Effects of phytosterol supplementation in a high-fat diet on growth performance, intestinal digestive enzyme activities, hepatic antioxidant capacity and lipid synthesis capacity in juvenile grass carp (Ctenopharyngodon idella)

High-fat diets are widely used in aquafeeds to increase dietary energy supply and spare protein, but excessive lipid intake may impair growth performance, induce hepatic lipid accumulation, and disturb antioxidant and digestive functions in fish. Phytosterols are natural plant-derived compounds with potential lipid-lowering and antioxidant effects, but their regulatory effects in C. idella fed high-fat diets remain unclear. This study investigated the effects of dietary phytosterol supplementation in a high-fat diet on growth performance, intestinal enzyme activities, hepatic antioxidant capacity, and lipid synthesis in juvenile C. idella. C. idella with an initial body weight of (84.89 ± 0.12) g were assigned to five dietary treatments: a control diet (CN, 5.0% crude lipid), a high-fat diet (HF, 10.7% crude lipid), and HF diets supplemented with 250, 500, or 1,000 mg/kg phytosterol product (PS250, PS500, and PS1 000). Each treatment had three replicates, and the feeding trial lasted 8 weeks. Growth performance, organ indices, intestinal digestive enzymes, plasma biochemical parameters, hepatic antioxidant and lipid indices, hepatic histology, and expression of lipid-synthesis-related genes were determined. Compared with CN, HF decreased final body weight (320.28 ± 3.82 vs. 286.96 ± 7.04 g), weight gain rate (276.35 ± 4.49% vs. 236.82 ± 8.40%), specific growth rate (2.21 ± 0.02%/d vs. 2.02 ± 0.38%/d), intestinal lipase activity (1.86 ± 0.25 vs. 0.97 ± 0.12 U/g protein), trypsin activity (7,470.34 ± 643.73 vs. 4,219.59 ± 371.07 U/g protein), and hepatic total superoxide dismutase activity (777.4 ± 33.5 vs. 662.3 ± 31.4 U/mg protein), while increasing feed conversion ratio (1.52 ± 0.04 vs. 1.77 ± 0.04), plasma alanine aminotransferase activity (1.19 ± 0.16 vs. 1.91 ± 0.27 U/L), plasma triglyceride (1.16 ± 0.05 vs. 1.44 ± 0.07 mmol/L), hepatic triglyceride (32.06 ± 1.14 vs. 35.98 ± 1.34 μmol/g protein), and hepatic malondialdehyde (4.52 ± 0.73 vs. 8.28 ± 1.01 nmol/mg protein) (P < 0.05). Phytosterol supplementation alleviated these adverse effects, with PS500 showing the most consistent response: final body weight, weight gain rate, feed conversion ratio, plasma alanine aminotransferase, plasma triglyceride, hepatic triglyceride, hepatic malondialdehyde, and hepatic total superoxide dismutase were 301.54 ± 14.79 g, 255.28 ± 18.53%, 1.62 ± 0.08, 1.28 ± 0.14 U/L, 1.15 ± 0.06 mmol/L, 32.56 ± 1.18 μmol/g protein, 3.25 ± 0.33 nmol/mg protein, and 982.5 ± 33.4 U/mg protein, respectively. The lipid droplet deposition area in PS500 was lower than in both CN and HF, and hepatic pparγ expression did not differ from CN. PS500 also downregulated fasn, acacα, pparγ, and gyk compared with HF (P < 0.05). Therefore, 500 mg/kg phytosterol product (54 mg/kg effective phytosterol) effectively alleviated growth suppression, digestive dysfunction, oxidative damage, and lipid accumulation caused by a high-fat diet. This dosage is recommended as a nutritional strategy for healthy C. idella aquaculture. These results provide a theoretical basis for using phytosterols as a green functional feed additive in high-energy aquafeeds.