L-Carnitine is an endogenous compound that can be synthesized in the liver and kidney from the essential amino acids lysine and methionine or ingested through diet. L-Carnitine plays a critical role in energy production and in fat metabolism through its function as a transporter of long-chain fatty acids into mitochondria for β-oxidation. In cells, it helps transport fatty acids into the mitochondria, where they can be burned for energy. Primary sources of dietary Carnitine are red meat and dairy products. L-Carnitine is stored primary in skeletal muscle (98%), but also in a much lower concentration found in plasma. L-Carnitine plays a crucial role in exercise and among athletes [1]. Human muscle contains a high concentration of carnitine. The first well-documented function of carnitine was found to be facilitation of the transport of long-chain fatty acids into the matrix of the mitochondrion, which is the site for beta-oxidation. Fatty acids are an important fuel for muscle metabolism. During endurance exercise, the oxidation of fatty acids spares the use of muscle glycogen and delays the onset of fatigue. Muscle tissue cannot synthesize carnitine and thus is dependent on the transport of carnitine from the bloodstream. Blood carnitine is derived from endogenous biosynthesis in the liver and kidney and from exogenous sources in the diet.

There is a decrease in both plasma and muscle L-Carnitine levels during high-intensity exercises. Therefore, it is suggested that supplemental L-Carnitine will greatly reduce potential effects of this decrease and to retain high muscle and plasma Carnitine levels [7].

Acetyl-L-Carnitine, or ALCAR, is L-Carnitine to which an acetyl group (-COCH3) has been added, and is thought to be the most bioavailable form of L-Carnitine. Acetyl-L-Carnitine is an amino acid the body uses to turn fat into energy [1]. Acetylcarnitine is a high-energy compound, the acetyl units stored as acetylcarnitine during heavy exercise would be available if the exercise intensity decreased. Also, the acetyl unit would not require metabolic energy to re-form acetyl-CoA [10].

A few studies demonstrated the effects of L-Carnitine in endurance athletes [1]. During low to moderate exercise, long chain fatty acids represent the main source of energy. L-Carnitine has been suggested to spare muscle glycogen and promote fat oxidation during exercise, and a proposed conversion of fat into energy seems likely to be reflected by a reduction in body weight. In addition, L-Carnitine supplementation has been shown to spare amino acid usage as a source of energy making them available potentially for new protein synthesis [1, 2]. L-Carnitine reduces the hazardous effects of hypoxic exercises and plays an important role in recovery from damage caused by exercise [7]. Study indicates that L-Carnitine supplementation improves exercise endurance under normobaric/normoxic and hypobaric hypoxic conditions [9]. L-Carnitine may enhance blood flow and oxygen supply to the muscle tissue via improved endothelial function, thereby reducing hypoxia-induced cellular and biochemical disruptions and, therefore, improving muscle recovery [11].

L-Carnitine crosses blood brain barrier where ALCAR is metabolized for energy and incorporated into the carbon skeleton of the neurotransmitters GABA which is key for brain focus and relaxation [3]. Study refer that acetyl moiety from ALCAR can be oxidized for energy, serve as a precursor for acetylcholine, and be incorporated into amino acid neurotransmitters and lipids in brain [4, 5]. Acetyl-L-Carnitine alters brain energy metabolism and increases noradrenaline and serotonin content. [6]

References:

1. Grivas, Gerasimos V. "The role of L-carnitine in distance athletes." Int. J. Sports Sci 8 (2018): 158-163.

2. Fielding, Roger, et al. "L-carnitine supplementation in recovery after exercise." Nutrients 10.3 (2018): 349.

3. Ferreira GC, McKenna MC. L-Carnitine and Acetyl-L-carnitine Roles and Neuroprotection in Developing Brain. Neurochem Res. 2017 Jun;42(6):1661-1675. doi: 10.1007/s11064-017-2288-7. Epub 2017 May 16. PMID: 28508995; PMCID: PMC5621476.

4. Scafidi et.al., in Ferreira GC, McKenna MC. L-Carnitine and Acetyl-L-carnitine Roles and Neuroprotection in Developing Brain. Neurochem Res. 2017 Jun;42(6):1661-1675. doi: 10.1007/s11064-017-2288-7. Epub 2017 May 16. PMID: 28508995; PMCID: PMC5621476.

5. Ricciolini et al., in Ferreira GC, McKenna MC. L-Carnitine and Acetyl-L-carnitine Roles and Neuroprotection in Developing Brain. Neurochem Res. 2017 Jun;42(6):1661-1675. doi: 10.1007/s11064-017-2288-7. Epub 2017 May 16. PMID: 28508995; PMCID: PMC5621476.

6. Smeland OB, Meisingset TW, Borges K, Sonnewald U. Chronic acetyl-L-carnitine alters brain energy metabolism and increases noradrenaline and serotonin content in healthy mice. Neurochem Int. 2012 Jul;61(1):100-7. doi: 10.1016/j.neuint.2012.04.008. Epub 2012 Apr 23. PMID: 22549035.

7. Orer, Gamze E., and Nevin A. Guzel. "The effects of acute L-carnitine supplementation on endurance performance of athletes." The Journal of Strength & Conditioning Research 28.2 (2014): 514-519.

9. Panjwani, Usha, et al. "Effect of L-carnitine supplementation on endurance exercise in normobaric/normoxic and hypobaric/hypoxic conditions." Wilderness & environmental medicine 18.3 (2007): 169-176.

10. Institute of Medicine (US) Committee on Military Nutrition Research; Marriott BM, editor. Food Components to Enhance Performance: An Evaluation of Potential Performance-Enhancing Food Components for Operational Rations. Washington (DC): National Academies Press (US); 1994. 21, The Role of Carnitine in Enhancing Physical Performance.

11. Wutzke, Klaus D., and Henrik Lorenz. "The effect of l-carnitine on fat oxidation, protein turnover, and body composition in slightly overweight subjects." Metabolism 53.8 (2004): 1002-1006.