Carnitine Metabolism and Human Nutrition

L-Carnitine - Is It Good or Bad for your Heart?
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Thiamin therapy for chronic heart failure: is there any future for this vitamin? Choline and phosphatidylcholine may maintain cognitive performance by multiple mechanisms. However, carnitine supplementation had no effects on lipid metabolism in African catfish 23 , hybrid tilapia 19 and hybrid striped bass Indeed, carnitine even reduced lipid metabolism in red sea bream 15 , yellow catfish 24 and rainbow trout The contradictory and complicated results of carnitine supplementation in fish have made the further utilization of carnitine in fish feed controversial.

This has largely restricted our understanding of the function of carnitine in fish, and an explanation of the conflicting results of the application of carnitine in fish remains elusive. Therefore, the mechanisms of the effects of carnitine in fish should be studied. As a widespread model animal in research areas such as development, molecular genetics, toxicology and biomedicine 26 , 27 , zebrafish Danio rerio has been used in mechanistic studies of fish nutrition 28 , In the present study, zebrafish was used to evaluate the systemic regulation of lipid, protein and carbohydrate metabolism by L-carnitine in fish in feeding and fasting states, respectively.

Moreover, the metabolic pathways associated with the nutritional regulation by dietary L-carnitine supplementation in zebrafish were determined. To the best of our knowledge, this is the first study to demonstrate the metabolic mechanisms of nutritional regulation by dietary L-carnitine supplementation in fish. During the experiment, the fish were in good health and showed no growth differences compared with control fish data not shown.

To test the effects of exogenous carnitine supplementation on the endogenous carnitine concentrations, the carnitine concentrations in various fish tissues were measured. In the liver and muscle, both in the feeding and fasting states, the free carnitine and total carnitine concentrations in the carnitine supplementation group were significantly higher than in the control group Fig.

The mRNA levels of BBOX1 , the key enzyme for carnitine synthesis, were not significantly different in the liver and in fasting state muscle; however, in feeding state muscle, the BBOX1 mRNA level in the carnitine supplementation group was significantly higher than that in the control group Fig. No significant difference was found in the whole fish lipid level in all groups Fig. Compared with the control group, the triglyceride TG content was significantly decreased in the carnitine supplementation group in the liver and muscle in both the feeding and fasting states; however, the TG content in the viscera was comparable in all groups Fig.

There were no interaction effects of the nutritional states and L-carnitine found in the whole fish lipid level and TG content in the different organs, except in the BBOX1 mRNA level Supplemental Table 1 , showing BBOX1 is simultaneously regulated by nutritional state and L-carnitine supplementation. The above data indicated that dietary carnitine supplementation could increase the in vivo carnitine concentration and reduce the TG content in the liver and muscle.

Carnitine Metabolism and Human Nutrition - CRC Press Book

As shown in Fig. Figure 3 shows the effects of dietary L-carnitine on the mRNA expressions of genes related to lipid metabolism. The mRNA level of LPL increased in the carnitine supplementation group in the liver in the feeding state but decreased in the muscle in the fasting state Fig.

The whole fish glycogen levels remained invariant in feeding states, but increased in fasting state Fig. However, the genes related to glucose metabolism were affected by dietary L-carnitine supplementation. The mRNA level of PFK , a key glycolysis-related gene, was significantly decreased fasting state and increased feeding state in the muscle, but was comparable in the liver Fig. The mRNA level of PK , another key glycolysis-related gene, was decreased in the muscle, and the decrease was significant in the feeding state Fig.

The mRNA expressions of insulin sensitivity-related genes, including insulin, Ira and Irb , were mostly unaffected by dietary L-carnitine supplementation Fig. In general, the above data suggested that dietary L-carnitine supplementation tended to increase gluconeogenesis, but decreased glycolysis; therefore, L-carnitine supplementation lowered the utilization of glucose.

The mRNA expressions of aminopeptidase N APN and oligopeptide transporter PEPT1 , which function in protein and amino acid digestion and absorption, and glutamate dehydrogenase 1a and 1b GDH1a, GDH1b , which are essential in amino acid catabolism, were not affected significantly by dietary L-carnitine supplementation in the liver and muscle Fig. The mRNA level of asparagine synthetase ASNS , which plays roles in protein synthesis, was significantly decreased in the L-carnitine group in the fasting state, both the liver and muscle Fig.

