Several natural compounds, such as vitamins C, E, and A and polyphenols, found in the majority of plant foods, interfere
with or scavenge ROS concentration within cells and subsequently protect the organism from the adverse effects of
oxidative stress. Indeed, as it has been shown by our group that the antioxidant activity of plasma in humans following
a diet rich in vegetables, fruits and olive oil was increased in comparison to a normal diet (Kampa et al., 2002). This increase can be mainly attributed to polyphenols which exhibit a wide range of biological activities, including anti-tumorigenic, anti-mutagenic, anti-inflammatory, and antiviral actions (Bravo, 1998; Hertog and Hollman, 1996) mainly due to their antioxidant properties and their ability to exert inhibitory effects by affecting basic cellular functions. Indeed the benefi- cial role of polyphenols in preventing cancer can be in part attributed to their ability to modify enzymes that activate or detoxify environmental carcinogens.