Dietary n-3/n-6 HUFA ratio modulates integrated physiological performance in hybrid grouper (Epinephelus fuscoguttatus ♀ × E. lanceolatus ♂): Defining a functional optimum for growth, hepatic health, and immune balance

This study investigated the effects of dietary n-3/n-6 highly unsaturated fatty acid (HUFA) ratios on growth performance, hepatic health, antioxidant capacity, and immune function in hybrid grouper ( Epinephelus fuscoguttatus ♀ × E. lanceolatus ♂). Six isonitrogenous and isolipidic diets were formulated with graded n-3/n-6 HUFA ratios of 25.29 (Y1), 19.54 (Y2), 13.27 (Y3), 7.74 (Y4), 3.96 (Y5), and 1.90 (Y6), and fed for 8 weeks. Optimal specific growth rate and protein efficiency ratio were observed at ratios 7.74–13.27, indicating superior nutrient utilization within this range. Hepatic lipid accumulation was lowest at a ratio of 19.54, as evidenced by minimal triglyceride content and lipid droplet area, whereas both excessively high and low ratios stimulated fatty acid synthase activity and promoted steatosis. The strongest antioxidant defense was observed at 7.74, characterized by upregulated glutathione peroxidase ( gpx ) and catalase ( cat ) gene expression, elevated glutathione peroxidase activity, induction of heat shock proteins ( hsp70 / hsp90 ), and the lowest malondialdehyde levels. Hepatocellular integrity was best preserved at 13.27, reflected by minimal glutamate pyruvate transaminase activity. Furthermore, this ratio enhanced humoral immunity—marked by the highest serum IgM—and shifted the inflammatory balance toward anti-inflammatory dominance through downregulation of interleukin 6 ( il-6 ) and interleukin 8 ( il-8 ) and upregulation of interleukin 10 ( il-10 ) and transforming growth-factor β ( tgf-β ). Collectively, dietary n-3/n-6 HUFA ratios modulate growth and liver health in hybrid grouper by coordinately regulating protein utilization, lipid metabolism, redox homeostasis, and immune responses, with a functional optimum between 7.74 and 13.27 for integrated physiological performance.