L-Theanine is an amino acid that is not common in the diet and not one of the essential amino acids or even one of the common nonessential amino acids. It is found in meaningful amounts in tea, but otherwise supplementation is the main source of it. L-Theanine has structural similarity to glutamine and both neurotransmitters that are produced from it (GABA and glutamate). It is known to reach the brain and influence its function following oral ingestion.*