Dietary Protein Requirement of Juvenile Tor sinensis Based on Growth Performance, Antioxidants, Digestive Enzyme Activity and Intestinal Morphology

Tor sinensis is an emerging aquaculture species in China, yet the effect of dietary protein on its hepatic and intestinal health remains unexplored. This study evaluated the effects of five diets designed to be isoenergetic and isolipidic with graded protein levels (28% to 40%) on juvenile T. sinensis (initial weight: 10 ± 0.4 g) over 60 days. Growth performance improved with increasing protein up to 34%, beyond which it plateaued. Regression analysis indicates that the optimal dietary protein level for both weight gain and specific growth rate is 34.3%. Hepatic antioxidant enzyme activities (SOD and CAT) were highest in the 34% protein group, while triglyceride content was lowest. Histological examination revealed minimal hepatocyte swelling, nuclear displacement, and lipid droplet accumulation at this protein level. Intestinal trypsin activity and villus morphology (height, width, and muscular thickness) were also optimized at 34% protein, though lipase and amylase activities remained unaffected. These findings demonstrate that a 34% dietary protein level supports optimal growth, enhances liver antioxidant capacity, and improves intestinal structure and function in juvenile T. sinensis, providing critical insights for its formulated feed development.