Natural Factors L-Tryptophan - A free-form amino acid used as a workout supplement to support muscle repair. L-tryptophan is one of nine essential amino acids, meaning that it cannot be synthesized by the body and must come from the diet. In addition to a healthy, balanced diet and regular resistance training, supplementing with L-tryptophan has been shown to help build lean muscle mass. L-tryptophan is a fundamental component of proteins and has numerous critical roles throughout the body, such as building and repairing muscle and other tissues, maintaining a healthy immune system, producing hormones, and supplying energy. L-tryptophan is also used to synthesize the neurotransmitter serotonin and the hormone melatonin in the brain, which play important roles in mood and sleep.