Because vitamin E is an antioxidant, it kinda makes sense that it should be good for you, and some studies have hinted that it can help prevent heart disease and cancer. So a lot of people take vitamin E, some at high doses. A meta-analysis of studies on vitamin E suggests those people may not have been doing themselves any favors.

What the researchers wanted to know: Does taking vitamin E increase mortality?

What they did: The researchers searched for all reports, in any language, of clinical trials in which people took vitamin E supplements.

They looked high, they looked low, they looked in the Cochrane database of randomized, controlled files, they looked in their own file cabinets. The researchers only used trials that randomly assigned participants to take vitamin E, didn’t look at pregnant women, followed participants for at least a year, and had at least 10 deaths. They analyzed the data from all 19 trials, looking at deaths and how much vitamin E people in the trial took. The studies used were looking at the effect of vitamin E on Alzheimer’s disease, heart disease, and cataracts, among other conditions.

What they found: It’s a little complicated. Study participants who took vitamin E were not more likely to die than people who didn’t take vitamin E. However, when the researchers looked at the amount of vitamin E participants were taking, people on 400 international units or more of vitamin E a day were more likely to die than people taking less than 400 IU a day. Further analysis suggested that the higher the dose of vitamin E, the higher the risk of death. This all points at some risk of taking a high dose of vitamin E. (For comparison, the RDA of vitamin E for adults is 22.5 IU, a little more for lactating women.)

What the study means to you: This should give pause to people who take high doses of vitamin E while thinking, “Well, it can’t hurt.” Well, maybe it can. The authors say that, given these data and an earlier report that high doses of beta carotene are harmful, people should be discouraged from taking high-dose vitamin supplements until there’s evidence that they work and they won’t kill you. The researchers don’t know why high doses of vitamin E might be harmful, although they suggest a few possibilities-like that having so much vitamin E somehow displaces other antioxidants, or that vitamin E might somehow interfere with blood clotting or vitamin A distribution. We clearly still have a lot to learn about antioxidants.

Caveats: A problem in any meta-analysis is that they rely mainly on published data, and people are more likely to publish interesting results than boring results; a study that finds vitamin E cures cancer and brings about world peace is more likely to get published than one that finds that vitamin E doesn’t really have any effect on heart disease. The researchers who did this meta-analysis almost definitely missed out on some studies that might have changed their results. Also, many of these studies were looking at vitamin E and chronic disease, so they used participants with chronic diseases; the results might be different for healthy adults. (But then, a lot of the people who take vitamin E have heart disease or cancer.)

Find out more: The National Institutes of Health’s Office of Dietary Supplements has a fact sheet on vitamin E (http://ods.od.nih.gov) that runs through the research on vitamin E and different chronic diseases.