Objective: The claimed action of conjugated linoleic acid as an antioxidant is unexpected and unclear, in view of its chemical structure - a conjugated diene, i.e., a fatty acid in its initial stage of autoxidation. Indeed, it can be speculated that it could act as a pro-oxidant, increasing oxidative stress in biological systems, nevertheless it has carbon-carbon bonds in the trans configuration. The objective of the present work was to evaluate the oxidative stability of commercial mixtures, and to investigate a possible correlation between conjugated linoleic acid supplementation and total hepatic lipid content, as well as some lipid oxidation indicators in rats. Methods: A biological assay was done with thirty rats divided into three groups (n=10) characterized as control and supplemented with the commercial mixtures AdvantEdge® and One®. The mixtures were administered in the concentration of 2% of the total diet consumption, and animals were supplemented for 42 days. The total liver lipid content was determined, and the morphology of the organ was examined by optical microscopy. Peroxide and malondialdehyde indexes were determined in vitro in order to evaluate the oxidative stability of the supplements. Peroxide, malondialdehyde and 8-iso-PGF2α isoprostane and catalase were determined as biological lipid oxidation indicators. Results: Results indicated a low in vitro oxidation stability of commercial mixtures. Associations between conjugated linoleic acid consumption and malondialdehyde (r=-0.7914, p<0.0001), and catalase (r=-0.5991, p=0.008) were moderate, negative and significant, demonstrating that conjugated linoleic acid reduced in vivo lipid oxidation. Total hepatic lipid content did not increase (22.42%, (SD=1.40%), and the organ remained morphologically undamaged. Conclusion: However, even though this CLA supplementation protocol did reduce lipid oxidation, a tendency was observed to an increase of 8-iso-PGF2 alpha isoprostane and peroxides induced by conjugated linoleic acid. Further research may be needed to verify its antioxidant effect. © 2009 Revista de Nutrição.2213949Van Den Berg, J.J.M., Cook, N.E., Tribble, D.L., Reinvestigation of the antioxidant properties of conjugated linoleic acid (1995) Lipids, 30 (5), pp. 599-605Cantwell, H., Devery, R., Stanton, C., Lawless, F., The effect of a conjugated linoleic acid on superoxide dismutase, catalase and glutathione peroxidase in oxidatively-challenged liver cells (1998) Biochem Soc T, 26 (1), p. 52Banni, S., Angioni, E., Contini, M.S., Carta, G., Casu, V., Iengo, G.A., Conjugated linoleic acid and oxidative stress (1998) J Am Oil Chem Soc, 75 (2), pp. 261-267Cantwell, H., Devery, R., O'shea, M., Stanton, C., The effect of conjugated linoleic acid on the antioxidant enzyme defense system in rat hepatocytes (1999) Lipids, 34 (8), pp. 833-839Yamasaki, M., Mansho, K., Mishima, H., Kimura, G., Sasaki, M., Kasai, M., Effect of dietary conjugated linoleic acid on lipid peroxidation and histological change in rat liver tissues (2000) J Agr Food Chem, 48 (12), pp. 6367-6371Basu, S., Risérus, U., Turpein, A., Vessby, B., Conjugated linoleic acid induces lipid peroxidation in men with abdominal obesity (2000) Clin Sci, 99 (6), pp. 511-516Basu, S., Smedman, A., Vessby, B., Conjugated linoleic acid induces lipid peroxidation in humans (2000) FEBS Letters, 468 (1), pp. 33-36Risérus, U., Basu, S., Jovinge, S., Fredrikson, G.N., Ärnlöv, J., Vessby, B., Supplementation with conjugated linoleic acid causes isomer-dependent oxidative stress and elevated C-reactive protein (2002) Circulation, 106 (15), pp. 1925-1929Risérus, U., Vessby, B., Ärnlöv, J., Basu, S., Effects of cis-9, trans-11 conjugated linoleic acid supplementation on insulin sensitivity, lipid peroxidation, and proinflammatory markers in obese men (2004) Am J Clin Nutr, 80 (2), pp. 279-283Smedman, A., Vessby, B., Basu, S., Isomer-specific effects of conjugated linoleic acid on lipid peroxidation in humans: regulation by a-tocopherol and cyclo-oxygenase-2 inhibitor (2004) Clin Sci, 106 (1), pp. 67-73Kim, J.H., Hubbard, N.E., Ziboh, V., Erickson, K.L., Attenuation of breast tumor cell growth by conjugated linoleic acid via inhibition of 5-lipoxygenase activating protein (2005) Biochim Biophys Acta, 1736 (3), pp. 244-250Arab, K., Rossary, A., Soulère, L., Steghens, J.P., Conjugated linoleic acid, unlike other unsaturated fatty acids, strongly induces glutathione synthesis without any lipoperoxidation (2006) Brit J Nutr, 96 (5), pp. 811-819Belury, M.A., Kempa-Steczko, A., Conjugated linoleic acid modulates hepatic lipid composition in mice (1997) Lipids, 32 (2), pp. 199-204Cherian, G., Holsonbake, T.B., Goeger, M.P., Bildfell, R., Dietary CLA alters yolk and tissue FA composition and hepatic histopathology of laying hens (2002) Lipids, 37 (8), pp. 751-757(1995) Official methods of analysis, , Association of Official Analytical Chemists. 