Do antioxidants promote health — or fuel cancer?

Gut Check is a periodic look at health claims made by studies and by newsmakers. We ask: Should we believe this?

The Claim: Antioxidants make malignant melanoma cells more invasive, causing them to metastasize farther, faster.

The Backstory: Antioxidants have become a multibillion-dollar business, sold as dietary supplements or added to foods such as juices and cereal. Their popularity is based on the claim that they protect cells from aging and help ward off cancer by mopping up “free radicals” — molecules that zip around harming whatever they encounter, including DNA and cell denizens called mitochondria.

Naturally, things are more complicated. Free radicals attack all kinds of cells — including cancer cells, said cancer biologist Zachary Schafer of the University of Notre Dame. So if antioxidants mop up free radicals, “that might help cancer cells,” he said, allowing them to proliferate and spread more easily. His research, using mice, has shown exactly that. So did a 2014 study, which found that antioxidants accelerated the spread of human lung cancer cells implanted in mice, partly by blocking a cancer-fighting gene called p53.

In the latest experiment, scientists led by Martin Bergo of Sweden’s University of Gothenburg exposed melanoma cells growing in a lab dish to two kinds of antioxidants, including a cousin of vitamin E, at doses comparable to those in supplements. The cancer cells again became more invasive and migrated more, the scientists reported in Science Translational Medicine.

Second Take: Here at STAT, we generally treat studies in lab animals as a great way to generate hypotheses about what might be true in humans — and an equally great way to be misled about that. But in this case, the mice results align with a growing body of evidence about antioxidants and cancer, including clinical trials. Way back in 1994, a large trial reported that supplement-size doses of beta carotene increased the risk of lung cancer in male smokers by 18 percent. One interpretation: Smoking triggered initially-undetectable cancer; antioxidants kept the body’s defenses from fighting it; the cancer grew enough to be detected. Two years later, another trial found that megadoses of beta-carotene increased lung cancer risk in smokers and people exposed to asbestos — populations likely to have incipient, if undiagnosed, lung tumors. And a 2011 study of 35,500 men found that large doses of vitamin E increased the risk of prostate cancer by 17 percent.

That said, a handful of studies have found that antioxidants inhibit melanoma and other malignant cells growing in lab dishes. Maybe different antioxidants (beta-carotene; vitamins A, C, E; others) act differently and are less harmful, or even beneficial. Or maybe studies in lab dishes, when they contradict human studies, aren’t relevant.

You might think that while antioxidants are a bad idea for cancer patients, they should help healthy people by preventing DNA damage that can trigger malignancies in the first place. Unfortunately, cancer turns out to be more prevalent than once thought: Many of us have undiagnosed micromalignancies that the immune system and other defenses keep in check. As a result, megadoses of antioxidants — in pills, not pomegranates — might be risky for everyone.

The Takeaway: “There is no credible evidence that antioxidant supplementation positively affects health in general, or cancer risk in particular,” said epidemiologist Dr. Michael Goodman of Emory University School of Public Health. And there’s some evidence that taking antioxidant supplements can be harmful, “particularly at high doses.”

This article was originally published on Nov. 5, 2015.