Ins and outs of dietary phytochemicals in cancer chemoprevention.

A voluminous number of evidence suggests that an increased consumption of fruit and vegetables is a relatively easy and practical strategy to reduce significantly the incidence of chronic diseases, such as cancer, cardiovascular diseases and other aging-related pathologies. This review will critically discuss the applications of chemical and dietary chemoprevention, intending the protecting effects against cancer of chemically synthesized molecules, or phytochemicals present in the regular diet. The length of chemopreventive treatments requires the administration of low doses of chemopreventive agents, to avoid toxic side effects. This poses the question, here discussed, of the bioavailability of these compounds, usually very modest. Another key issue is whether purified phytochemicals have the same protective effects, as do the whole food or mixture of foods in which these compounds are present. These aspects will be analysed at the light of the "antioxidant hypothesis" in cancer prevention and the "combination chemoprevention", both referring to the pleiotropic and synergistic effects of compounds present in the diet. Single molecules may evolve in perfect chemopreventive agents, as in the case of tamoxifen, or generate ambiguity. Resveratrol and quercetin represent two paradoxes, discussed here.