Abstract
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 a 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 for phase 1 followed by a FM-based diet for phase 2(FM2); F1S2, FM1 followed by a 25% SBM diet (SBM2), S1F2 (SBM1 followed by FM2), and S1S2 (SBM1 followed by 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 il-10, and lzm. In contrast, it upregulated the expression of pro-inflammatory genes, including nf -κ b, il -15, and 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 f oxp3 and h elios; 2) the s tat5b; 3) immunoregulatory cytokines, including tgf -β and il -10; and 4) the immune tolerance-related molecule tnfaip 2, without a concomitant increase in the pro-inflammatory 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.