Amazon.com: Force Factor L Carnitine Supplement, Liquid L Carnitine 3000 mg to Help Turn Fat Into Energy, Support Muscle Recovery, and Boost Cellular Energy, Maximum Strength, Non GMO, Orange Flavor, 16 Oz. : Health L Carnitine Plus Power Blendz Carnitine supplementation improves metabolic flexibility and skeletal muscle acetylcarnitine formation in volunteers with impaired glucose tolerance: A randomised controlled trial eBioMedicine 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.