Over 3 decades of research on dietary flavonoid antioxidants and cancer prevention: What have we achieved?

Epidemiological studies keep confirming that the so-called Mediterranean Diet, which is characterised by a relatively high intake of fruit and vegetables, enhances health and provides protection against cancer. The first step in carcinogenesis, and possibly in a range of other degenerative diseases such as heart disease or degenerative dementia, is most likely damage to DNA and other macromolecules. Radical oxygen species - i.e. superoxide anion, hydrogen peroxide, and the hydroxyl radical - are generally considered a major cause of damage to macromolecules. It was long suggested that the antioxidant properties of food ingredients are essential to understanding the mechanism of action of what constitutes a healthy diet, i.e. a diet that prevents the onset of degenerative diseases. However, since the levels of antioxidants in blood plasma required to see any health benefits is much higher that what we get through our diet, the role of dietary phytochemicals acting as anti-oxidants is now in in doubt. Nevertheless, a correlation between presence of flavonoids in the diet and prevention of degenerative diseases has remained. Though there is a putative role for dietary flavonoids in the prevention of degenerative diseases, the exact mechanism of action of these phytonutrients is still a matter of debate. The human body has its own defences against oxidative stress in the form of the enzymes superoxide dismutase (SOD) and catalase (CAT), and reduced glutathione (GSH). Rather than being antioxidants in their own right, plant constituents are more likely to act as triggers or inducers of expression of the human antioxidants SOD, CAT, and GSH.