Reevaluation of the dietary methionine requirement of gilthead seabream (Sparus aurata) juveniles: A comparison of the efficiency of hydroxy methionine with DL-methionine as a dietary methionine source

Abstract

This work aimed to estimate the dietary methionine (Met) requirement of gilthead seabream (Sparus aurata) juveniles using hydroxy methionine (OH-Met) as Met source and compare its efficiency with the DL-Methionine form (DL-Met). For that purpose, triplicate groups of fish with 17 g initial body weight were fed for 83 days with diets including increasing dietary Met levels (0.66, 0.80, 0.94, 1.08, 1.22, and 1.36% - sulfur-equivalent) using OH-Met as a Met source. Three other diets were also formulated with 0.94, 1.08, and 1.22% DL-Met, to compare OH-Met with DL-Met as dietary Met source. Cysteine was maintained at 0.54% in all the diets.Fish growth increased with dietary OH-Met inclusion up to 1.22% plateauing thereafter. No effect on feed intake was observed between fish fed the different dietary Met levels and sources. Feed and protein efficiencies increased with the increase in dietary Met level. Dietary Met source did not affect growth performance, feed utilization, nitrogen, and energy retention. Based on the four-parameter logistic model, the dietary Met requirement for maximum growth was estimated to be 1.05%. The lowest hepatic level of reduced glutathione (GSH), oxidized glutathione (GSSG), total glutathione (tGSH), and malondialdehyde (MDA) were observed in fish fed the OH-Met 1.36 diet, but the oxidative stress index (OSI) and GSH/GSSG ratio were not affected by the dietary OH-Met level. In the muscle, GSH, GSSG, OSI, MDA, and GSH/GSSG ratio were not affected by the OH-Met level, while tGSH linearly decreased with increasing dietary OH-Met level. Dietary Met source did not affect the liver and muscle oxidative stress-related parameters measured. Overall, it is concluded that the Met requirement for growth of gilthead seabream juveniles is satisfied with 1.05% Met in the diets (corresponding to 1.59% total sulfur amino acids, TSAA). This corresponds to a Met requirement of 2.3 g/16 g N (3.5 g/ 16 g N of TSAA). It is also concluded that OH-Met and DL-Met can be used interchangeably as dietary Met sources and that the liver and muscle antioxidant status of fish is not affected by the dietary Met sources tested.