16th ed. Virgínia: AOAC InternationalSinnhuber, R.O., Yu, T.C., 2-Thiobarbituric acid method for the measurement of rancidity in fishery products. II. The quantitative determination of malonaldehyde (1958) Food Technol, 12 (1), pp. 9-12Reeves, P.G., Nielsen, F.H., Fahey, G.C.J., AIN-93 Purified diets for laboratory rodents: final report of the American Institute of Nutrition ad hoc Writing Committee on the Reformulation of the AIN-76A rodent diet (1993) J Nutr, 123 (11), pp. 1939-1951Goena, M., Mazo, F., Fernández-González, L., Tosar, A., Fruhbeck, G., Santidrián, S., Effect of the raw legume Vicia ervilha on muscle and liver protein metabolism in growing rats (1989) Rev Española Fisiol, 45, pp. 55-60Bligh, E.G., Dyer, W.J., A rapid method of total lipid extration and purification (1959) Can J Biochem Physiol, 37 (8), pp. 911-917Goth, L., A simple method for determination of serum catalase activity and revision of reference range (1991) Clin Chim Acta, 196 (2-3), pp. 143-152Behmer, O.A., Tolosa, E.M.C., Neto, A.G.F., (1976) Manual de técnicas para histologia normal e patológica, , São Paulo: Edart Editora(2003) SAS Institute Project for Windows: user's guide statistics, , Version 8.0. Cary (USA)Yang, L., Leung, L.K., Huang, Y., Chen, Z.Y., Oxidative stability of conjugated linoleic acid isomers (2000) J Agric Food Chem, 48 (8), pp. 3072-3076Tsuzuki, T., Igarashi, M., Iwata, T., Ymauchi-Sato, Y., Yamamoto, T., Ogita, K., Oxidation rate of conjugated linoleic acid and conjugated linolenic acid is slowed by triacylglycerol esterification and a-tocopherol (2004) Lipids, 39 (5), pp. 475-480Miyazawa, T., Tsuzuki, T., Nakagawa, K., Igarashi, M., Fatty acids with conjugated unsaturation: relationship between oxidative stability and physiological activities (2005) Lipid Technol, 17 (10), pp. 221-225Ha, Y.L., Storkson, J., Pariza, M., Inhibition of benzo(a)pyrene-induced mouse forestomach neoplasia by conjugated linoleic acid (1990) Cancer Res, 50 (4), pp. 1097-101Flintoff-Dye, N.L., Omaye, S.T., Antioxidant effects of conjugated linoleic acid isomers in isolated human low-density lipoproteins (2005) Nutr Res, 25 (4), pp. 1-12Roberts, L.J., Morrow, J.D., Measurement of F2-isoprostanes as an index of oxidative stress in vivo (2000) Free Radical Bio Med, 28 (1), pp. 505-513Cherian, G., Goeger, M.P., Hepatic lipid characteristics and histopathology of laying hens fed CLA or n-3 fatty acids (2004) Lipids, 39 (4), pp. 31-36Macarulla, M.T., Fernández-Quintela, A., Zabala, A., Navarro, V., Echevarría, E., Churruca, I., Effects of conjugated linoleic acid on liver composition and fatty acid oxidation are isomer-dependent in hamster (2005) Nutrition, 21 (1), pp. 512-519Moya-Camarena, S.Y., Vanden Heuvel, J.P., Blanchard, S.G., Leesnitzer, L.A., Belury, M.A., Conjugated linoleic acid is a potent naturally occurring ligand and activator of PPARa (1999) J Lipid Res, 40 (4), pp. 1426-1433Choi, Y.J., Kim, Y.C., Han, Y.B., Park, Y., Pariza, M.W., Ntambi, J.M., The trans-10, cis-12 isomer of conjugated linoleic acid downregulated stearoyl-CoA desaturase gene expression in 3T3-L1 adipocytes (2000) J Nutr, 130 (8), pp. 1920-1924Belury, M.A., Moya-Camarena, S.Y., Lu, M., Shi, L., Leesnitzer, L.M., Blanchard, S.G., Conjugated linoleic acid is na activator and ligand for peroxissome proliferator-activated receptor-gama (PPARg) (2002) Nutr Res, 22 (7), pp. 817-824Kang, K., Liu, W., Albright, K.J., Park, Y., Pariza, M.W., Trans-10, cis-12 inhibits differentiation of 3T3-L1 adipocytes and decrases PPAR gamma expression (2003) Biochem Bioph Res Co, 303 (3), pp. 795-799Cimini, A.M., Cristiano, L., Colafarina, S., Benedetti, E., Di Loreto, S., Festuccia, C., PPARg-dependent effects of conjugated linoleic acido in the human glioblastoma cell line (ADF) (2005) Int J Cancer, 117 (6), pp. 923-933Bassaganya-Riera, J., Hontecillas, R., CLA and n-3 PUFA differentially modulate clinical activity and colonic PPAR-responsive gene expression in a pig model of experimental IBD (2006) Clin Nutr, 25 (3), pp. 454-465Ringseis, R., Müller, A., Herter, C., Gahler, S., Steinhart, H., Eder, K., CLA isomers inhibit TNFa-induced eicosanoid release from human vascular smooth muscle cells via a PPARg ligand-like action (2006) Biochim Bioph Acta, 1760 (2), pp. 290-300