Evaluation of Silymarin–L-Carnitine as a Dietary Supplement on Growth Performance, Antioxidants and Immunity, Gut/Liver Health, and Gene Expression in Nile Tilapia (Oreochromis niloticus)

Silymarin and L-carnitine are individually used in fish diets, yet whether they exert interactive or additive effects when combined remains unclear. This study aimed to investigate the individual and combined impacts of dietary silymarin (S), L-carnitine (LC), and their combination (S + LC) on growth performance, digestive enzyme activity, antioxidant status, immune response, and gene expression in Nile tilapia. A total of 360 fish (initial body weight: 10.01 ± 0.03 g) were randomly allocated into 12 fiberglass tanks (30 fish/tank) and fed one of four diets for 84 days: control (basal diet), S (850 mg/kg), LC (500 mg/kg), and S + LC (425 mg/kg S + 250 mg/kg LC). Fish fed S and S + LC diets exhibited significantly higher final body weight, weight gain, and specific growth rate (SGR), along with improved feed conversion ratio (FCR) compared to the control (p < 0.05). All supplemented groups exhibited enhanced digestive enzyme activities (amylase, lipase, protease), with the S + LC group showing the highest values. Serum biochemical profiles revealed increased total protein and globulin and reduced glucose and cortisol levels. Innate immune responses (IgM, lysozyme activity, NBT%, and bactericidal activity) were significantly elevated, especially in the S + LC group. Antioxidant enzyme activities (SOD, CAT, GPx) increased, while malondialdehyde (MDA) levels declined. Gene expression analysis showed significant upregulation of IGF-1, IFNA-1, SOD, CAT, and Gsr, with the greatest expression in the S + LC group. These findings indicate that dietary silymarin and L-carnitine, particularly when provided together, produced complementary and enhanced effects on growth, immune competence, antioxidant capacity, and gene regulation in Nile tilapia.