Known as a double-blind control trial, this is the gold standard of pharmaceutical research. Since the 1970s, there have been many trials like this trying to figure out what antioxidant supplementation does for our health and survival. The results are far from heartening.

In 1994, for example, one trial followed the lives of 29,133 Finish people in their 50s. All smoked, but only some were given beta-carotene supplements. Within this group, the incidence of lung cancer increased by 16%.

A similar result was found in postmenopausal women in the U.S. After 10 years of taking folic acid (a variety of B vitamin) every day their risk of breast cancer increased by 20% relative to those women who didn’t take the supplement.

It gets worse. One study of more than 1,000 heavy smokers published in 1996 had to be terminated nearly two years early. After just four years of beta-carotene and vitamin A supplementation, there was a 28% increase in lung cancer rates and a 17% increase in those who died.

These aren’t trivial numbers. Compared to placebo, 20 more people were dying every year when taking these two supplements. Over the four years of the trial, that equates to 80 more deaths. As the authors wrote at the time, “The present findings provide ample grounds to discourage use of supplemental beta-carotene and the combination of beta-carotene and vitamin A.”

Fatal ideas

Of course, these notable studies don’t tell the full story. There are some studies that do show benefits of taking antioxidants, especially when the population sampled doesn’t have access to a healthy diet.

But, according a review from 2012 that noted the conclusions of 27 clinical trials assessing the efficacy of a variety of antioxidants, the weight of evidence does not fall in its favour.

Just seven studies reported that supplementation led to some sort of health benefit from antioxidant supplements, including reduced risk of coronary heart disease and pancreatic cancer. Ten studies didn’t see any benefit at all – it was as if all patients were given the sugar pill also (but, of course, they weren’t). That left another 10 studies that found many patients to be in a measurably worse state after being administered antioxidants than before, including an increased incidence of diseases such as lung and breast cancer.