Bioavailability of free lysine and protein-bound lysine from casein and fishmeal in juvenile turbot ( Psetta maxima )

• Published in: British Journal of Nutrition
• Main Authors: Kroeckel, Saskia, Dietz, Carsten, Schulz, Carsten, Susenbeth, Andreas
• Format: Article in Journal/Newspaper
• Language: English
• Published: Cambridge University Press (CUP) 2015
• Subjects: Nutrition and Dietetics; Medicine (miscellaneous); Turbot
• Online Access: http://dx.doi.org/10.1017/s000711451400381x; https://www.cambridge.org/core/services/aop-cambridge-core/content/view/S000711451400381X

In the present study, a linear regression analysis between lysine intake and lysine retention was conducted to investigate the efficiency of lysine utilisation ( k Lys ) at marginal lysine intake of either protein-bound or free lysine sources in juvenile turbot ( Psetta maxima ). For this purpose, nine isonitrogenous and isoenergetic diets were formulated to contain 2·25–4·12 g lysine/100 g crude protein (CP) to ensure that lysine was the first-limiting amino acid in all diets. The basal diet contained 2·25 g lysine/100 g CP. Graded levels of casein (Cas), fishmeal (FM) and l -lysine HCl (Lys) were added to the experimental diets to achieve stepwise lysine increments. A total of 240 fish (initial weight 50·1 g) were hand-fed all the experimental diets once daily until apparent satiation over a period of 56 d. Feed intake was significantly affected by dietary lysine concentration rather than by dietary lysine source. Specific growth rate increased significantly at higher lysine concentrations ( P < 0·001). CP, crude lipid and crude ash contents in the whole body were affected by the dietary treatments. The linear regression slope between lysine retention and lysine intake ( k Lys ) was similar between all the dietary lysine sources. The k Lys values for the diets supplemented with Cas, Lys or FM were 0·833, 0·857 and 0·684, respectively. The bioavailability of lysine from the respective lysine sources was determined by a slope-ratio approach. The bioavailability of lysine (relative to the reference lysine source Cas) from FM and Lys was 82·1 and 103 %, respectively. Nutrient requirement for maintenance was in the range of 16·7–23·4 mg/kg 0·8 per d, and did not differ between the treatments. There were no significant differences in lysine utilisation efficiency or bioavailability of protein-bound or crystalline lysine from the respective sources observed when lysine was confirmed to be the first-limiting nutrient.