What is L Carnitine Fumarate L-carnitine fumarate is a stable form of L-carnitine. L-carnitine is a compound derived from an amino acid that is involved in metabolism in most mammals and plants. It is produced naturally by the body and exists in two biologically active isomers, L-carnitine and D-carnitine, which have drastically different effects on the body. L-carnitine (not to be confused with D-carnitine) is sometimes referred to as simply "carnitine." L-Carnitine Fumarate Benefits L-carnitine fumarate is commonly used by bodybuilders as part of a preworkout regimen to help boost energy and aid exercise recovery. It also helps support heart health and brain function.*