Different metabolomic responses of grass carp (Ctenopharyngodon idellus) to dietary tannin and rapeseed meal

In order to study the metabolomic responses of rapeseed meal on grass carp and the role that tannin plays in those responses, grass carp were fed four diets for an eight week trial. Two were practical diets: FM diet contained 10% fishmeal without rapeseed meal, RM diet contained 50% rapeseed meal without fishmeal; the other two were semi-purified diets without (T0) or with 1.25% (T1) of supplemental hydrolysable tannin. The tannin content in the RM diet was close to that of T1. The rate of weight gain for RM was significantly lower than FM, while the feed conversion ratio in the T1 was significantly higher than T0. The muscle lipid and protein content was significantly lower and glycogen higher in the RM and T1 compared to the other treatments. The percent of muscle-saturated fatty acid (SFA) in the RM and T1 groups was significantly lower than FM and T0 groups, respectively, while the mono-unsaturated fatty acid (MUFA) in the RM group was significantly higher than FM. The hepatic activities of alanine aminotransferase and aspartate aminotransferase were lower and the hepatic glycogen and malonaldehyde were higher in the T1 and RM than the other treatments. The total serum protein, globulin, and triglycerides were significantly higher in T1 and RM; the alanine aminotransferase in the T1 was significantly higher than T0; and the aspartate aminotransferase in the RM and T0 groups were significantly higher than the other treatments. In a metabolomic assay, 29 differential metabolites were identified between T0 and T1, 23 metabolites were downregulated, and 6 metabolites were upregulated in T1 compared to T0. Ninety-two differential metabolites were identified between FM and RM, 31 metabolites were downregulated, and 61 metabolites were upregulated in RM compared to FM. These indicated that lipid utilization and carbohydrate metabolism might be improved, while the protein metabolism was suppressed by rapeseed meal. The abnormal protein and lipid metabolism may be partly caused by tannin in the rapeseed meal, as the tannin contained within the rapeseed meal seemed to bring different effects that the tannin additive.