Acetyl-L-carnitine or ALCAR, is an acetylated form of L-carnitine. It is a dietary supplement and naturally occurs in plants and animals.

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.