Background – Phytochemicals are abundant micronutrients in fruit and vegetables. There is an emerging body of evidence regarding their health benefits, some of which may be due to their antioxidant properties.
Objective – To compare chemical antioxidant assays with more biologically relevant cell-based assay systems for measuring antioxidant activities of food polyphenols.
Design – The free radical scavenging activities of three flavonoids: quercetin, rutin and catechin, commonly found in apple, onions and tea respectively, were measured. The three flavonoids were evaluated using both oxygen radical absorbance capacity (ORAC) and lipid peroxidation inhibition capacity (LPIC) assays. Cytoprotective effects were measured by the degree of protection against H2O2-induced damage of human Jurkat cells.
Outcomes – As expected all compounds exhibited activity in these assays. Quercetin offered the strongest protection against H2O2-induced cell death. A comparison of the results of the assays showed that the ability to inhibit peroxidation of lipids in a liposomal system (LPIC) correlated well with the cytoprotective activities (expressed as EC50), but not with the ability to protect an aqueous fluorescent substrate in the ORAC assay. Conclusions – In vitro assays can only rank antioxidant activity for their particular reaction system and their relevance to in vivo health-protective activities is uncertain. Therefore, it is prudent to use more than one type of antioxidant assay to measure antioxidant activities, and to include at least one assay that has biological relevance.