Dietary L-Carnitine and energy and lipid metabolism in African catfish (Clarias gariepinus) juveniles

Summary

As the lipid content of the diet increases so does the requirement for certain components involved in lipid metabolism. Carnitine is a normal constituent of animal tissues and plasma, which is required for the transport of long-chain fatty acids (LCFAs) to the site of oxidation. To avoid accumulation of lipids, supplementation of dietary carnitine may be used to stimulate fatty acid oxidation and to regulate lipolysis. Previous studies have demonstrated that responses to carnitine administration were ambiguous. Many reports demonstrated that dietary carnitine supplements improved growth and body lipid oxidation, however, others studies did not observe such effects.

The aim of this study is to assess the effects of dietary L-Carnitine on lipid metabolism in African catfish ( Clarias gariepinus , Burchell).

The working hypotheses of this thesis were the following. Firstly, growth and fatty acid concentrations in fish are positively related to dietary carnitine levels. Carnitine supplementation is expected to increase fatty acid oxidation and as a result the protein:fat ratio in the fish body will increase. Secondly, one may expect that nutritional conditions that result in decreased carnitine synthesis (e.g. dietary lysine deficiency) will enhance the effect of dietary carnitine. Thirdly, the increased lipid oxidation resulting from the extra dietary carnitine may result in a protein-sparing effect (i.e. reduced amino acid oxidation). Fourthly, the carnitine effect will be more pronounced when dietary protein is in shortage, i.e. at a low dietary protein to fat ratio. Finally, extra dietary carnitine may improve the energetic status of the working muscle, thus protecting it against a sudden energy depletion often experienced by fish exposed to prolonged exercise.

The first step of this study aimed to determine whether dietary carnitine effects on African catfish performance is associated to fish size and/or dietary protein to non-protein ratios. Carnitine supplemented fish accumulated 4 times more carnitine in their tissues than fish fed a basal level. The smallest fish (< 60g) fed 660mg carnitine/kg diet showed significantly higher growth rates and lower feed conversion rates than 80mg carnitine supplemented fish, only when dietary PE:NPE ratio was low. Additionally, high dietary carnitine supplements increased body protein:fat ratios and decreased both total ammonia nitrogen (TAN) excretion and respiratory quotient (RQ) rates in fish < 80g.

In Chapter 3 and 4, our working hypothesis was that the effects of dietary carnitine supplements would be measurable only when fish were preconditionally fed to be metabolically incapable to synthesise carnitine endogenously. Therefore, we tested the effect of dietary carnitine and fat supplementation on growth and fatty acid concentrations in fish fed either with a low- (13g/kg diet) or a high-lysine (21g/kg diet) diet. Dietary lysine clearly affected the growth performance and feed conversion ratios, but dietary carnitine supplements had no effect. High-carnitine supplements, however, reduced the tissue free- to acyl-carnitine ratio and showed to increase polyunsaturated fatty acid (PUFAs) transport to the liver. Moreover, carnitine supplements raised the concentration of several amino acids (glutamic acid, aspartic acid, glycine, alanine, arginine, serine and threonine) in muscle tissue. Total amino acid concentration in muscle and liver tissues (dry-matter basis) increased from 506 to 564mg.g ^-1and from 138 to 166mg.g ^-1, respectively, when diets were offered with high-carnitine, low-lysine and low-fat levels.

We additionally tested if lipid metabolism in fish tissues (muscle and liver) would respond differently to dietary carnitine when dietary composition and the metabolic state of the fish change. In Chapter 5, we investigated the influence of dietary carnitine and dietary energy sources on lipid metabolism. Plasma carnitine level was increased (7.7 vs. 16.5mmol/ml) and plasma protein level decreased (36.0 v. 32.9 g/L) by high-carnitine supplements in diet. In addition, a decrease in plasma glucose (206.2 v. 61.4 mg d/L), plasma lactate dehydrogenase (944.3 v. 591 U /L) and plasma leptin (16.1 v. 5.2 ng/ml) were also observed in fish fed high-carnitine and high-fat diets.

At the end of the experimental phase of this study we investigated the interaction effect between diet composition and exercise on the energy state of the fish (Chapter 6). High-energy phosphates [adenosine triphosphate (ATP) and phosphocreatine (PCr) levels], adenilate energy charge index (AEC) and ammonia concentrations in white muscle were affected by the carnitine-fat-exercise interaction. Exercise caused a decrease in the PCr level, followed by a decrease in the ATP level (p<0.05). At the same time, a drastic increase in the iosine monophosphate (IMP) level was observed. Carnitine supplements showed to be effective to improve the recovery of the high-energy phosphates.

In conclusion, carnitine biosynthesis by the animal may be sufficient to maintain growth during standard husbandry conditions. Nevertheless, extra dietary carnitine altered the non-protein energy metabolism, body protein:fat rate, and decreased postprandial ammonia synthesis and ammonia excretion. These effects are conditioned by nutritional factors, such as the natural carnitine content and fat in the feeds. In addition, the presence of factors that modulate the action of dietary carnitine, including size-related metabolic differences, high metabolic state, and their interactions have been shown. In such cases, an extra dietary requirement for carnitine was observed.