L-theanine is an amino acid found most commonly in tea leaves and in small amounts in Bay Bolete mushrooms. It can be found in both green and black tea.
Theanine has a chemical structure very similar to glutamate, a naturally occurring amino acid in the body that helps transmit nerve impulses in the brain. Some of the effects of theanine appear to be similar to glutamate, and some effects seem to block glutamate.