No wonder the role of antioxidants during cancer treatment is controversial; on one hand, they may protect the healthy cells from the harsh effects and damage of chemotherapy and radiation. On the other, they may protect the cancer cells as well, rendering treatment less effective. Without large trials to provide answers, most oncologists recommend that patients lay off the antioxidant supplements during treatment. But Block says harmful effects were limited to smokers, and he believes the available evidence supports the use of antioxidants in nonsmoking patients undergoing chemotherapy and, probably, radiation as well. By reducing the toxic side effects of treatment, he says, antioxidants can help patients complete their full regimens, which has been linked to lower mortality. (He says the risk-benefit ratio better supports the use of antioxidants in patients with metastatic disease; he'd be more likely to hold off until the question is settled among patients for whom cancer treatment is likely to offer a cure.)
Specific supplements may interact with specific anticancer drugs; scientists said earlier this year that in mice, an ingredient in green tea extract blocked the effects of Velcade, used to treat multiple myeloma. Green tea, however, has been associated with lower rates of cancer while taken in tea form, and some research suggests that its active ingredients, taken as a supplement, can slow the progression of prostate cancer. Like other compounds, it's probably helpful in some contexts and harmful in others.
There's not a silver bullet. It is not likely that one isolated nutrient is going to have a large effect in preventing cancer, say many researchers. "I personally think we will find that individual supplements are not the answer," says Kara Kelly, medical director of the Integrative Therapies Program for Children With Cancer at Children's Hospital of NewYork-Presbyterian. To her, that suggests focusing far more on diet (more on that later). As far as supplements go, many experts say there's a movement toward products that more closely mimic whole foods, delivering nutrients together in a state more similar to, say, how you'd get them if you ate a piece of fruit. But no supplement is likely to make a big difference if accompanied by a poor diet, lack of exercise, smoking, or other harmful factors. "If I live on burgers and fries, loading my body not only with fats but with the wrong fats, one fish oil capsule isn't going to do the trick," says Block.
So what's a person concerned about cancer prevention—or who is fighting the disease—to do?
Research. Two good databases of information on specific supplements are produced by the American Cancer Society and Memorial Sloan-Kettering Cancer Center. Both outline the evidence for the role of herbals, botanicals, and vitamin/mineral supplements in preventing and treating cancer, plus any possible risks and drug interactions. ConsumerLab.com, which requires an annual subscription, also has that information, in addition to guidance on specific brands.
Fully disclose to your doctor. Because there's the potential for side effects and drug interactions with so many supplements, from ginger to St.-John's-wort, it's imperative that people being treated for cancer tell their physicians what they're taking. That doesn't mean you will be greeted by a lecture; you may find your doctor recommends or OKs certain supplements. Lorenzo Cohen, director of the integrative medicine program at M.D. Anderson Cancer Center in Houston, says that while he tends to err on the side of caution, he's not going to tell people to avoid anything for which there's no evidence of harm.
An earlier version of this article misstated Peter Gann's title. He is director of the division of pathology research, not director of pathology.