Effects of different levels of methionine and taurine in all-plant-protein diets on growth, physiology and antioxidant capacity of juvenile Megalobrama amblycephala

To explore the effect of supplementing methionine (Met) and taurine (Tau) in the all-plant-protein diet of juvenile Megalobrama amblycephala, six experimental diets were formulated with three Met supplementary levels (2.5, 5.0 and 10.0 g/kg) and two Tau supplementary levels (1.0 and 2.0 g/kg), yielding measured concentrations of: M₁T₁ (Met 7.5 g/kg and Tau 1.7 g/kg), M₁T₂ (Met 7.2 g/kg and Tau 2.7 g/kg), M₂T₁ (Met 7.9 g/kg and Tau 1.7 g/kg), M₂T₂ (Met 8.6 g/kg and Tau 2.7 g/kg), M₃T₁ (Met 11.8 g/kg and Tau 1.6 g/kg) and M₃T₂ (Met 12.5 g/kg and Tau 2.8 g/kg). After a 10-week feeding trial, growth performance, plasma indices, and hepatic and intestinal antioxidant capacity were evaluated. Weight gain rate (WGR), specific growth rate (SGR), and feed conversion ratio (FCR) were affected by dietary Met and by Met × Tau interaction; the M₂T₂ group exhibited the highest WGR and SGR and the lowest FCR. Plasma alkaline phosphatase (ALP) was influenced by Met and the Met × Tau interaction, whereas plasma alanine aminotransferase (ALT) was affected by Tau. At Tau level T₁, ALP and ALT declined as Met increased. Hepatic and intestinal catalase (CAT) activity was highest in M₂T₂ and was influenced by the Met × Tau interaction. Intestinal glutathione (GSH) responded to Met and Tau, whereas malondialdehyde (MDA) content was affected by the interaction. At Tau level T₂, GSH and MDA decreased with increasing Met. Intestinal amylase (AMS) and trypsin (TPS) activities were affected by the Met × Tau interaction; at Tau level T₁, TPS peaked in the M₂T₁ group. Dietary inclusion of Met 8.6 g/kg and Tau 2.7 g/kg in an all-plant-protein diet significantly improved growth, feed utilization and antioxidant status, indicating a synergistic benefit of optimal Met and Tau supplementation.