't drink fortified milk, and make sure the supplement does not exceed 100% of the RDA.
- Folic Acid: During their reproductive years, women should get 400ug/day to help prevent neural tube birth defects. But megadoses don't give you more protection. In fact, they can cause harm by interacting to create dangerous zinc deficiencies.
- Iron: For adults, daily intakes from 25 to 75 milligrams appear to be safe. However, iron supplements can be deadly to children and poisoning occurs when children accidentally eat iron supplement pills. A dose of 3 grams can be fatal to a two year old. A dose of 1,000 mg for every 10 pounds of body weight is fatal to an adult. Even at lower doses, too much iron from supplements can cause constipation, upset stomach, diarrhea and abdominal pain. Some people have enhanced iron absorption and are genetically at risk from iron overload or hemochromatosis, which can result in the failure of multiple organ systems.
- Calcium: Supplements much above the RDA aren't recommended. There is no evidence of ill effects up to 2500mg/day, but very large amounts can induce constipation, result in the deterioration of kidney function and put some men at risk of kidney stones.