The anticarcinogenic effect of conjugated linoleic acids (CLA) has been attributed to their antioxidant activity. Strong evidence is lacking, however, to substantiate that CLA is an antioxidant. The objective of this study was, therefore, to test whether CLA is an antioxidant or a prooxidant in canola oil. The oxidation was conducted at 90°C by monitoring oxygen uptake and changes in linoleic acid and α-linolenic acid. Free CLA and CLA methyl ester (CLAME) accelerated lipid oxidation in canola oil. The prooxidant activity of CLA and CLAME was dose-dependent at concentrations ranging from 0.1 to 1.0% in canola oil heated at 90°C. CLA-containing triacylglycerol, however, was neither an antioxidant nor a prooxidant. Under the present experimental conditions, CLA was not an antioxidant in fats and oil. JAOCS 73, 749-753 (1997).

Contrary to current opinion, conjugated linoleic acids (CLA) as a mixture of several isomers have been previously shown to function as prooxidants in the form of free fatty acids and methyl esters in heated canola oil. Furthermore, CLA oxidizes considerably faster than linoleic acid. However, stability of CLA relative to other polyunsaturated fatty acids remains undetermined. The present study was therefore undertaken to examine the relative oxidation rate of CLA compared with that of linolenic acid (LNA), arachidonic acid (AA), and docosahexaenoic acid (DHA) in air at 90°C. CLA, both in the form of free fatty acids and triacylglycerols, were extremely unstable to the same extent as DHA, but they oxidized considerably faster than LNA and AA. The mechanism by which CLA were readily decomposed was probably due to formation of the unstable free-radical intermediate.

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