Notably, in the L-carnitine supplementation group, the mRNA expression of mTOR , the regulatory factor for protein synthesis, was significantly increased in the liver in the feeding state Fig. These data suggested that dietary L-carnitine supplementation tended to increase protein degradation by inhibiting protein synthesis in the fasting state. However, the increased crude protein levels in whole fish and the increased mTOR expression in the liver of carnitine-fed zebrafish also showed that L-carnitine has positive effects on protein synthesis in the normal feeding period.

The expressions of three inflammation-related genes were measured to investigate the potential effects of dietary L-carnitine on inflammation. These data indicated that dietary L-carnitine is likely to play roles in inflammation processes. However, the effects of dietary carnitine supplementation in different fish species are contradictory 31 , but few previous studies performed comprehensive assays of lipid metabolism.

In the present study, although growth promotion was not observed in zebrafish, dietary L-carnitine supplementation decreased the TG content in liver and muscle significantly, but did not affect the level in the viscera. Indeed, the carnitine concentration in the liver and muscle of the experimental zebrafish increased with dietary L-carnitine supplementation. This is the first report of this anti-inflammatory effect of L-carnitine in fish.

Lipid accumulation-related metabolic dysfunctions have been observed widely in aquatic animals; therefore, the anti-inflammatory effects of L-carnitine in fish require further study. Compared with mammals, fish have lower abilities to use carbohydrates as energy sources, thus fish cannot use glucose to produce energy efficiently A number of studies have indicated that, as compared with mammals, the responses of the activities of many glucose metabolism-related key enzymes, such as GK, PFK and PK, are not sensitive to the increased dietary carbohydrate content in fish 36 , In many fish species, high dietary carbohydrate induced lower growth, excess lipid deposition, and decreased stress resistance 38 , 39 , However, in many fish species, these efforts are not ideal, and the regulatory mechanisms of carbohydrate metabolism are still poorly understood in fish.

Recently, the interaction between lipid metabolism and carbohydrate metabolism has been observed in fish 47 , The phenomena that high-fat diet impairs glucose homeostasis has been reported in rainbow trout 49 , Our recent work further illustrated that glycolysis-related genes are upregulated or downregulated during low-fat diet or high-fat diet feeding, respectively In the present study, dietary L-carnitine supplementation improved lipid catabolism significantly, but increased the whole body glycogen deposition in the fasting state, indicating that glucose degradation was inhibited.

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Similarly, dietary L-carnitine supplementation also increased the pyruvate carboxylase PC activity and glucose production in the liver in Atlantic salmon This evidence indicated that in fish, increasing the catabolism of lipids decreases the energy portion sourced from glucose degradation, resulting in increased gluconeogenesis and glycogen synthesis. This could be explained if, during energy homeostasis in fish, the endogenous acetyl-CoA concentration is relatively stable; therefore, if the increased lipid catabolism produces more acetyl-CoA for energy production, the portion of the acetyl-CoA sourced from other nutrients, such as glucose, would decrease correspondingly.

Thus, the glycolysis pathway would be downregulated and gluconeogenesis would be upregulated. The relationships of the metabolic pathways are illustrated in Fig. A The regulation of L-carnitine in feeding state. B The regulation of L-carnitine in fasting state. ACS, acyl-CoA synthetase. However, the underlying molecular basis remained unknown. In the present study, dietary L-carnitine supplementation increased the protein content of the whole body in the feeding trial, and did not affect the mRNA expressions of genes associated with protein and amino acid catabolism.

Health & Nutrition : Acetyl-L-Carnitine Side Effects

The mTOR protein is one of the most important regulatory elements of protein synthesis 53 ; therefore, dietary supplementation with L-carnitine might have positive effects on protein synthesis in zebrafish. In Atlantic salmon, dietary L-carnitine supplementation increased the amino acid concentration in plasma and the liver, especially the three branched-chain amino acids, including leucine, isoleucine, and valine, and increased the protein synthesis capacity, accompanied by the accumulation of protein in organs The studies in mammals and rainbow trout reported that leucine can effectively stimulate mTOR signalling 54 , Thus, we deduced that dietary L-carnitine supplementation can stimulate the expression of mTOR in fish by increasing the leucine concentration in the feeding state.

