Diet digestible energy content affects lysine utilization, but not dietary lysine requirements of rainbow trout (Oncorhynchus mykiss) for maximum growth

The effect of diet composition on essential amino acid (EAA) utilization and requirements is a topic of some controversy in fish. A growth trial was conducted to examine the effect of diet digestible energy (DE) content on lysine utilization and lysine requirement in rainbow trout (Oncorhynchus mykiss). Twelve isoproteic (40% digestible protein (DP)) practical wheat-gluten-based diets, with two DE levels (16 and 20 MJ/kg) and six lysine levels (1.2% to 2.5%), were hand fed to near-satiety to triplicate groups of rainbow trout (initial weight 24 g) over 12 weeks. Across dietary lysine levels, higher (P<0.05) weight gain and feed efficiency (FE) were observed at the higher diet DE, yet there was no effect (P>0.10) of diet DE content on whole body protein deposition. Based on body weight gain, diet DE content did not affect (P>0.10) lysine requirements, expressed as dietary concentration, while different statistical models yielded different (P<0.05) estimates of lysine requirements (% in diet, low vs. high diet DE content): broken line regression, 1.74 vs. 1.86; exponential, 2.28 vs. 2.33; asymptotic, 2.29 vs. 2.13. These results indicate that expressing lysine requirements in relation to DE content of the diet is not appropriate. At marginally deficient lysine intake levels, efficiency of lysine utilization for whole body protein deposition was affected both by lysine and DE levels in the diet. Effect of diet DE content on lysine utilization was also confirmed through liver glutamate dehydrogenase (GDH) activity. The effect of diet DE content on essential AA utilization of rainbow trout appears to be different from that seen in pigs and poultry. (C) 2004 Elsevier B.V. All rights reserved.