Effects of dietary oxidized rice bran on the coloration, antioxidant capacity, and immunity in channel catfish (Ictalurus punctatus)

To examine the impact of oxidized rice bran on body pigmentation and immune-antioxidant responses in channel catfish, four diets containing rice bran with varying oxidation levels were formulated for a 9-week feeding trial. The oxidation values were 1.28 mmol/kg (Control), 12.9 mmol/kg (TSR1), 24.3 mmol/kg (TSR2), and 36.7 mmol/kg (TSR3). Compared with the control, TSR3 exhibited a significantly reduced FCR, while plasma AST and ALT levels were markedly elevated in TSR2 and TSR3. Regarding skin pigmentation indices, L* values were significantly lower in TSR1 and TSR2, whereas a* values increased in TSR1–TSR3 and b* values in TSR2–TSR3. In muscle tissue, a* value was significantly higher in TSR3, and b* values in TSR1–TSR3. Both skin and muscle uranidin contents rose significantly in TSR1–TSR3, accompanied by a decline in tyrosinase levels that paralleled the reduction in muscle melanin content. Skin melanin concentration was significantly reduced in TSR2–TSR3. The expression of pigmentation-related genes ( mitf , tyr , α-msh , etb , and camk2 ) in skin and muscle was markedly suppressed in TSR1–TSR3, while xdh expression was up-regulated. Concerning antioxidant parameters, MDA concentrations in skin and muscle were elevated in TSR1–TSR3, whereas T-AOC levels and the activities of skin T-SOD, GPx, and muscle CAT declined significantly. Skin CAT and muscle GPx activities were further reduced in TSR2–TSR3. Gene expression analyses indicated significant down-regulation of ifn-γ , nrf2 , and il-10 in skin and muscle, as well as gpx in muscle within TSR1–TSR3, alongside increased keap1 and nfκb expression in skin. In TSR2–TSR3, gpx expression in skin was significantly decreased, and keap1 and nfκb were up-regulated in muscle. Overall, oxidized rice bran lowered FCR but adversely affected pigmentation, antioxidant defenses, and immune function in channel catfish • Oxidized rice bran with a high POV (36.7 mmol/kg) didn't affect the growth of channel catfish and reduced the FCR. • Oxidized rice bran reduced melanin and promoted uranidin in the skin and muscle of channel catfish, causing yellowing. • Oxidized rice bran damaged the immune-antioxidant systems of channel catfish, potentially contributing to the yellowing.