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). Supplementation with d‐ lysine or l‐ lysine (but not dl‐ lysine) generally increased the weight gain, specific growth rate, feed efficiency ratio and protein efficiency ratio. Dietary lysine supplementation generally increased the apparent digestibility of dry matter, crude protein and crude lipid, but no statistically significant differences were observed between the control group and the lysine‐supplement groups (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.