Overfeeding in rainbow trout (Oncorhynchus mykiss): Metabolic disruptions, impaired immunity, and increased infection risk.

Excess adiposity impairs immune function and host defense in obese individuals, but studies on this concept in fish remain limited. In aquaculture, rapid growth is often encouraged through intensive farming practices, leading to overfeeding and negatively impacting production. This study aimed to induce obesity in rainbow trout through overfeeding, exploring metabolic abnormalities, immune response alterations, and infection susceptibility via transcriptomic and metabolomic analyses. In the overfed group, fish were fed until they refused to eat, while the control group was fed according to recommended feeding rates for four weeks. Sampling was conducted at weeks 1, 2, and 4 for serological, histopathological, metabolomic, and transcriptomic analyses. After four weeks, mortality rates were compared following Aeromonas salmonicida challenge, and immunological changes assessed one day post-infection. Overfed fish exhibited significant increases in weight gain (WG), body mass index (BMI), elevated AST/ALT levels, hepatocyte hypertrophy, lipid droplet formation, and triglyceride accumulation. At 1, 2, and 4 wpf, the overfed group exhibited distinct metabolic changes, with key alterations in glycolysis/gluconeogenesis, lipid metabolism and amino acid metabolism. KEGG analysis of transcriptomic data revealed a significant decrease in complement and coagulation cascades, including C3, FB, FH, an FI, accompanied by heightened TNF and IL-17 signaling pathways, involving the upregulation of genes such as TNF-α, IL-1β, and IL-6, indicating an enhanced inflammatory response. The overfed group experienced higher mortality post-infection. Excess energy from overfeeding led to hepatic fat accumulation, liver damage, and reduced innate immune responses, particularly in complement activation. These physiological disruptions compromised immune function, highlighting the detrimental effects of overfeeding-induced obesity on fish health. This study offers critical insights into the immunological mechanisms linking obesity to increased disease susceptibility.
Competing Interests: Declaration of competing interest The authors declared no competing interest exists.
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