The effects of conjugated linoleic acid isomers (CLA) and endurance training on lean body mass are expected to result from their action on tissue protein metabolism. The aim of this study was to analyze their effects on protein metabolism in 2 muscles, the small intestine and liver of adult rats. Four-month-old male Wistar rats were fed diets containing either no CLA, cis-9, trans-11 CLA isomer (1 g.100 g −1 ), trans-10, cis-12 CLA isomer (1 g.100 g −1 ) or both isomers (1 g.100 g −1 each) for

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... weeks. Half of the rats were subjected to endurance training by running on a treadmill. At the end of this period, the rats were injected with a flooding dose of 13 C-valine to determine protein synthesis rates in the post-absorptive (experiment 1) and in the post-prandial (experiment 2) states. No effect of CLA or endurance training were detected in the small intestine. Training reduced food intake and protein synthesis rates in the liver but no effect was found on the protein synthesis rates in muscles. In the post-absorptive state, protein synthesis rate was increased by feeding the trans-10, cis-12 CLA isomer alone in the liver (+9%) or in combination with the cis-9, trans-11 isomer in the gastrocnemius (+30%), mostly in sedentary rats. In the post-prandial state, the cis-9, trans-11 CLA isomer tended to reduce the protein synthesis rate in the gastrocnemius muscle. However, no effect of CLA was found on muscle protein amounts. In conclusion, CLA isomers would have limited but differential effects on tissue protein metabolism in adult rats. conjugated linoleic acid / protein metabolism / exercise / muscle / liver / intestine * Corresponding author: patureau@clermont.inra.fr mans (see [2, 3] for reviews). However, in the latter species, the effects were less marked than in mice. This fat-lowering effect appeared to be mainly related to lipid metabolism alterations induced by the trans-10, cis-12 isomer. The effects of CLA feeding on lean body mass and on its main constituents, muscle and protein masses are less consistent. In some studies, Article published by EDP Sciences and available at http://www.edpsciences.org/rnd or http://dx.doi.org/10.1051/rnd:2006040 622 P. Patureau Mirand et al.