Integrative unveiling of the effects of low dietary protein levels on grass carp (Ctenopharyngodon idella) fingerlings: Growth performance, antioxidant capacity, hepatic and intestinal function, and gut microbiota

A 60-days feeding trial was conducted to evaluate low dietary protein diets on the growth, antioxidant status, hepatic and intestinal function, and gut microbiota of grass carp fingerlings (28.07 ± 0.12) g. Following the experimental period, final body weight, weight gain and specific growth rate in the P30 and P35 groups were observed to be significantly higher than P20 and P25, while feed conversion rate was lower (P < 0.05). Whole-body and hepatic crude lipid, as well as serum triglycerides and total cholesterol were down-regulated with higher protein. In addition, the P20 group suppressed the activity of intestinal lipase and serum high-density lipoprotein, while enhanced the serum low-density lipoprotein and aspartate aminotransferase (P < 0.05). Further, the P20 group had poorer antioxidant capacity, whereas increasing protein also up-regulated intestinal barrier function with the higher expression of intestine tight junction protein (claudin-b) and lower serum LPS content, and inhibited intestinal inflammation with the lower expression of pro-inflammatory cytokines (il-8, il-1β) and higher expression of anti-inflammatory cytokines (tgf-β1) (P < 0.05). The intestinal microbiome results demonstrated that compared with the P20 group, the P35 group significantly increased the α-diversity, the abundance of Fusobacteriota, Cetobacterium, Enterobacter, Dielma, Epulopiscium, and Defluviicoccus, ameliorated the structure and metabolic systems of intestinal microbiome. In summary, low protein diets (P20 and P25) adversely affected the growth and feed efficiency of grass carp fingerlings. Notably, P20 could induce hepatic lipid accumulation and further trigger oxidative stress, leading to intestinal barrier injury and inflammatory response, which were resulted in microbiome disorder.