Dietary supplementation of kaempferol improved the growth, lipid metabolism and flesh quality of juvenile grass carp (Ctenopharyngodon idellus) based on metabolomics

Kaempferol is a common flavonoid with various biological and pharmacological functions. To investigate the effects of dietary kaempferol on growth, lipid metabolism and flesh quality of juvenile grass carp (Ctenopharyngodon idella), five diets with kaempferol inclusion at 0 (control diet), 0.2, 0.4, 0.6 and 0.8 g/kg were fed to fish (17.0 ± 0.2 g) for 60 days. The results indicated that dietary kaempferol level linearly affected weight gain (WG) and feed conversion ratio (FCR), and the supplementation of 0.6 g/kg, 0.8 g/kg kaempferol increased WG by 4.9 %, 5.6 % and decreased FCR by 0.08, 0.08, respectively. Flavor amino acids content and activities of superoxide dismutase, catalase and glutathione peroxidase in flesh were linearly increased, and total free amino acids content, hardness, chewiness and adhesiveness of flesh were linearly and quadratically increased with the increasing kaempferol level. Moreover, the increasing dietary kaempferol linearly decreased intraperitoneal fat ratio, malondialdehyde content in flesh and triacylglycerol and cholesterol in serum and liver. In metabolomics profiling of flesh, 102 and 133 named differential metabolites were identified in 0.4 g/kg and 0.8 g/kg kaempferol groups, and these metabolites were mainly involved in the second-grade pathways of “lipid metabolism” and “amino acid metabolism”. In gene expression, the mRNA levels of mammalian target of rapamycin (mTOR), catalase (CAT), peroxisome proliferators-activated receptor α (PPARα) in flesh as well as PPARα in liver were up-regulated, and FAS gene expression in liver was down-regulated by dietary supplementation of 0.4 and 0.8 g/kg kaempferol. In summary, dietary kaempferol improved the growth, lipid metabolism and flesh quality of juvenile grass carp, which might be associated with the insulin-like growth factor 1/mammalian target of rapamycin (IGF-1/mTOR), PPAR and nuclear factor-E2 related factor 2 (Nrf2) pathways. The recommended supplementation level of kaempferol in diet was 0.8 g/kg for juvenile grass carp.