Antioxidant effects of conjugated linoleic acid isomers in isolated human low-density lipoproteins

The possible beneficial health effects of conjugated linoleic acid (CLA) have been the subject of extensive investigation. The antioxidant capacity of CLA is one possible action to explain its protective effect in cancer and atherosclerosis. The objective of this study was to assess, through enrichment of human low-density lipoproteins (LDLs) with CLA isomers, the antioxidant and pro-oxidant properties of CLA isomers (9-cis, 11-trans-CLA; 9-trans, 11-trans-CLA; 9-cis, 11-cis-CLA; and 10-trans, 12-cis-CLA) and to determine at what concentrations the effects are most prominent. Lipid oxidation of human LDL was determined by the thiobarbituric acid reactive substances assay. In all isomers tested, the enrichment with CLA at 2 μmol/L resulted in a pro-oxidant response when compared to LDL not enriched with CLA. Reduced lipid peroxidation was observed at the middle enrichment levels for each isomer, suggesting a promotion-inhibition-promotion action of CLA, perhaps reflecting biphasic effects consistent with the phenomenon defined as hormesis. In general, enrichment of isolated human LDL with CLA isomers promoted oxidation.