Plant Secondary Metabolites: Their Role in the Human Diet

There is increasing evidence that plant secondary metabolites such as flavonoids and phenolic compounds have a key role in the reduced incidences of cardiovascular disease and cancer that are associated with the long-term consumption of a fruit and vegetable-rich diet. Originally these effects were thought to be due the antioxidant properties of these compounds but it is becoming evident that the diverse protective effects are not due solely to in vivo increases in antioxidant levels and that other mechanisms are also involved. Qualitative and quantitative variations in the amounts of flavonoids and phenolic compounds in different fruits and vegetables as well as beverages such as red wine, fruit juices, tea and coffee, will be discussed. The bioavailability of these compounds is an important issue with some components being absorbed in the small intestine and others in the large intestine where they are also subjected to catabolism by colonic bacteria. This topic will be discussed with reference to feeding studies with flavonols (onions and tomato juice), anthocyanins (strawberries) and green tea (flavan-3-ols) with healthy human subjects and volunteers with an ileostomy.