Antioxidant evaluation protocols: Food quality or health effects

Increasing research on natural antioxidants in foods and development of new assays has prompted critical reflection on the field. It has been common practice to identify health benefits from antioxidant activity on the cellular level with antioxidant capacity of food measured in vitro. The use of antioxidants and their positive effects on food quality has been demonstrated in a large variety of foods and beverages using various methods for detection of lipid and protein oxidation or various assays based on electron transfer or hydrogen-atom transfer. A direct positive effect on markers of oxidative status after dietary intervention has, however, been difficult to confirm and much has still to be learnt about antioxidant action in vivo including synergistic or inhibitory roles, the uptake, biotransformation, and tissue distribution of potential antioxidants. This review critically evaluates various types of assays for antioxidative capacity, i.e. the stoichiometry, and antioxidative activity, i.e. the kinetics of the antioxidant action, with focus on the antioxidant mechanism of natural dietary antioxidants, particularly phenolic compounds, on lipid oxidation. It is concluded that it is difficult to transfer antioxidant mechanisms established in model systems and in foods to the in vivo situation and that no simple relationship has been recognized so far between antioxidant capacity determined for various foods and beverages and health benefits for humans. Screening of antioxidant capacity using simple assays in order to predict positive health effects of food are not scientifically justified. Different protocols will have to be used for evaluation of the protection of food by antioxidants and for evaluation of the health effect of antioxidants.