We have all heard the claim that eating vegetables and fruits is good for us and that their consumption may help ward off many illnesses, including cancer. This receives endorsements from all sorts of health organizations and scientists so frequently that it may have reached the state of being a conditional truth. For example, here is a U.S. Government page that says "...those who eat more generous amounts as part of a healthful diet are likely to have reduced risk of chronic diseases, including stroke and perhaps other cardiovascular diseases, and certain cancers".
In reality, preventive nutrition is very much a science of uncertainties. This is so probably because of the enormous among-person variability that exists in any human population that results from the innumerable external and internal factors that control our phenotypes. Another confounding factor is the practical difficulty of monitoring and controlling peoples' diets for research purposes.
Therefore, it should not come as a surprise that a recent review* makes the bold counter claim that there is not much experimental support behind the assertion that eating plenty of fruits and vegetables reduces the risk of getting cancer.
Nutritional principles indicate that healthy diets should include at least moderate amounts of fruit and vegetables, but the available data suggest that general increases in fruit and vegetable intake would not have much effect on cancer rates, at least in well-nourished populations.The author then goes on to review the supposed associations of specific cancers with fruit and vegetable consumption. We learn the following about prostate cancer:
There has been much interest in the possibility that fruits and vegetables, such as tomatoes, which are rich in the carotenoid lycopene might reduce the risk for prostate cancer, but overall the data do not support this hypothesis...Having written this, I will now take a break to enjoy a platefull of fruits with a glass of wine. They do taste good and they won't harm me. That's all that matters.
*T. J. Key. Fruit and vegetables and cancer risk. British Journal of Cancer, advance online publication 30 November 2010. Freely available here.