from Scientific American:
Antioxidants May Make Cancer Worse
New animal studies explain why supposedly healthy supplements like beta-carotene could exacerbate a dread disease
By Melinda Wenner Moyer | October 7, 2015
Antioxidants are supposed to keep your cells healthy. That is why millions of people gobble supplements like vitamin E and beta-carotene each year. Today, however, a new study adds to a growing body of research suggesting these supplements actually have a harmful effect in one serious disease: cancer.
The work, conducted in mice, shows that antioxidants can change cells in ways that fuel the spread of malignant melanoma-- the most serious skin cancer-- to different parts of the body. The progression makes the disease even more deadly. Earlier studies of antioxidant supplement use by people have also hinted at a cancer-promoting effect. A large trial reported in 1994 (pdf) that daily megadoses of the antioxidant beta-carotene increased the risk of lung cancer in male smokers by 18 percent and a 1996 trial was stopped early after researchers discovered that high-dose beta-carotene and retinol, another form of vitamin A, increased lung cancer risk by 28 percent in smokers and workers exposed to asbestos. More recently, a 2011 trial involving more than 35,500 men over 50 found that large doses of vitamin E increased the risk of prostate cancer by 17 percent. These findings had puzzled researchers because the conventional wisdom is that antioxidants should lower cancer risk by neutralizing cell-damaging, cancer-causing free radicals.
But scientists now think that antioxidants, at high enough levels, also protect cancer cells from these same free radicals. "There now exists a sizable quantity of data suggesting that antioxidants can help cancer cells much like they help normal cells," says Zachary Schafer, a biologist at the University of Notre Dame, who was not involved in the new study. Last year the scientists behind the melanoma study found that antioxidants fuel the growth of another type of malignancy, lung cancer.
The entire post can be read here.