Effects of dietary emodin supplementation on the growth performance, serum biochemical parameters, and intestinal health of Pengze crucian carp (Carassius auratus var. Pengze) fed with high plant-protein diets

Emodin supplementation could relieve the adverse impacts induced by a high plant-protein diet on intestinal health and growth performance of fish, while its application in Pengze crucian carp (Carassius auratus var. Pengze) is lacking. Pengze crucian carp, an omnivorous fish, belongs to the freshwater Cyprinidae family. In this study, Pengze crucian carp were fed with a normal fishmeal diet (18% fishmeal, FM), a high plant-protein diet (4% fishmeal, HP), and four different emodin supplementation diets (HP diet supplemented with 25, 50, 75, and 100 mg kg−1 emodin); the feeding trial lasted 8 weeks. Compared with the FM group, the HP diet dramatically inhibited the growth performance of carp, while an appropriate addition of emodin could restore its growth performance. Emodin could improve carp morphology in terms of a higher value of the hepatosomatic index (HSI), the viserosomatic index (VSI), and the condition factor (CF), and increase the crude protein content concurrent with a lower crude lipid content in body compared with HP and FM groups. The HP diet induced the oxidative stress of carp, whereas emodin supplementation enhanced antioxidant parameters in serum and intestinal tissues together with a remarkable reduction of malondialdehyde content, thereby improving antioxidant status. The antioxidant property of emodin triggered the keap1-nrf2 signal pathway and then induced the overexpression of antioxidant genes (cuznsod, cat, and gpx) in intestinal tissue. Additionally, emodin attenuated the intestinal inflammation induced by the HP diet by suppressing the expression of pro-inflammatory-related factors (tlr4, myd88, tnf-α, il-1β, and ifn-γ) and upregulating anti-inflammatory-related factors (tlr2, tgf-β, and il-10), followed by an improvement of the intestinal barrier function. Moreover, emodin supplementation could induce the transcriptional level of intestinal alkaline phosphatase (iap) through the pparγ-klf4 signal pathway and enhance IAP activity ultimately. In summary, a suitable amount of emodin can elevate the growth performance and improve the intestinal health of Pengze crucian carp fed a high plant-protein diet.