Tolerance assessment of dietary bile acids in common carp (Cyprinus carpio L.) fed a high plant protein diet

Abstract This study was conducted to investigate the tolerance dose of bile acids as a feed additive in high plant protein (PP) diets of common carp (Cyprinus carpio L.), by evaluating the responses in growth performance, hepatopancreas and intestinal histology, and glucose and lipid metabolism regulation. Three isonitrogenous and isoenergetic diets (36% PP and 10% fish meal) were prepared with bile acids supplementation of 0 (B0), 60 (B60), and 600 (B600) mg/kg, in which B600 was assayed as 10-fold the maximum recommended level (60 mg/kg) obtained by a previous efficacy test. Each diet was fed to six replicates of 30 common carps (initial body weight = 13.7 ± 0.01 g). Bile acids supplementation decreased feed conversion rate, hepatopancreas-somatic index, and hepatopancreas lipid content significantly. Dietary bile acids improved histopathological symptoms induced by the high PP diet, accompanied with down-regulated inflammatory responses (tnfα, il1β, il10, and tgfβ1) in distal intestine; and up-regulated anti-inflammatory cytokines (il10), but reduced apoptosis response (cleaved caspase3, detected by immunofluorescence staining) in the hepatopancreas. The ameliorated pathological symptoms and liver function in the B60 and B600 groups were closely related to the improved glucose metabolism regulation, with decreased fasting blood glucose and increased gluconeogenesis enzyme activity (G6Pase). Simultaneously, improved lipid metabolism was observed with up-regulated lipolysis (atgl) and bile acids synthesis (cyp7a1 and cyp8b1), but down-regulated cholesterol synthesis (hmgcr) gene-related mRNA, together with enhanced cholesterol transportation and bile acid recirculation to reduce lipid accumulation in the hepatopancreas. The phosphorylation of ERK1/2 was positively regulated by the systematic recovery of metabolic liver disease induced by the high PP diet. In conclusion, addition of 60 mg/kg bile acids is effective in common carp, with a 10-fold safety margin in commercial feed.