Changes in fatty acid concentrations in tissues of African catfish, Clarias gariepinus Burchell, as a consequence of dietary carnitine, fat and lysine supplementation

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.

Functional differences between l- and d-carnitine in metabolic regulation evaluated using a low-carnitine Nile tilapia model

l-Carnitine is essential for mitochondrial β-oxidation and has been used as a lipid-lowering feed additive in humans and farmed animals. d-Carnitine is an optical isomer of l-carnitine and dl-carnitine has been widely used in animal feeds. However, the functional differences between l- and d-carnitine are difficult to study because of the endogenous l-carnitine background. In the present study, we developed a low-carnitine Nile tilapia model by treating fish with a carnitine synthesis inhibitor, and used this model to investigate the functional differences between l- and d-carnitine in nutrient metabolism in fish. l- or d-carnitine (0·4 g/kg diet) was fed to the low-carnitine tilapia for 6 weeks. l-Carnitine feeding increased the acyl-carnitine concentration from 3522 to 10 822 ng/g and alleviated the lipid deposition from 15·89 to 11·97 % in the liver of low-carnitine tilapia. However, as compared with l-carnitine group, d-carnitine feeding reduced the acyl-carnitine concentration from 10 822 to 5482 ng/g, and increased lipid deposition from 11·97 to 20·21 % and the mRNA expression of the genes involved in β-oxidation and detoxification in the liver. d-Carnitine feeding also induced hepatic inflammation, oxidative stress and apoptosis. A metabolomic investigation further showed that d-carnitine feeding increased glycolysis, protein metabolism and activity of the tricarboxylic acid cycle and oxidative phosphorylation. Thus, l-carnitine can be physiologically utilised in fish, whereas d-carnitine is metabolised as a xenobiotic and induces lipotoxicity. d-Carnitine-fed fish demonstrates increases in peroxisomal β-oxidation, glycolysis and amino acid degradation to maintain energy homeostasis. Therefore, d-carnitine is not recommended for use in farmed animals.

Effects of L-carnitine against H2O2-induced oxidative stress in grass carp ovary cells (Ctenopharyngodon idellus)

This study was designed in vitro to investigate the effects of L-carnitine against H2O2-induced oxidative stress in a grass carp (Ctenopharyngodon idellus) ovary cell line (GCO). GCO cells were pre-treated with different concentrations of L-carnitine, followed by incubation with 2.5 mM H2O2 for 1 h to induce oxidative damage. The results indicated that adding L-carnitine at concentrations of 0.01-1 mM into the medium for 12 h significantly increased cell viability. Pre-treatment with L-carnitine at concentrations of 0.1-5 mM for 12 h significantly inhibited 2.5 mM H2O2-induced cell viability loss. The significant decreases in the level of reactive oxygen species and cell apoptosis were observed in 0.5 mM L-carnitine group compared to the H2O2 group. Malondialdehyde values of all of the L-carnitine groups were significantly lower than those of the H2O2 group, while total glutathione levels of all of the L-carnitine groups were significantly higher than of the H2O2 group. The activity of antioxidant enzymes, such as total superoxide dismutase (0.1 and 0.5 mM L-carnitine), catalase (0.5 mM L-carnitine) and γ-glutamyl cysteine synthetase (0.5 and 1 mM L-carnitine), was significantly increased. In addition, pre-treatment of L-carnitine in GCO cells exposed to 2.5 mM H2O2 significantly increased the mRNA expression of copper, zinc superoxide dismutase, catalase (0.5 mM L-carnitine), glutamate cysteine ligase catalytic subunit (0.1-1 mM) and glutathione peroxidase (0.1 mM L-carnitine). In conclusion, L-carnitine promotes GCO cell growth and improves antioxidant function, it plays a protective role against oxidative stress induced by H2O2 in GCO cells, and the appropriate supplemental amount of L-carnitine is 0.1-1 mM.

