The Role of l Carnitine in Mitochondria, Prevention of Metabolic Inflexibility and Disease Initiation Vitacost Acetyl L Carnitine HCl 500 mg 300 Capsules Vitacost L carnitine is a naturally occurring amino acid structure that is made in the human brain, liver, and kidneys. L carnitine plays a critical role in energy production, as it converts fat into energy. Carnitine supplementation improves metabolic flexibility and skeletal muscle acetylcarnitine formation in volunteers with impaired glucose tolerance: A randomised controlled trial eBioMedicine