We speak of "oxidative stress" when the production of reactive oxygen species exceeds the organism's antioxidant defense capacities.

Our organism continually tends to re-establish the equilibrium between pro-oxidative molecules and antioxidant molecules using the mechanisms which we all possess: the production of antioxidant enzymes or molecules, the activation of damage repair systems, etc. Obviously this endogenous production is helped by exogenous sources of antioxidants, provided by a balanced diet, in particular, rich in fruit and vegetables.