Phytic acid disrupted intestinal immune status and suppressed growth performance in on-growing grass carp (Ctenopharyngodon idella).

Phytic acid (PA) is one of the most common anti-nutritional factors in plant-derived protein feeds, and it poses considerable threats to aquaculture production. However, little is known about the effects of PA on fish intestinal health. This study aimed to investigate the impacts of PA on intestinal immune function in on-growing grass carp. To achieve this goal, a growth trial was conducted for 60 days by feeding 540 fish (120.56 ± 0.51 g) with six semi-purified diets containing graded levels of PA (0, 0.8, 1.6, 2.4, 3.2 and 4.0%). Then fish were challenged with Aeromonas hydrophila for 6 days. The results indicated that, compared with the control group (0% PA), PA did the following: (1) suppressed fish growth performance (percentage weight gain and feed efficiency) and reduced their ability to resist enteritis; (2) decreased fish intestinal antimicrobial ability by reducing intestinal lysozyme (LZ) activities, the contents of complement 3 (C3), C4 and immunoglobulin M (IgM), and downregulating the mRNA levels of hepcidin, liver-expressed antimicrobial peptide 2A (LEAP-2A), LEAP-2B, and β-defensin-1; and (3) aggravated fish intestinal inflammation responses by upregulating the mRNA levels of pro-inflammatory cytokines including tumour necrosis factor α (TNF-α), interleukin 1β (IL-1β) (except in the DI), interferon γ2 (IFN-γ2), IL-8, IL-12p40, IL-15 (except in the DI) and IL-17D, which is partly related to the nuclear factor kappa B (NF-κB) signalling pathway, whereas downregulating the mRNA levels of anti-inflammatory cytokines including transforming growth factor β1 (TGF-β1), IL-4/13A, IL-4/13B, IL-10 and IL-11, which is partially associated with the target of rapamycin (TOR) signalling pathway. The possible reasons for some distinctive gene expression patterns in fish three intestinal segments were discussed. Finally, based on the percent weight gain, enteritis morbidity, IgM content and LZ activity in the PI, the maximum tolerance levels of PA for on-growing grass carp were estimated to be 2.17, 1.68, 1.47 and 1.18% of the diet, respectively.