Dietary l-carnitine supplementation increases lipid deposition in the liver and muscle of yellow catfish (Pelteobagrus fulvidraco) through changes in lipid metabolism

Carnitine has been reported to improve growth performance and reduce body lipid content in fish. Thus, we hypothesised that carnitine supplementation can improve growth performance and reduce lipid content in the liver and muscle of yellow catfish (Pelteobagrus fulvidraco), a commonly cultured freshwater fish in inland China, and tested this hypothesis in the present study. Diets containing l-carnitine at three different concentrations of 47 mg/kg (control, without extra carnitine addition), 331 mg/kg (low carnitine) and 3495 mg/kg (high carnitine) diet were fed to yellow catfish for 8 weeks. The low-carnitine diet significantly improved weight gain (WG) and reduced the feed conversion ratio (FCR). In contrast, the high-carnitine diet did not affect WG and FCR. Compared with the control diet, the low-carnitine and high-carnitine diets increased lipid and carnitine contents in the liver and muscle. The increased lipid content in the liver could be attributed to the up-regulation of the mRNA levels of SREBP, PPARγ, fatty acid synthase (FAS) and ACCa and the increased activities of lipogenic enzymes (such as FAS, glucose-6-phosphate dehydrogenase, 6-phosphogluconate dehydrogenase and malic enzyme) and to the down-regulation of the mRNA levels of the lipolytic gene CPT1A. The increased lipid content in muscle could be attributed to the down-regulation of the mRNA levels of the lipolytic genes CPT1A and ATGL and the increased activity of lipoprotein lipase. In conclusion, in contrast to our hypothesis, dietary carnitine supplementation increased body lipid content in yellow catfish.

Failure of carnitine in improving hepatic nitrogen content in alcoholic and non-alcoholic malnourished rats

AIMS

To investigate the effect of carnitine supplementation on alcoholic malnourished rats' hepatic nitrogen content.

METHODS

Malnourished rats, on 50% protein-calorie restriction with free access to water (malnutrition group) and malnourished rats under the same conditions with free access to a 20% alcohol/water solution (alcohol group) were studied. After the undernourishment period (4 weeks with or without alcohol), both groups were randomly divided into two subgroups, one of them nutritionally recovered for 28 days with free access to a normal diet and water (recovery groups) and the other re-fed with free access to diet and water plus carnitine (0.1 g/g body weight/day by gavage) (carnitine groups). No alcohol intake was allowed during the recovery period.

RESULTS

The results showed: i) no difference between the alcohol/no alcohol groups, with or without carnitine, regarding body weight gain, diet consumption, urinary nitrogen excretion, plasma free fatty acids, lysine, methionine, and glycine. ii) Liver nitrogen content was highest in the carnitine recovery non-alcoholic group (from 1.7 to 3.3 g/100 g, P<0·05) and lowest in alcoholic animals (about 1.5 g/100g). iii) Hepatic fat content (~10 g/100 g, P>·05) was highest in the alcoholic animals.

CONCLUSION

Carnitine supplementation did not induce better nutritional recovery.

Effects of exercise on L-carnitine and lipid metabolism in African catfish (Clarias gariepinus) fed different dietary L-carnitine and lipid levels

African catfish (Clarias gariepinus) were fed four isonitrogenous diets (34 % crude protein), each containing one of two lipid (100 or 180 g/kg) and two L-carnitine (15 or 1000 mg/kg) levels. After 81 d of feeding, thirty-two fish (body weight 32 g) from each dietary group were randomly selected, sixteen fish were induced to a 3-h swim (speed of 1.5 body length (BL)/s), while the other sixteen fish were kept under resting condition. Fish fed 1000 mg L-carnitine accumulated 3.5 and 5 times more L-carnitine in plasma and muscle, respectively, than fish fed the 15 mg L-carnitine. Muscle L-carnitine content was significantly lower in exercised fish than in rested fish. High dietary lipid level (fish oil) led to an increase in muscle n-3 PUFA content and a decrease in SFA and MUFA content. In liver, the increase in dietary lipid level resulted in an increased levels of both n-6 and n-3 PUFA. L-carnitine supplementation significantly decreased n-3 PUFA content. Exercise decreased n-3 PUFA in both muscle and liver. Plasma lactate and lactate dehydrogenase, normally associated with increased glycolytic processes, were positively correlated with exercise and inversely correlated with dietary L-carnitine level. L-carnitine supplementation reduced significantly the RQ from 0.72 to 0.63, and an interaction between dietary L-carnitine and lipid was observed (P < 0.03). Our results indicate that an increase in fatty acids (FA) intake may promote FA oxidation, and both carnitine and exercise might influence the regulation of FA oxidation selectivity.