What is L-Carnitine? You’ve probably heard the L-carnitine is a fat-burner and can assist in performance, but probably unaware of the science behind the amino acid. Firstly an amino acid is the building block of proteins. L-carnitine is an amino acid that is naturally produced in the body within the liver and kidneys. Although L-carnitine is primarily stored within the muscle, including the brain. Now let’s deep dive into the questions of what is l-carnitine and how it works in your body.