Effects of supplemental dietary L-carnitine and ractopamine on the performance of juvenile rainbow trout, Oncorhynchus mykiss Sayed Mohammad Ali Jalali Haji-abadi 1,2 , Nasrollah Mahboobi Soo¢ani 3 , Ali Asghar Sadeghi 2 , Mohammad Chamani 2 & Gholam Hossein Riazi 4 1 Department of Animal Science, Faculty of Agriculture, Shahrekord Branch, Islamic Azad University, Shahrekord, Iran 2 Department of Animal Science, Faculty of Agriculture, Science and Research Branch, Islamic Azad University,Tehran, Iran 3 Department of Natural Resources, Isfahan University of Technology (IUT), Isfahan, Iran 4 Institute of Biochemistry and Biophysics, University of Tehran,Tehran, Iran Correspondence: S M A Jalali, Department of Animal Science, Faculty of Agriculture, Shahrekord Branch, Islamic Azad University, Rahmatieh, PO Box 166, Shahrekord, Iran. E-mail: ¢sh.nutritionist@gmail.com Abstract This study was carried out to investigate a possible protein-sparing action of L-carnitine and ractopa- mine in rainbow trout, Oncorhynchus mykiss. An 8- week feeding trial was carried out to evaluate the e¡ects of supplementation of three levels of L-carni- tine (0,1 and 2 g kg À 1 ) and two levels of ractopamine (0 and 10 mg kg À 1 ) on growth performance, ¢llet fatty acid compositions and blood biochemical para- meters in a 3 Â 2 factorial experimental design. Ractopamine and 1g kg À 1 carnitine improved the speci¢c growth rate (1.03% and 1.05% day À 1 ), feed conversion ratio (FCR,1.3 and1.29), protein e/ciency ratio (PER, 1.88 and 1.85) of ¢sh and crude protein (73.5 and 73.8) content of ¢sh ¢llet. L-carnitine and ractopamine increased the levels of albumin, total protein and globulin in the serum of ¢sh. Apart from eicosapentaenoic acid and docosahexaenoic acid, other fatty acids of ¢sh ¢llet were increased by racto- pamine, while total saturated fatty acids were almost intact. However, the total n-3 poly unsaturated fatty acids were reduced by L-carnitine supplementation ( Po0.05). The present study showed that 1g kg À 1 L-carnitine and 10 mg kg À 1 ractopamine each can improve the performance of rainbow trout and their combination in diet could enhance the protein level and change the fatty acids pro¢le in ¢llet muscle. Keywords: L-carnitine, ractopamine, growth, fatty acids pro¢le, rainbow trout Introduction Protein and fat content of ¢sh diet are the most im- portant dietary energy sources; nevertheless, protein is also the most expensive feed component. An in- creased dietary fat content could induce a protein- sparing action and improve the utilization of dietary protein (Torreele, Van der Sluiszen & Verreth 1993). Oxidation of fat and fatty acids provides the most cost^e¡ective energy yield per unit weight of dietary ingredient, and this may be promoted by a dietary supplementation of L-carnitine and ractopamine hy- drochloride. L-carnitine is required for the transfer of long-chain fatty acids from the cytosol to the mito- chondrial matrix during lipid catabolism (b -oxida- tion); therefore, it plays a vital role in fat combustion (Borum1987 cited in Harpaz 2005). It has also been suggested that L-carnitine supplementation may sti- mulate the protein-sparing action by increasing the energy derived from lipids (Ozo¤ rio, Van Ginneken, Van den Thillart, Verstegen & Verreth 2005). Some reports have indicated that L-carnitine has no e¡ects on the growth performance of rainbow trout fry and ¢ngerlings (Rodehutscord 1995; Chatzifotis, Takeu- chi, Watanabe & Satoh 1997) and tilapia (Yang, Wen, Liou & Liu 2009), but improved the performance of other species including African cat¢sh (Torreele et al . 1993), hybrid striped bass (Twibell & Brown 2000), red sea bream (Chatzifotis,Takeuchi & Seikai 1995), beluga sturgeon (Mohseni, Ozo¤ rio, Pourkzemi & Bai 2008) and black sea bream (Ma, Xu, Shao, Xu, Aquaculture Research, 2010, 41 , 1582^1591 doi: 10.1111/j.1365-2109.2009.02462.x r 2010 The Authors 1582 Aquaculture Research r 2010 Blackwell Publishing Ltd