Acetylcarnitine is broken down in the blood by plasma esterases to carnitine which is used by the body to transport fatty acids into the mitochondria for breakdown and energy production.

Carnitine is both a nutrient and made by the body as needed; it serves as a substrate for important reactions in which it accepts and gives up an acyl group.

[2] Within cells, carnitine plays a key role in importing acyl-CoA into mitochondria; the acyl-group of the acyl-CoA is transferred to carnitine, and the acyl-carnitine is imported through both mitochondrial membranes before being transferred to a CoA molecule, which is then beta oxidized to acetyl-CoA.

A separate set of enzymes and transporters also plays a buffering role by eliminating acetyl-CoA from inside mitochondria created by the pyruvate dehydrogenase complex that is in excess of its utilization by the Krebs cycle; carnitine accepts the acetyl moiety and becomes ALCAR, which is then transported out of the mitochondria and into the cytosol, leaving free CoA inside the mitochondria ready to accept new import of fatty acid chains.

[3] ALCAR in the cytosol can also form a pool of acetyl-groups for CoA, should the cell need it.