The gastrointestinal tract: A major site of antioxidant action?

Diets rich in fruits and vegetables delay the onset of many age-related dis eases, and contain a complex mixture of antioxidants (including ascorbate, carotenoids, vitamin E and other phenolics such as the flavonoids). However , diet also contains pro-oxidants, including iron, copper, H2O2, haem, lipi d peroxides and aldehydes. Nitrite is frequently present in diet, leading t o generation of reactive nitrogen species in the stomach. In considering th e biological importance of dietary antioxidants, attention has usually focu ssed on those that are absorbed through the gastrointestinal tract into the rest of the body. In the present paper we develop the argument that the hi gh levels of antioxidants present in certain foods (fruits, vegetables, gra ins) and beverages (e.g. green tea) play an important role in protecting th e gastrointestinal tract itself from oxidative damage, and in delaying the development of stomach, colon and rectal cancer. Indeed, carotenoids and fl avonoids do not seem to be as well absorbed as vitamins C and E. Hence thei r concentrations can be much higher in the lumen of the GI tract than are e ver achieved in plasma or other body tissues, making an antioxidant action in the GI tract more likely. Additional protective mechanisms of these diet ary constituents (e.g. effects on intercellular communication, apoptosis, c yclooxygenases and telomerase) may also be important.