Nutritional programming through early exposure to soybean meal: Effects on growth performance, intestinal health, metabolism, and microbiota of juvenile largemouth bass (Micropterus salmoides)

This study investigated nutritional programming (NP) in largemouth bass ( Micropterus salmoides ) by examining the long-term effects of early soybean meal (SBM) exposure on growth, immunity, and intestinal health. In Phase 1 (fry stage, 28 days), 720 fry (28 days post-hatch, initial body weight 0.17 ± 0.06 g) were divided into two groups (six replicates per group). They were fed either a fishmeal (FM)-based diet (FM1) or 15% SBM diet (SBM1). In Phase 2 (juvenile stage, 56 days), a cross-over feeding trial was conducted, generating four groups (four replicates per group): F1F2 (FM1→FM2,), F1S2 (FM1→25% SBM [SBM2]), S1F2 (SBM1→FM2), and S1S2 (SBM1→SBM2). Phase 1 SBM feeding (SBM1) impaired fry survival, intestinal relative weight, and villus width ( P < 0.05), and triggered intestinal inflammation and microbial dysbiosis. Compared to the FM1 group, the SBM1 group exhibited downregulated expression of key intestinal health-related genes, including the tight junction protein gene claudin-1 , the anti-inflammatory factor interleukin-10 ( IL-10 ), and lysozyme ( LZM ). In contrast, it upregulated the expression of pro-inflammatory genes, including nuclear factor kappa-B ( NF-κβ ), interleukin-15 ( IL-15 ), and interferon gamma 1 ( IFN-γ1 ) ( P < 0.05). In Phase 2, continuous SBM exposure (S1S2 group) impaired growth performance, as evidenced by significantly lower final body weight and a poorer feed conversion ratio ( P < 0.05), but enhanced hepatic alkaline phosphatase (AKP) and LZM activities ( P < 0.001). Concurrently, the S1S2 group developed an immunologically tolerant intestinal phenotype, characterized by a significant upregulation ( P < 0.05) of: 1) regulatory T cell (Treg)-related transcription factors, including forkhead box P3 ( Foxp3 ) and Ikaros family zinc finger protein elios ( Helios ); 2) the Treg-associated signaling molecule signal transducer and activator of transcription 5b ( Stat5b ); 3) immunoregulatory cytokines, including transforming growth factor-β ( TGF-β ) and interleukin-10 ( IL-10 ); and 4) the immune tolerance-related molecule tumor necrosis factor alpha-induced protein 2 ( TNFAIP2 ), without a concomitant increase in the pro-inflammatory cytokine interleukin-15 ( IL-15 ). Continuous SBM exposure (S1S2) also improved gut microbiota composition, characterized by increased abundance of Firmicutes and Streptococcus and decreased abundance of Proteobacteria. These changes were accompanied by a significant downregulation of the arachidonic acid metabolic pathway ( P < 0.05 for all observed changes). In contrast, switching to a fishmeal diet in Phase 2 (S1F2 group) rescued growth, increased intestinal villus length and hepatic AKP activity, and alleviated inflammation and apoptosis ( P < 0.05). These results suggest that NP during the juvenile stage induces physiological adaptations and tolerance to SBM-based diets, highlighting its potential to improve sustainable plant protein use in aquafeeds.