d-lysine can be effectively utilized for growth by common carp (Cyprinus carpio)

An 8-week feeding trial was conducted to compare the effects of different lysine isomers on growth, nutrient utilization and metabolic enzyme activity in common carp (Cyprinus carpio). A basic experimental diet (the control group) with 30% of dietary protein from fish meal and the remaining 70% from soybean meal was used either with or without supplementation of d-lysine, l-lysine or dl-lysine, respectively. The total feed intake was significantly improved by supplementation with lysine regardless of isomeric form (P 0.05). The d-amino acid oxidase and d-aspartate oxidase activities were generally higher in the liver, kidney and intestine of fish fed diet containing d-lysine compared to fish fed diet containing l-lysine. The lysine, protein and lipid gains in fish were markedly increased by the supplementation with d-lysine or l-lysine, and the lysine (r = 0.618, P < 0.05) and protein gains (r = 0.671, P < 0.05) were linearly related to dietary lysine intake. Lysine retention efficiency linearly decreased with the increase of dietary lysine intake (r = −0.579, P < 0.05). These results indicate that common carp are capable of utilizing the supplemental crystalline d-lysine·HCl (4 g kg⁻¹ dry diet) for growth when the rest of dietary amino acids are protein-bound forms.