If antioxidant supplement labels are to be believed, you should stop reading this article and gobble down some pills: Spurred by the rising sales of antioxidant supplements, Pom Wonderful, makers of pomegranate juice, now makes an antioxidant supplement that they claim has "extraordinary health benefits."

This proclamation is echoed by numerous health supplement ads in health food stores and on the Internet. For instance, Source Naturals Resveratrol advises on the General Nutrition Centers Web site that taking antioxidants "…may help prevent free-radical damage throughout the body and provide protective support to the cardiovascular system.*" Problem solved. Except a bit of a buzz-kill is delivered by the asterisked footnote: "These statements have not been evaluated by the Food and Drug Administration. This product is not intended to diagnose, treat, cure or prevent any disease."

So, do the purported health benefits of antioxidants actually exist?

Here's the theory: Just as the name suggests, antioxidants slow down oxidation, a process that is part of normal bodily functions but can also damage cells. Oxidation can even increase the stickiness of cholesterol, upping the risk that it will block circulation and cause heart attacks or strokes.

So it at least theoretically makes sense that antioxidants such as vitamin C, vitamin E and other antioxidant compounds found in carrots and other vegetables, are good for you. Ditto antioxidants found in pomegranates, red wine and licorice root. And early studies in the 1990s showed that people who ate more antioxidants had a lower risk of heart disease and stroke.

But those findings didn't hold up for antioxidant supplements. In later studies, such supplements did not affect risk of—and in some studies actually increased—heart attacks and strokes.

Nancy Cook, an epidemiologist at Brigham and Women's Hospital in Boston and a co-author of one of these studies, suggests two possible explanations for these results: It could be that studies of supplements are using the wrong doses and combinations of antioxidants. Or, people who eat lots of antioxidants—in foods, not supplements—are already doing the kinds of things that lower heart disease risk, namely exercise and, well, eating plenty of fruits and vegetables.

Biochemist Michael Aviram of the Rambam Medical Center in Haifa, Israel, suggests another alternative. His research focuses on pomegranates. In a recent study, he found that mice bred to have blockages in their arteries and developed fewer blockages in their arteries after they were fed parts of pomegranates. Because such blockages can cause heart attacks and strokes, he says his studies suggest antioxidants work against such events. And although earlier studies found that vitamin E—another antioxidant—didn't clear such blockages, he found that the kinds of antioxidants in pomegranates do. His theory: there are many sources of oxidative stress—viruses, toxins, physical strain—and each antioxidant might be effective against a particular type of stress, but not the others.

In other words, it depends whether the antioxidants you're taking are fighting against the good, normal oxidation in your body or the bad oxidation. "The devil's in the details in a lot of these things," says Andrew Shao of the Council for Responsible Nutrition, a trade association in Washington, D.C., representing supplement manufacturers. "If you were to take all the marketing at face value, you might think that the ideal situation is to have no oxidation whatsoever. That is not what you want. It's part of normal biochemistry, part of the immune system."

"Most [antioxidants] don't work alone," he adds. "They're not drugs."

Like Cook, Shao recommends a diet high in antioxidants and a healthy lifestyle that includes regular exercise. "That may run counter to what you see in some marketing," he says, although he refused to say which ads he meant.

Marketing also makes it difficult for consumers to know what they are getting. "Is the product on the shelf the same one that was studied in clinical trials?" Shao asks. For foods, the Food and Drug Administration (FDA) and the Federal Trade Commission monitor claims made on the label or in advertising, but "it very much depends on the strength and specificity of the language in the claim," he says. A product that claims "antioxidant support" will raise less ire than one that claims to lower the risk of a particular disease.

In any case, the research is clear: Large, carefully controlled studies and trials have consistently found no benefit to antioxidant supplements, says Alice Lichtenstein of Tufts University.* "You have to take the totality of the data, and that's what we normally do [in science]," she says. "Why are they popular? I don't know. Maybe because it sounds like the easy answer."

With reporting by Willa Austen Isikoff

* Note (7/7/08): This sentence has been modified since the original posting.