Can antioxidants prevent cancer?

Question

Antioxidants are often touted as some kind of miracle cure for all kinds of illnesses. They are supposed to prevent damage caused by reactive oxygen species (ROS) to the cells by scavenging those ROS before they can cause any damage.

The link between antioxidants and cancer seems plausible at first glance, ROS can react with DNA and cause mutations through that mechanism. The accumulation of certain mutations can lead to the development of cancer, so the idea that preventing the damage caused by ROS to the DNA by consuming antioxidants could prevent cancer seems plausible.

Is there any hard evidence that consuming antioxidants can prevent the development of cancer?

Answer 1

No hard evidence yet.

From the National Cancer Institute:

Considerable laboratory evidence from chemical, cell culture, and animal studies indicates that antioxidants may slow or possibly prevent the development of cancer. However, information from recent clinical trials is less clear. In recent years, large-scale, randomized clinical trials reached inconsistent conclusions.

There is a 2003 article at the American Association for Cancer Research, the last part of the abstract states:

The use of antioxidants during cancer therapy is currently a topic of heated debate because of an overall lack of clear research findings. Some data suggest antioxidants can ameliorate toxic side effects of therapy without affecting treatment efficacy, whereas other data suggest antioxidants interfere with radiotherapy or chemotherapy. Overall, examination of the evidence related to potential interactions between ROS and dietary antioxidants and effects on human health indicates that consuming dietary antioxidant supplements has pros and cons for any population and raises numerous questions, issues, and challenges that make this topic a fertile field for future research. Overall, current knowledge makes it premature to generalize and make specific recommendations about antioxidant usage for those at high risk for cancer or undergoing treatment.

Answer 2

Although I'm aware this doesn't answer the question, I think it's worth mentioning that laboratory research in the past 5 years has identified some of the reasons why antioxidants aren't as beneficial as hoped. Some of the clinical trials from NCI's list (from ghoppe's answer) were aborted after a iatrogenic rather than the hoped therapeutic effect of antioxidants was observed.

Why anti-oxidants had the opposite effect on the development of some cancers baffled people a long time... or more precisely until 2012, when a mice model of lung cancer was shown to be accelerated by anti-oxidants. And in 2015, anti-oxidants were found to promte metastasis of human melanoma (again in a mouse model); there was in fact a second study by a different group on the exact same topic in the same year. The findings were in line with the knwolege that the anti-metastatic drug methotrexate has some pro-oxidative effects.

So, the current picture seems to be that ROS is sometimes cancer-promoting (in the initial stages--if we stick to the conventional assumptions) but also prevents rapid tumor growth and spreading, whereas antioxidants have opposite effects, at least in some cancers. This is obviously an area of active research so more will probably be known in the upcoming years. It's also an area of high-stakes research because metastasis is the main cause of death in cancer patients.

A 2016 review ends with this advice

the multitude of studies discussed here suggest that patients diagnosed with (or at risk for) cancers should avoid unnecessary supplementation of antioxidants in order [to] ensure that enhanced antioxidant activity does not inadvertently facilitate tumor progression and metastasis.