Antioxidants have been pitched from grocery store shelves, vitamin supplement labels, and by nutritionist sketchballs for as long as I can remember. Indeed, the basic idea traces all the way back to the 1920s and the identification of vitamin E and its benefits for the reproductive health of mice. The idea would peak in the early 00's with the sudden ascendancy of the "superfoods" marketing movement—largely premised on the idea of beneficial antioxidants—and it's waned only slightly since.
As described in the current issue of Science Translational Medicine, researchers have found that antioxidants as much as double the rate of melanoma metastasis in mouse models. The results align well with other recent research by the same group finding that antioxidant supplements hasten and aggravate the progression of lung cancer, the most common form of the disease. Both sets of findings sit atop a growing body of evidence that antioxidant supplementation is a sketchy, possibly even dangerous practice for those without pre-existing antioxidant deficits.
The antioxidant idea is based on often trouble-making sorts of molecules called free radicals. The danger of free radicals is in their high degree of reactivity and instability, owing to the presence of a single loner electron hanging out in a member atom's outermost electron shell. The free radical is always on the lookout for nearby compounds from which it might "steal" away an electron, resulting in a more stable not-so-free radical, but also a new unstable atom and a new free radical, which then repeats the process. This goes on and on in a chain reaction with one possible outcome of this cascade being cellular damage, perhaps involving a cell's genetic material.
Based on this knowledge, it seems reasonable to try and get rid of free radicals, which is where antioxidants come in. Antioxidants, which are a natural part of the body's defense mechanisms, function by "donating" electrons to free radical particles, leaving them in a more stable, less dangerous state.
It's not that easy, however. "Melanoma has steadily increased in incidence and lethality over several decades, and identifying factors that affect metastasis would be valuable," the authors behind the current study note. "Here, antioxidant supplementation was found to increase metastasis in mice with endogenous malignant melanoma and increase the invasive behavior of human melanoma cells." The reason likely has to do with antioxidants' effects on certain proteins in the body regulating cytoskeletal changes in migrating cells—by triggering these proteins, an antioxidant may be triggering a cancer cell's migration to a new home in the body.
"As opposed to the lung cancer studies, the primary melanoma tumor was not affected," Martin Bergö, the new study's lead author, explains in a statement. "But the antioxidant boosted the ability of the tumor cells to metastasize, an even more serious problem because metastasis is the cause of death in the case of melanoma. The primary tumor is not dangerous per se and is usually removed."
Antioxidant hype was never really based on science. A handful of studies in the early '90s suggested that people will low intanks of fruits and vegetables known to be high in antioxidants were at greater risk for cancer, and the supplement industry went all out in selling the idea (it's still going all out). Follow up studies, however, focused on individual antioxidant compounds and found not much benefit. These were easily drowned out by a towering wave of antioxidant hype.
Since that first wave, further research has continued to find mixed to negative results. Most of the benefits that have been observed (and there are some) involve patients with preexisting deficits in the antioxidant in question. A Harvard School for Public Health summary explains:
Few trials have gone on long enough to provide an adequate test for cancer. In the long-term Physicians' Health Study, cancer rates were similar among men taking beta-carotene and among those taking a placebo. Other trials have also largely showed no effect, including HOPE. The SU.VI.MAX trial showed a reduction in cancer risk and all-cause mortality among men taking an antioxidant cocktail but no apparent effect in women, possibly because men tended to have low blood levels of beta-carotene and other vitamins at the beginning of the study. A randomized trial of selenium in people with skin cancer demonstrated significant reductions in cancer and cancer mortality at various sites, including colon, lung, and prostate. The effects were strongest among those with low selenium levels at baseline.
Basically, when it comes to antioxidants, be skeptical as shit. Same goes for the rest of the supplement industry, for that matter.