Effects of dietary l-carnitine on growth performance, lipid metabolism, oxidative stress, mitochondrial function and endoplasmic reticulum stress of rice field eel (Monopterus albus) fed high-carbohydrate-diets

Abstract

To comprehensively investigate effects of dietary l-carnitine (LC) on growth performance, lipid metabolism, oxidative stress, mitochondrial function and endoplasmic reticulum (ER) stress in farmed fish fed high-carbohydrate-diets (HCD), 960 rice field eels (17.09 ± 0.14 g) were chosen and randomly assigned to 16 cages. These fish were fed a control diet (15% α-starch, CON), a high-carbohydrate-diet (35% α-starch, HCD), a HCD supplemented with 1.2‰ LC (HC/LLC), and a HCD supplemented with 2.4‰ LC (HC/HLC). Results of 8-week feeding trial showed that HCD increased liver lipid content, induced liver dysfunction, oxidative stress and endoplasmic reticulum (ER) stress, but did not affect growth performance. Dietary LC supplementation increased significantly weight gain rate and specific growth rate and decreased liver lipid content. Concurrently, LC reduced the activity of alanine transaminase enzyme in the serum, improved liver's antioxidant capacity, thereby mitigating liver damage. Dietary LC positively regulated the expression of genes related to oxidative stress, such as Kelch-like ECH-associated protein 1, nuclear factor erythroid 2-related factor 2, catalase and superoxide dismutase in fish. Furthermore, LC supplementation upregulated expression of genes related to the mitochondrial function, such as cardiolipin synthase 1 and lysocardiolipin acyltransferase 1 (cardiolipin metabolism), peroxisome proliferator-activated receptor gamma coactivator 1 alpha, peroxisome proliferator-activated receptor alpha and carnitine palmitoyltransferase 1 (mitochondrial β-oxidation) in fish. Intriguingly, administering LC-fortified diets resulted in downregulation of ER stress-related gene expression, such as glucose regulated protein 78, inositol-requiring enzyme 1 and activating transcription factor 6 in fish. For the first time, this study reports that the incorporation of LC into HCD facilitates lipid metabolism of fish through concurrent enhancement of mitochondrial function, reduction of oxidative stress, and alleviation of ER stress.