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Antioxidants have been touted as the answer to everything from heart disease to erectile dysfunction. But in fact, antioxidant supplements have been studied for almost 20 years and the results have been overwhelmingly poor, ranging from having no effect to significantly increasing the risk of death. Here's what the science tells us:

What are antioxidants?

Antioxidants fight free radicals, which are unstable chemicals that are made when you eat, exercise and when you're exposed to pollution, cigarette smoke, alcohol and the sun. These molecules grab electrons from other cells, a process that is thought to contribute to cancer, cardiovascular disease, diabetes, Alzheimer's, Parkinson's and eye diseases.

Antioxidants are the peacemakers in this scenario. Created naturally by your body or ingested through the food you eat, antioxidants contain extra electrons. Lab studies have shown that antioxidants such as vitamins A, C and E give these extra electrons to free radicals, which neutralizes them.

Fruits and vegetables are high in antioxidants, and studies have shown that people who eat plenty of fruits and vegetables are less likely to get diseases such as heart disease, vision loss and cancer. But many of those are cohort studies, where researchers look at how the habits of large groups of people relate to diseases. While they try to control for all relevant factors, it's difficult to prove cause and effect this way.

Are antioxidants good for you?

That uncertainty that comes with cohort studies is the reason why, in the 1990s, researchers moved on to the next step: randomized controlled trials that gave participants antioxidants in the form of supplements. That allowed the antioxidants to be compared against placebos, and to be tested in higher doses. Studies looked at a variety of antioxidants, from beta-carotene and retinol to vitamins E and C.

It didn't go well. In the mid-1990s, one of the first trials, the Beta-Carotene and Retinol Efficacy Trial, was stopped early after it became clear that the group taking the supplements had a 28 per cent higher chance of getting lung cancer, and were 17 per cent more likely to die.

"If you protect the body's cells, you may protect the bad as well as the good," explains David Jenkins, the creator of the GI diet and a scientist at the Li Ka Shing Knowledge Institute of St. Michael's Hospital. Other large-scale studies followed, and found, among other things, no effect on the risk of cancers or of cardiovascular disease.

A 2012 Cochrane review on antioxidants covered 78 randomized clinical trials. Together, those trials included more than 200,000 healthy people and more than 80,000 people with stable diseases, such as cardiovascular disease. On the whole, the review found a slightly negative effect. People who had taken antioxidants were 3 per cent more likely to die. That risk was true for beta-carotene, possibly true for vitamin E and vitamin A, and wasn't seen in vitamin C or selenium.

The bottom line

All of this evidence suggests that antioxidants – when taken as a supplement – aren't good for you. So how does that fit with the research that shows fruits and vegetables are good at preventing disease?

There are many theories. The lower doses of antioxidants in fruits and vegetables might make a difference. Antioxidants could be combined in a different way in whole foods. Or something else in those foods could be what's beneficial – or could be making the antioxidants effective.

Antioxidants may turn out to be acting differently in foods to offer a health effect – maybe there are synergistic effects, for example. But right now, that's all very much unproven.

Based on the studies that we do have, the takeaway is simple: don't take antioxidant supplements, especially since the research suggests they may be harmful. And don't be swayed by processed foods that tout antioxidant benefits. But do eat plenty of fruits and vegetables, because something in them really does seem to fight disease. A 2014 systematic review published in the British Medical Journal found that every additional serving of fruits and vegetables reduces your risk of death by 5 per cent, up to five servings a day.

This story first appeared in Healthy Debate, an online publication guided by health care professionals and patients that covers health policy and evidence-based medicine in Canada

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