Randomized controlled trials are regarded as the most definitive of study designs. The randomized controlled trials that have tested nutritional factors for cancer prevention are reviewed. Trials that have tested the effects of nutrients given as high-dose supplements have been largely disappointing, typically showing either no or harmful effects. Possible benefits of vitamin E for prostate cancer prevention and selenium for prostate, colorectal, and lung cancer prevention have emerged only as secondary endpoints in trials conducted for other purposes; confirmatory new trials for these nutrients are now underway or are planned.

The limitations of both past and current randomized controlled trials for studying diet-cancer relationships are discussed. The disappointing findings that have emerged from short-term studies of high-dose supplements cannot be interpreted as direct tests of the diet-cancer relationship because high-dose supplements cannot fully simulate the effects of whole foods on cancer risk.

As we await findings from current and future trials, we should not forget that the ample evidence from observational epidemiologic research—suggesting that diets rich in fruits and vegetables can reduce the risk of many of the most common cancers—can provide a sound basis for nutritional recommendations aimed at reducing cancer risk.