Evaluation of the protein-sparing effects of carbohydrates in the diet of the crayfish, Procambarus clarkii

To explore the effects of dietary protein (P)/carbohydrate (C) ratios on the growth performance, body compo-sition, immunity, and metabolism of red swamp crayfish (Procambarus clarkii), soybean meal, peanut meal, and cottonseed meal were used to adjust the P content of the feed, and corn starch was used to adjust the C content. Five groups of crayfish were fed diets with different P/C ratios (P25%/C39%, P28%/C36%, P31%/C33%, P34%/ C30%, and P37%/C27%) for 8 weeks. The growth performance, body composition, and immunity of the crayfish, as well as the enzyme activities and expression of genes in the hepatopancreas, were assessed at the end of the feeding period. While the P/C ratio had no significant effect on growth performance, immunity, or body composition, a decrease in the P/C ratio resulted in a significant increase in the expression of glucose transporter 1 (GLUT1), as well as gradual increases in the activities and expression of hexokinase and pyruvate kinase in the hepatopancreas. There were significant differences in these values between the highest P/C ratio group (P37%/ C27%) and the lowest P/C ratio group (P25%/C39%). However, the activities of the glycolytic enzymes glucose-6-phosphatase (G6P) and pyruvate carboxylase (PC) showed the opposite trend, indicating that a decrease in the P/C ratio inhibited gluconeogenesis. Increases in the dietary P/C ratio were associated with increases in aspartate aminotransferase (AST) and alanine aminotransferase (ALT) activities, which correlated with increases in the expression of AST, ALT, and glutamate dehydrogenase (GDH). An increase in dietary P content was also asso-ciated with significant increases in expression of the mechanistic target of rapamycin (mTOR) and 4E binding protein 1 (4EBP1) genes, which promote protein deposition. By contrast, while increasing C content decreased and then increased hepatopancreatic malondialdehyde content, it increased and then decreased superoxide dismutase and glutathione peroxidase activities. There were no significant differences in circulating acid phos-phatase, alkaline phosphatase, AST, and ALT activities among the groups. Thus, the addition of carbohydrate to crayfish diets has a protein-sparing effect, and a diet containing 31% P and 33% C is optimal for maintaining antioxidative capacity, while a diet with a 25%P/39%C ratio results in maximum cost saving.