Diet formulations with graded C:N ratios are associated with intestinal microbiota and health responses in GIFT tilapia (Oreochromis niloticus)

Abstract

This research examined the effects of graded dietary carbon to nitrogen (C:N) ratios, by decreasing protein sources and increasing wheat starch and wheat bran, on growth performance, intestinal health, and intestinal microbiota in genetically improved farmed tilapia (GIFT; Oreochromis niloticus ). Six experimental diets were formulated by adjusting dietary protein and carbohydrate to achieve C:N ratios of 7.34, 8.13, 9.20, 10.70, 12.15, and 14.47, and juvenile GIFT (initial mean body weight 5.30 g) were fed these diets for 58 days (6 groups, 3 replicates per group, 30 fish per cage). Growth performance indicators decreased significantly as the dietary C:N ratio increased, whereas the feed conversion ratio increased ( P < 0.001). Diets with C:N ratios ≥ 10.7 were associated with more pronounced adverse responses, including impaired intestinal morphology, reduced trypsin activity, downregulated tight junction–related gene expression, decreased antioxidant enzyme activities, and enhanced inflammatory responses ( P < 0.05). The graded diet formulations were associated with changes in intestinal microbial composition: higher C:N ratios were associated with positive linear trends in Mycobacterium , Dietzia , and Actinobacteriota abundance ( P < 0.05). Parallel to these taxonomic shifts, short-chain fatty acid analysis showed significant among-diet differences in butyric acid levels ( P < 0.05), with the highest numerical value observed in the CN10.7 treatment. Functional prediction indicated that higher C:N diets were associated with shifts in predicted microbial metabolic functions. Collectively, although diets with C:N ratios ≥ 10.7 were associated with predicted enrichment of carbohydrate-related microbial functions, these higher-ratio diets were also associated with impaired growth and intestinal health. Under the present experimental conditions and formulation strategy, the CN7.3–CN9.2 treatments showed less pronounced adverse host responses than the CN10.7–CN14.5 treatments, whereas further increases were associated with impaired growth, intestinal barrier function, and inflammatory and oxidative stress responses.