Abstract
l-Carnitine is an amino acid derivative widely known for its involvement in the transport of long-chain fatty acids into the mitochondrial matrix, where fatty acid oxidation occurs. Moreover, l-Carnitine protects the cell from acyl-CoA accretion through the generation of acylcarnitines. Circulating carnitine is mainly supplied by animal-based food products and to a lesser extent by endogenous biosynthesis in the liver and kidney. Human muscle contains high amounts of carnitine but it depends on the uptake of this compound from the bloodstream, due to muscle inability to synthesize carnitine. Mitochondrial fatty acid oxidation represents an important energy source for muscle metabolism particularly during physical exercise. However, especially during high-intensity exercise, this process seems to be limited by the mitochondrial availability of free l-carnitine. Hence, fatty acid oxidation rapidly declines, increasing exercise intensity from moderate to high. Considering the important role of fatty acids in muscle bioenergetics, and the limiting effect of free carnitine in fatty acid oxidation during endurance exercise, l-carnitine supplementation has been hypothesized to improve exercise performance. So far, the question of the role of l-carnitine supplementation on muscle performance has not definitively been clarified. Differences in exercise intensity, training or conditioning of the subjects, amount of l-carnitine administered, route and timing of administration relative to the exercise led to different experimental results. In this review, we will describe the role of l-carnitine in muscle energetics and the main causes that led to conflicting data on the use of l-carnitine as a supplement.
Highlights
Carnitine (3-hydroxy-4-N-trimethylaminobutyrate) represents an amino acid derivative and a micronutrient that plays a key role in intermediary metabolism with the main function being the transport of long-chain fatty acids from the cytosol to the mitochondrial matrix where fatty acid β-oxidation occurs
Carnitine is mainly found in a free form and in the form of acylcarnitine esters, a pool of carnitine bounded to various acyl groups that are delivered throughout the body for a wide range of functions [4]
The raised availability of carbohydrate during exercise increases glycolysis and pyruvate flux through the pyruvate dehydrogenase with a corresponding decrease in the rate of fat oxidation. This observation should lead to paying attention to the use of carnitine as a supplement to increase the burning of fatty acids during high-intensity exercise, in particular when carnitine administration is preceded by a load of carbohydrates before exercise
Summary
Carnitine (3-hydroxy-4-N-trimethylaminobutyrate) represents an amino acid derivative and a micronutrient that plays a key role in intermediary metabolism with the main function being the transport of long-chain fatty acids from the cytosol to the mitochondrial matrix where fatty acid β-oxidation occurs. The skeletal muscle carnitine pool is distributed as Molecules 2020, 25, 182; doi:10.3390/molecules25010182 www.mdpi.com/journal/molecules. It has been estimated that the total carnitine content in the human body is about 300 mg/kg, with about 95% stored intracellularly in the heart and skeletal muscle, and the remaining part in the liver, kidney, and plasma [6]. In light of the fundamental role of carnitine in fatty acids β-oxidation for energy production, studies have been conducted to understand whether carnitine supplementation can affect skeletal muscle function and athletic performance in healthy individuals [2,9]. We will summarize the main roles of carnitine in the skeletal muscle energetics and the principal pharmacokinetic characteristics of carnitine in order to highlight the main critical points for carnitine supplementation during exercise
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