Dietary L-carnitine supplementation changes lipid metabolism and glucose utilization of Rhynchocypris lagowskii fed diets with different lipid sources.

The widely available crop oil is an effective alternative to the increasingly scarce marine fish oil. However, simple alternative strategies have led to declining growth and the edible value of farmed fish. It is worthwhile to explore the effects of micro supplements in diets to improve the tolerance of fish to different dietary lipid sources, which finally optimizes the feeding strategies. This study aimed to investigate the regulation of L-carnitine and dietary oil conditions on nutrient composition, lipid metabolism, and glucose regulation of Rhynchocypris lagowskii. Four diets were prepared according to fish oil, fish oil supplemented with L-carnitine, corn oil, and corn oil supplemented with L-carnitine, and FO, LCFO, CO, and LCCO were labeled, respectively. R. lagowskii was fed experimental diets for 8 weeks, and the glucose tolerance test was performed. The CO diet significantly resulted in higher crude lipid content in muscle but a lower level of serum lipid parameters of R. lagowskii than the FO diet. However, dietary L-carnitine supplementation significantly reduced the crude lipid content in the hepatopancreas and muscle of the fish fed with the CO diet yet increased the serum lipid parameters. Additionally, the crude lipid content of muscle was reduced in the fish fed with an FO diet supplemented with L-carnitine. Compared with the FO diet, the CO diet significantly reduced the ratio of n3/n6 polyunsaturated fatty acid in the hepatopancreas and muscle of R.lagowskii. Dietary L-carnitine supplementation significantly reduced the contents of total saturated fatty acids and total monounsaturated fatty acids in hepatopancreas under both dietary lipid sources. The CO diet significantly up-regulated the expression of genes related to lipid uptake and adipogenesis in hepatopancreas, including lipoprotein lipase (lpl), acetyl-coenzyme A carboxylase alpha (accα), and sterol regulatory element binding protein-1 (srebp1), compared with the FO diet. While dietary L-carnitine supplementation significantly down-regulated the expressions of lpl, accα, srebp1, and fatty acid synthase in hepatopancreas and muscle of fish under both dietary lipid sources, along with up-regulated expression of carnitine palmitoyltransferase 1 in hepatopancreas. Moreover, the fish fed with a CO diet significantly increased the expression of glucose uptake and clearance and significantly down-regulated the expressions of glucose regulation-related genes, including glucose transporter 1, glycogen synthase 1, and phosphofructokinase in hepatopancreas and muscle, resulting in slower glucose uptake and clearance than fish fed with FO diet. Nevertheless, dietary L-carnitine supplementation up-regulated the expression of gluconeogenesis-related genes, including glucose-6-phosphatase and phosphoenolpyruvate carboxykinase in the hepatopancreas of R. lagowskii under both dietary lipid sources. In conclusion, a higher dietary n6 PUFA resulted in lipid deposition, decreased serum lipid parameters, and limited serum glucose utilization of R. lagowskii. While the regulatory effect of L-carnitine on lipid metabolism and glucose utilization of R. lagowskii varies with dietary lipid sources and tissues.