L-carnitine is a natural amino acid derivative that helps the body turn fat into energy. It moves fatty acids into cell power plants called mitochondria. Your body makes it, and you can also find it in foods or take it as a pill or liquid. People use it to boost workout performance and help muscles recover, though it is not a magic fix for weight loss.Medical uses: Doctors prescribe it for real carnitine deficiencies and sometimes for heart or kidney conditions.