Effects of dietary carbohydrate/lipid ratios on non-specific immune responses, oxidative status and liver histology of juvenile yellow catfish Pelteobagrus fulvidraco

This study aimed to investigate the effects of dietary carbohydrate/lipid (CHO:L) ratios on immune responses, oxidative status, and liver histology of juvenile Pelteobagrus fulvidraco. Six isonitrogenous and isoenergetic diets were formulated, containing various CHO:L ratios from 1.11 to 9.50. Each diet was tested in four replicates for 8 weeks. The results indicated that the leucocyte count, plasma globulin content and lysozyme activity of fish fed the 5.58 CHO:L diet were significantly (P < 0.05) higher than those of fish fed 1.11 and 1.67 CHO:L diets. A similar trend was observed in plasma alternative complement (ACH50) pathway activity, while the erythrocyte count decreased significantly (P < 0.05) as the dietary CHO:L ratios increased from 1.11 to 2.45. Moreover, reduced glutathione (GSH) contents, catalase (CAT), glutathione peroxidase (GPX) activities and malondialdehyde (MDA) contents in the liver decreased significantly (P < 0.05) as dietary CHO:L ratios increased from 1.11 to 5.58. A similar trend was observed in total superoxide dismutase (t-SOD) (P < 0.05). Liver histology was normal in fish fed 2.45, 3.63 and 5.58 CHO:L diets; however, hepatic steatosis scores and hepatocyte apoptosis rates of fish fed 1.11 and 1.67 CHO:L diets were significantly (P < 0.05) higher than those of fish fed with other diets. On the whole, the diets with CHO:L ratios of 2.45-5.58 (approximately 24.5-33.5% carbohydrate and 6-10% lipid) are optimal for physiological condition and health status of juvenile yellow catfish.