Researchers have long focused on the study and application of nutritional supplements, achieving significant advancements. In 1905, Russian scientists Gulewitsch and Kvimberg first extracted carnitine from muscles. The chemical structure of carnitine was determined in 1927, revealing that it consists of two isomers: L-carnitine and D-carnitine. Research has shown that only L-carnitine has nutritional benefits for animals. In 1985, the Chicago International Nutrition Conference recognized L-carnitine as an essential nutrient under specific conditions. Further research has revealed that supplementing L-carnitine can improve exercise fatigue and enhance athletic performance.
L-carnitine and its functions
- Chemical structure of L-carnitine
L-carnitine, also known as carnitine or vitamin BT, has a chemical name of B-hydroxy-Y-trimethylaminobutyric acid. Its molecular formula is C7H15NO3, and it is a white crystalline or transparent powdery substance. It has a molecular weight of 161.2 and is easily soluble in water and ethanol. L-carnitine exhibits good stability but is prone to hygroscopicity.
- Forms of L-carnitine supplementation
Generally, the L-carnitine reserve in adult males is about 20-25g, with 95% of it being present in skeletal muscles. Supplementing L-carnitine can increase both plasma-free L-carnitine and muscle carnitine levels. In laboratory studies involving human subjects, the dosage of L-carnitine supplementation typically ranges from 1 to 4g, but the optimal dosage and duration of intake are still unclear.
Due to the lively chemical nature of L-carnitine and its susceptibility to degradation, a stable form called L-carnitine tartrate is often used as a substitute for L-carnitine in experimental and clinical settings. Other forms of supplementation include its esterified forms such as acetyl-L-carnitine, propionyl-L-carnitine, and Glycine Propionyl-L-Carnitine (GPLC). Among them, GPLC is a muscle-specific form of carnitine that enhances endothelial function and promotes the maximal utilization of muscle energy. Additionally, a high-carbohydrate diet or insulin can increase the expression of carnitine transport protein (OCTN) mRNA, thereby increasing carnitine levels in skeletal muscles.
The effect of exercise on L-carnitine
Many studies have shown that exercise training has little impact on the total amount of L-carnitine. Generally, plasma-free carnitine levels decrease, while muscle carnitine content increases. Research has shown that plasma L-carnitine levels are lower in regularly trained soccer players compared to the general population. Exercise can increase the excretion of L-carnitine from 55mg/d to 94mg/d, primarily in the form of acylcarnitine, indicating the need for L-carnitine supplementation before and after high-intensity exercise. However, trained individuals have a greater ability to maintain free carnitine during acute exercise compared to untrained individuals.
The impact of L-carnitine supplementation on athletic performance
There are reports suggesting that patients with carnitine deficiency experience excessive lipid accumulation in skeletal muscles, particularly in type I muscle fibers. After exercise, there is an abnormal increase in blood lactate concentration, an elevated lactate/pyruvate ratio, and impaired mitochondrial respiratory chain function. However, most views suggest that under normal conditions, there is no need for additional supplementation of high doses of carnitine. However, supplementing a certain dose of carnitine during moderate-intensity exercise can effectively delay fatigue and improve both aerobic and anaerobic capacity. Studies have shown that L-carnitine supplementation is a long-term process, and a one-time supplementation of L-carnitine before exercise does not have an impact on athletic performance.
- The impact of L-carnitine supplementation on aerobic capacity
In recent years, several studies have found that supplementing L-carnitine can improve aerobic capacity in terms of anaerobic threshold, maximal oxygen uptake, lactate clearance ability, and cardiac function. It is effective for high-level athletes, and L-carnitine supplementation can also enhance exercise endurance under low-pressure and low-oxygen conditions, making it applicable to high-altitude training.
- The impact of L-carnitine supplementation on anaerobic capacity
Most reports suggest that supplementing L-carnitine can increase power output during anaerobic exercise. Long-term supplementation of L-carnitine can increase power output by 11% during high-intensity exercise (80% VO2max). Daily supplementation of 1.5g of GPLC for 28 days can improve maximum power output during exercise by 3% to 6%.
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