Fruit, vegetables, and the prevention of cancer: research challenges.

Objective: A great deal of epidemiologic evidence has indicated that fruits and vegetables are protective against numerous forms of cancer. However, there are many gaps in our knowledge.

Methods: In this pilot study we reviewed more than 200 cohort and case-control studies to determine the shape of the dose-response relationship (i.e., how the risk reduction per extra serving of fruits and vegetables changes with the actual intake of these foods). We found major barriers to investigating this. As part of this pilot study we also investigated whether specific fruits and vegetables are responsible for the anticancer action of these foods or whether a wide variety is required for optimal protection. If the former is correct, then fruits and vegetables may contain one or a small number of "magic bullets"; if the latter is correct, then a "teamwork" concept may be valid.

Results: Different findings suggested that the teamwork concept is much more likely. Many studies, especially older ones, have ignored potential confounding variables such as energy intake, alcohol consumption, physical activity, body mass index, smoking, and socioeconomic status (although many recent studies have adjusted for education). Other potential confounders that have generally been ignored are consumption of whole grain cereals and the use of vitamin and mineral supplements.

Conclusions: The inverse association between intake of fruits and vegetables and the risk of cancer of the colon, breast, and stomach has generally been much stronger in case-control than in cohort studies. We have no clear explanation for this.