Is it possible to take too many vitamins?

DEAR DOCTOR K:

I take vitamin and mineral supplements. Do I need to worry about getting too much of certain nutrients?

DEAR READER:

Many people take individual vitamin and mineral supplements in addition to a powerful multivitamin. But ingesting too much of certain micronutrients can be dangerous. It’s harder — but not impossible — to get dangerously high amounts of micronutrients from food alone.

To play it safe, avoid taking more than the recommended dietary allowance (RDA) of any micronutrient through supplements. (To check the RDA for any supplement, visit: ods.od.nih.gov/factsheets/list-all.)

It is especially important to avoid taking too much of the vitamins and minerals listed below:

Vitamin A. Most of us get plenty of vitamin A in foods — liver, milk, eggs, carrots, spinach and many other foods. Too much vitamin A in supplements can harm bones and can lead to birth defects. About 20 years ago there was some evidence that beta carotene supplements, which the body converts to vitamin A, might have health benefits. However, newer studies in the past two decades do not show clear benefits. Too much beta carotene can also give your skin and eyes a yellowish hue.

VitaminĀ E. Too much vitamin E can cause bleeding, headache, fatigue and blurred vision. Until about 10 years ago, I thought the weight of scientific evidence suggested that vitamin E supplements might have health benefits. But bigger and better studies in the past decade generally do not show benefits.

Calcium (for men). Recent studies have found that excess intake of calcium appears to increase the risk of prostate cancer. While not all scientists agree, I think that most men should avoid taking calcium supplements and should not consume too many dairy products. An exception is men who have osteoporosis, or thin bones, who may need a combination of calcium and vitamin D.

Iron. Large doses of iron supplements can trigger an iron overload. Some people inherit a genetic condition that causes them to absorb more iron from the gut than most people. This can damage body tissues and can raise the risk of heart disease, liver cancer, infections and arthritis. Your body can’t easily shed excess iron. Also, taking high doses of vitamin C allows your body to absorb more iron than it normally would. In my opinion, the only people who should take iron supplements are people who have a clear iron deficiency, as shown by blood tests.

Zinc. Getting enough but not too much zinc is a bit of a high-wire act. The RDA for zinc is 8 mg for women and 11 mg for men. Yet levels higher than 15 mg can trigger side effects, such as a depressed immune system, poor healing, hair loss and interference with taste and smell. It’s best to get zinc from food sources rather than supplements.

We have more information on dietary supplements in our Special Health Report, “Vitamins and Minerals: Choosing the nutrients you need to stay healthy.” You can learn more about this report here