Acetyl-L-carnitine or ALCAR, is an acetylated form of L-carnitine. It is a dietary supplement and naturally occurs in plants and animals.
Biochemical production and action
ALCAR is an acetylated derivative of L-carnitine. During strenuous exercise, a large portion of L-carnitine and unused acetyl-CoA are converted to ALCAR and CoA inside mitochondria by carnitine O-acetyltransferase. The ALCAR is transported outside the mitochondria where it converts back to the two constituents. The L-carnitine is cycled back into the mitochondria with acyl groups to facilitate fatty acid utilization, but excess acetyl-CoA may block it. Excess acetyl-CoA causes more carbohydrates to be used for energy at the expense of fatty acids. This occurs by different mechanisms inside and outside the mitochondria. ALCAR transport decreases acetyl-CoA inside the mitochondria, but increases it outside. Glucose metabolism increases with administration of either ALCAR or L-carnitine. A portion of L-carnitine is converted to ALCAR after ingestion in humans.
Absorption compared to L-carnitine
It has been claimed ALCAR is superior to L-carnitine in terms of bioavailability. Both use the same mechanism for intestinal absorption that improves with sodium. One study shows ALCAR has a lower blood concentration in humans after ingestion possibly because ALCAR is hydrolyzed more in blood. This means it has less bioavailability unless it is entering cells (e.g., brain or muscle) more efficiently from the blood than L-carnitine. L-carnitine is known to not absorb into cells unless there is an insulin spike such as from a carbohydrate load.
ALCAR has the ability to cross the blood–brain barrier and enter the brain, where it acts as a powerful antioxidant, which helps in prevention of the brain cells' deterioration. Its supplementation has been shown to be neuroprotective in instances of cerebral ischemia in rats and may be useful in treating peripheral nerve injury as well as spinal cord injury. It may have some neuroprotective benefit in the treatment of Parkinson's disease, but further research is required. ALCAR is also known to increase sperm motility, which describes the ability of sperm to move vigorously. Since motility is among the most important factors that help in the determination of sperm’s fertilization capability, acetyl-L-carnitine can help sperm cells move more actively, which consequently leads to the improved male fertility.
ALCAR has been shown to be more effective than tamoxifen in improving the curvature and reducing the pain and plaque sizes for men who sought treatment for their Peyronie's disease early and having low curvature deformities. ALCAR has also been shown to improve insulin response and is proved to have a positive effect on various muscle diseases as well as heart conditions.  
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