We tend to think of vitamins as healthy. But can you have too much of a good thing?
New research suggests this may be the case when it comes to supplements.
In a meta-analysis study published in the Journal of the American Medical Association, researchers reviewed 68 studies involving more than 200,000 patients to determine whether taking high-dose vitamin supplements -- in particular, beta carotene, vitamin A, vitamin C, vitamin E and selenium -- affected your risk of dying.
What they found was that some of these supplements actually increased risk of death by a small, but significant, amount.
Taking large doses of vitamin A increased the risk of death by 16 percent. Smaller increases were seen for vitamin E (4 percent) and beta carotene (7 percent).
Vitamin C and selenium did not appear to affect the risk of dying.
But exactly how the high doses of supplements affect the risk of death is not clear. The study authors speculate that perhaps the vitamins interfere with the body's defense mechanisms.
The researchers arrived at their conclusions by pooling the results from many different, previously published studies.
Because each of the studies involved in this meta-analysis was very different, though, it is hard to generalize the findings to one particular person, such as you or a family member.
Dr. Meir Stampfer, professor of nutrition and epidemiology at the Harvard School of Public Health, states, "The effects might well differ according to particular characteristics of the population, such as enhanced risk among smokers for beta carotene, but no increased risk among nonsmokers."
Select people should still take supplements. For instance, people who have had gastric bypass surgery need large amounts of vitamin A, says internist Dr. Tina Dobsevage, assistant clinical professor of medicine at Mount Sinai School of Medicine.
She says she also prescribes vitamins D and B12 to patients who have low blood levels of these nutrients.
However, others have conditions that increase the health risk associated with certain supplements. Vitamin E, for example, has blood-thinning properties and may increase the risk of bleeding in people taking prescription blood thinners.
Information that large doses of some vitamins can be harmful is not new. When taken in excess, the fat-soluble vitamins -- vitamins A, D, E and K -- are stored in body fat tissues. This can lead to toxic buildup in the liver, brain and heart.
Excess amounts of water-soluble vitamins, on the other hand, are less hazardous because they are eliminated from the body in the urine.
It is also known that vitamins taken as supplements, rather than in whole foods, tend to be less beneficial. Thus, the study authors say, people should not shy away from fruits and vegetables for fear that they are overloading on vitamins.
An estimated 10 percent to 20 percent of U.S. adults -- 80 million to 160 million people -- look to vitamin supplements for a nutritional boost.
Supposed health benefits include longer life, a healthier heart and stronger bones.
Antioxidant supplements, which include the vitamins in the current study, are thought to fight off substances called "free radicals" and improve immune function.
Dr. Kathy Helzlsouer, a women's health specialist and director of the Prevention and Research Center at Mercy Medical Center in Baltimore, thinks the study has "extremely significant findings."