Theanine is an amino acid commonly found in tea (infusions of Camellia sinensis). Theanine is related to glutamine, and can cross the blood-brain barrier. Because it can enter the brain, theanine has psychoactive properties. Theanine has been shown to reduce mental and physical stress and may produce feelings of relaxation.
Theanine is speculated to produce these effects by increasing the level of gamma-aminobutyric acid (GABA) production. Theanine increases brain serotonin, dopamine, GABA levels and has micromolar affinities for AMPA, Kainate and NMDA receptors. It has also been found that injecting spontaneously hypertensive mice with theanine significantly lowered levels of 5-hydroxyindole in the brain. Researchers also speculate that it may inhibit glutamic acid excitotoxicity. Theanine also promotes alpha wave production in the brain.
Studies on test rats have shown that even repeated, extremely high doses of theanine cause little to no harmful psychological or physical effects.
L-theanine may help the body's immune system response when fighting infection by boosting the disease-fighting capacity of gamma delta T cells. The study, published in 2003 by the Brigham and Women's Hospital, included a four-week trial with 11 coffee drinkers and 10 tea drinkers, who consumed 600 milliliters of coffee or black tea daily. Blood sample analysis found that the production of anti-bacterial proteins was up to five times higher in the tea-drinkers, an indicator of a stronger immune response.
Recently, marketing campaigns from tea companies have been emphasizing the natural presence of L-theanine in their tea and that L-theanine boosts the production of alpha brainwaves.