The specific mechanism of the protein sparing effect of L-carnitine is detailed in Fig. However, the present study also indicated that dietary L-carnitine supplementation significantly decreased the whole body protein content in the fasting state, and the gene expressions of ASNS and mTOR in the L-carnitine groups were also downregulated in the muscle in the fasting state. This might be explained if L-carnitine accelerated the degradation of lipids and proteins in the fasting state, and inhibited the synthesis of lipids and proteins to larger degree compared with the control.

fotodialogue.com/map5.php Compared with the whole body glycogen content, which was increased in the L-carnitine supplementation group in the fasting state, the decreased lipid and protein content in L-carnitine supplementation group in the fasting state confirmed that fish prefer to utilize proteins and lipids rather than carbohydrates. To illustrate the systemic regulation of nutritional metabolism by L-carnitine in zebrafish, the interaction of different metabolic pathways in the L-carnitine-fed fish is shown in Fig.

Meanwhile, dietary L-carnitine supplementation also inhibited glycolysis and enhanced gluconeogenesis to decrease glucose-derived energy production Fig. In the fasting state, dietary L-carnitine supplementation had similar effects on lipid and glucose metabolism to the feeding state, but also increased protein degradation and reduced protein synthesis Fig. Nevertheless, our results also indicated that the interaction effects between nutritional state and L-carnitine supplementation existed in many parameters, including nutrient compositions, biochemical activities and metabolism-related gene expressions.

This indicates that the regulatory mechanisms of L-carnitine in feeding or fasting states as shown in Fig. Furthermore, it is of note that the changes of gene mRNA expression do not directly reflect the enzyme activities or protein functions, therefore, the regulatory mechanisms of L-carnitine at the transcriptional level still need further functional validation. Dietary L-carnitine supplementation also increased the expression of mTOR in the liver, suggesting that it has positive roles in protein synthesis.

However, dietary L-carnitine supplementation decreased glycolysis, because the increase in lipid-sourced ATP from L-carnitine supplementation might change the balance of energy homeostasis between lipids and carbohydrates. This is the first study to explore the mechanism of the positive effect of L-carnitine on the nutritional metabolism at transcriptional and biochemical levels in fish.

In order to avoid the metabolic disturbance of estrogen during the sexual maturation of female fish, only male zebrafish 0. After acclimation, six hundred zebrafish were randomly divided into 2 groups 3 tanks per group, fish per tank : control group and carnitine group Fig. In the feeding trial, the carnitine was given by feeding fish with small wheat flour-dough particles containing L-carnitine carnitine group or not control group , before basic diet feeding.

In the preparation of the L-carnitine-contained wheat flour-dough particles, L-carnitine was first dissolved in pure water, and the carnitine solution was mixed with given amount of wheat flour to make wet dough. The formulations of the basal diet and wheat flour-dough particle are listed in Table 1.

The basal diet and dough particles were pelleted to a proper size about 0. In the two feeding groups, the feeding strategy was the same as that in the previous 6 weeks. In the two fasting groups, only dough particles were fed in the morning and the amount was decreased to 0.

Carnitine metabolism and human nutrition

Because the physiological effects of dietary L-carnitine supplementation were normally observed after 6—8 weeks in other animals 24 , 57 , the duration of the present experiment was 7 weeks. The experimental design was shown in Fig. The weight of the fish in each tank was recorded every one week, and the feeding amount was adjusted correspondingly.

At the end of the experiment, the whole fish in each tank were anthesthetized on ice, and sampled to collect liver, muscle and visceral for molecular and biochemical indexes. Hepatic, muscle and visceral triglyceride TG and whole fish glycogen were assessed by the commercial kit Jiancheng Biotech Co. The crude lipid of the whole fish body was tested by using methanol and chloroform as previously described Because of the little mass of zebrafish organs e.

Total carnitine was defined as the concentration of carnitine in sample A, free carnitine was defined as the concentration of carnitine in sample B. The data was acquired and analyzed using MassHunter software version 5. The total run time for sample analysis was 4. After the feeding trial, the whole liver and parts of muscle of 3 fish collected from each group were weighted and homogenized in the ice-cold 0. After 0. The pure radioactive ASP medium was collected using 0.

The melting curves of amplified products were generated to ensure the specificity of assays at the end of each PCR. Two-way ANOVA analysis was used to explore the possible interactions existing between nutritional states and L-carnitine supplementation in all parameters. How to cite this article : Li, J.

Systemic regulation of L-carnitine in nutritional metabolism in zebrafish, Danio rerio. Publisher's note: Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

Book Description

Summary. Carnitine Metabolism and Human Nutrition offers a contemporary and in-depth look at the biological effects of carnitine metabolism and its application. Carnitine Metabolism and Human Nutrition offers a contemporary and in-depth look at the biological effects of carnitine metabolism and its application to clinical .

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