Diet energy source affects lysine utilization for protein deposition in rainbow trout (Oncorhynchus mykiss) Pedro Encarnação ⁎ , Cornelis de Lange, Dominique P. Bureau Department of Animal and Poultry Science, University of Guelph, Guelph, Ontario, N1G 2W1, Canada Received 13 April 2006; received in revised form 1 August 2006; accepted 1 August 2006 Abstract We have previously shown that lysine utilization for body protein deposition (PD) in rainbow trout was affected by digestible energy (DE) content of the diet. When lysine intake limits PD, additional DE intake from fish oil improves efficiency of lysine utilization for PD. It is unclear whether other energy-yielding nutrients have the same effect on lysine utilization. Different energy sources that may be converted to different tricarboxylic acid (TCA) cycle metabolic intermediates can have different abilities to spare lysine for PD. An experiment was therefore carried out to examine the effects of different energy-yielding nutrients, on growth, nitrogen gain and lysine utilization by rainbow trout. Fish weighing 5.0 g ± 0.1, were fed diets with either limiting (1.5%) or marginally adequate (2.0%) lysine levels. Acetyl-CoA precursors (fish oil), pyruvate precursors (mixture of alanine, serine, glycine, and cystine) and α-ketoglutarate + oxaloacetate precursors (mixture of aspartate, glutamine and glutamate) were then added to the control diets (at the expense of raw corn starch) to produce a series of experimental diets with 20 MJ DE. Fish were pair-fed for 16 weeks and then to satiation for an additional 6 weeks. Results suggest that different energy sources affect lysine utilization differently. When feed intake was similar, additional DE from pyruvate and α-ketoglutarate/oxaloacetate precursors did not improve PD. When lysine was limiting, pyruvate precursors reduced feed intake and PD. Fatty acids, which similarly to lysine may be converted to acetyl-CoA, appear to be more effective in sparing lysine for PD than other metabolic substrates that have different entry points into the TCA cycle. Rainbow trout appears to have a higher tolerance for an excess of amino acids that are α- ketoglutarate/oxaloacetate precursors (glu, gln, asp) than pyruvate precursors. © 2006 Elsevier B.V. All rights reserved. Keywords: Lysine; Energy; Metabolism; Amino acid; Lipids 1. Introduction Lysine is the first limiting essential amino acid (EAA) in many protein sources available for fish pro- duction, especially plant proteins. Since the first limiting EAA will determine body protein deposition (PD) and ultimately fish growth, it is of particular interest to understand the effects of diet composition on lysine utilization. In a recent study Encarnação et al. (2004) have shown that diet digestible energy (DE) content affected the marginal efficiency of lysine utilization for PD in rainbow trout. At limiting lysine intake levels, addition- al DE, supplied by fish oil, allowed lysine to be spared for PD. It is unclear whether other energy-yielding Aquaculture 261 (2006) 1371 – 1381 www.elsevier.com/locate/aqua-online ⁎ Corresponding author. Current address: Biomin Laboratory Singapore. 3791 Jalan Bukit Merah #08-08, E-Center@ Redhill, 159471 Singapore. Tel.: +65 6275 0903; fax: +65 6275 4743. E-mail address: pencarna@gmail.com (P. Encarnação). 0044-8486/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.aquaculture.2006.08.001.