L-carnitine supplementation in high-fat diets on growth performance, digestive enzyme activities, antioxidant capacity, and lipid metabolism in bullfrogs (Lithobates catesbeianus)

This study aimed to investigate the effects of dietary L-carnitine supplementation at different levels in high-fat diets on growth performance, digestive enzyme activities, serum biochemical indices, antioxidant capacity, and expression of hepatic lipid metabolism-related genes in bullfrogs (Lithobates catesbeianus). A total of 234healthy bullfrogs with an initial average body weight of approximately 26.83 g were randomly divided into 6 groups, which were fed a normal-fat diet (NFD group, crude lipid 7.01%), a high-fat diet (HFD group, crude lipid 10.56%), and high-fat diets supplemented with 0.075%, 0.150%, 0.225%, and 0.300% L-carnitine (HR1 to HR4 groups), respectively, with a feeding trial period of 56 days. The results showed that compared with the HFD group, weight gain rate, specific growth rate, and feed efficiency of bullfrogs in HR1-HR3 groups were significantly increased (P<0.05). Among them, the above indicators in the HR1 group were significantly higher than those in the NFD group (P<0.05), and there was no significant difference between the other supplemented groups and the NFD group (P>0.05). No significant difference in whole-body crude lipid of bullfrogs in HR1-HR4 groups compared with the NFD group. Furthermore, no significant difference in muscle crude lipid content of bullfrogs in HR1-HR3 groups compared with the NFD group (P>0.05). Bullfrogs in HR1-HR4 groups showed significantly decreased serum total cholesterol content compared with the HFD group (P<0.05); however, no significant differences in these parameters were observed between any of the L-carnitine-supplemented groups and the NFD group (P>0.05). Only the intestinal trypsin activity and whole-body crude protein content in the HR1 group were significantly higher than those in the HFD group (P<0.05); the intestinal lipase activity of all supplemented groups was significantly lower than that of the HFD group (P<0.05), and there was no significant difference in the above digestive enzyme indicators between all supplemented groups and the NFD group (P>0.05). Bullfrogs in HR1-HR3 groups showed significantly increased the activities of hepatic catalase and superoxide dismutase, and significantly decreased the malondialdehyde content (P<0.05), with no significant difference in the above indicators between all supplemented groups and the NFD group (P>0.05). Oil Red O staining showed that the HR1 and HR2 groups had the optimal alleviation effect on hepatic lipid deposition. Compared with the HFD group, bullfrogs in HR1-HR4 groups showed significantly up-regulated the expression of the hepatic lipolysis-related gene cpt1α, and significantly down-regulated the expression of the lipogenesis-related genes fas, acc1, and pparγ (P<0.05); the expression level of atgl in HR3 and HR4 groups was significantly higher than that in the HFD group (P<0.05). No significant differences were observed in the expression of lipogenic genes in all supplementation groups, as well as the cpt1α gene in HR1-HR3 groups, compared with the NFD group (P>0.05). Additionally, there were no significant differences in the expression of the hsl gene among all groups (P>0.05). In conclusion, 0.075% L-carnitine supplementation in high-fat diets can effectively alleviate the growth inhibition and excessive lipid deposition in bullfrogs caused by high-fat intake by regulating the expression of hepatic lipid metabolism-related genes, improving intestinal digestive function, and enhancing antioxidant capacity.