Effects of different levels of stevioside on growth performance, digestive enzyme activity, antioxidant capacity and gene expression of juvenile mirror carp (Cyprinus carpio)

Abstract

The study evaluated the effects of different levels of stevioside (STE) on the growth performance, intestinal digestive enzyme activity, liver antioxidant indices, and gene expression of juvenile songpu mirror carp (Cyprinus carpio). The carp were fed with six diets supplemented with 0 (control), 80, 160, 240, 320, and 400 mg/kg of STE, respectively. 540 juvenile mirror carp with an average initial weight of 3.00 ± 0.15 g were randomly divided into 6 groups, 3 replicates in each group and 30 fish in every replicate. The carp were fed to satiation for 56 days. The results showed that the weight gain rate (WGR), the condition factor (CF), and the ash content were increased linearly (P < 0.05), but the hepatosomatic index (HSI) was decreased linearly (P < 0.05). Dietary STE supplementation tended to improve the total protein (TP), total cholesterol (TCHO), and low-density lipoprotein (LDL) content (P < 0.10). The activity of superoxide dismutase (SOD) was increased quadratically (P ≤ 0.05). The addition of STE tended to increase the activity of protease and decrease amylase activity in the foregut (P < 0.10). The expression of tumor necrosis factor-alpha (TNF-α), interleukin-1 beta (IL-1β), and myeloid differentiation factor88 (MyD88) mRNA did not vary significantly (P > 0.05). It is confirmed from the study that the addition of STE could improve growth performance, liver antioxidant capacity, and immune function. Based on a broken-line regression analysis between the WGR and SOD activity against the dietary STE levels, the optimal dietary STE is 217.68 and 215.21 mg/kg, respectively.