Dietary L-carnitine optimizes lipid homeostasis in juvenile Trachinotus ovatus: a balancing strategy for promoting muscle fat deposition and hepatic lipid catabolism

Given that functional additives have species-specific effects on lipid metabolism in fish muscles and livers, determining the appropriate addition amount is of great significance for a single species. This study aimed to investigate the regulatory effects of dietary L-carnitine supplementation on juvenile Trachinotus ovatus, focusing on promoting muscle fat deposition and hepatic lipid catabolism to achieve cross-tissue lipid homeostasis, while optimizing growth performance and antioxidant status.Five iso-nitrogenous and iso-lipidic diets (D1 to D5) were formulated with L-carnitine levels of 0, 200, 400, 600, and 800 mg/kg, respectively. An 8-week feeding trial was conducted in sea cages.As dietary L-carnitine levels increased, the final body weight (FBW), weight gain rate (WGR), and specific growth rate (SGR) showed an initial increase followed by stabilization, reaching their peak in the D2 group（P<0.05）. In terms of lipid metabolism, increasing L-carnitine supplementation resulted in a decrease in hepatic crude fat content and lipid deposition, while muscle crude fat content and lipid deposition increased. No significant effect was observed on liver fatty acid composition (P>0.05). The expression levels of hepatic lipid metabolism-related genes-carnitine palmitoyltransferase 1 (cpt1), apolipoprotein B-100 (*apob-100*), liver-type fatty acid-binding protein (l-fabp), and peroxisome proliferator-activated receptor alpha (ppar-α)—increased with the supplementation level, peaking in group D2 before gradually declining in group D4（P<0.05）. In muscle, the expression levels of cpt1 and ppar-α in groups D3 to D5 were significantly lower than those in groups D1 and D2（P<0.05）, reaching the minimum in group D5. Regarding antioxidant indices, serum total antioxidant capacity was lowest in group D1（P<0.05）. Serum superoxide dismutase (SOD) activity decreased with increasing L-carnitine levels（P<0.05）, while malondialdehyde (MDA) levels increased（P<0.05） Hepatic gene expression of thioredoxin (trx1) and peroxiredoxin (prx1) was lowest in group D3（P<0.05）, and heat shock protein 70 (hsp70) reached its highest level in group D4（P<0.05）. For serum physiological and biochemical parameters, high-density lipoprotein (HDL) cholesterol was significantly higher in group D2 than in groups D1 and D5（P<0.05）, and total protein was lowest in group D4（P<0.05）.In summary, dietary L-carnitine supplementation increased muscle crude fat content and decreased hepatic crude fat content in juvenile T. ovatus, promoting muscle fat deposition and hepatic lipid catabolism. The optimal growth performance was achieved at the supplementation level of 400 mg/kg. This level also supported favorable hepatic lipid metabolism and systemic antioxidant capacity. These findings provide valuable scientific data on lipid regulation by L-carnitine in T. ovatus.