L-tryptophan is an essential amino acid, meaning that the body cannot produce it and must obtain it from the diet. It is a precursor to the neurotransmitter serotonin, which plays a crucial role in regulating mood, sleep, and appetite.L-Tryptophan is an essential amino acid for making protein. It is found naturally in red meat, poultry, eggs, and dairy products. L-Tryptophan is important for many organs in the body. L-Tryptophan is not made by the body and must be obtained from the diet.