L-Theanine was discovered as a constituent of green tea in 1949 and was approved in Japan in 1964 for unlimited use in all foods, including chocolates, soft drinks, and herb teas, except infant foods. It also provides a unique umami (brothy or savory) taste and flavor to green tea infusion.^[4]

In 1950, the tea laboratory of Kyoto successfully separated theanine from gyokuro leaf, which has high theanine content.^{[citation needed]} Theanine is an analog to glutamine and glutamate, and can cross the blood–brain barrier.^[5] It is sold in the US as a dietary supplement, and is classified by the Food and Drug Administration (FDA) as a generally recognized as safe (GRAS) ingredient.^[6] However, the German Federal Institute for Risk Assessment (Bundesinstitut für Risikobewertung, BfR) has objected to the addition of isolated theanine to beverages.^[7]

Early studies of theanine involved much larger doses than are found in an everyday cup of tea. Researchers wonder whether drinking tea might have the same effects found in those studies.^[8] However, one recent study by Unilever found that smaller doses typical of those found in a cup of tea did induce changes in alpha waves as shown by EEG.^[9] Alpha waves occur in the brain and are associated with relaxation.

Effects on the brain[edit]

Able to cross the blood–brain barrier, theanine has psychoactive properties.^[10] Theanine has been studied for its potential ability to reduce mental and physical stress,^[11] improve cognition,^[12] and boost mood and cognitive performance in a synergistic manner with caffeine.^{[13][14][15][16][17][18]}

While structurally related to the excitatory neurotransmitter glutamate, theanine only has weak affinity for the glutamate receptor on postsynaptic cells.^[19] Rather, its primary effect seems to increase the overall level of the brain inhibitory transmitter GABA. It also increases brain dopamine levels and has a low affinity forAMPA, kainate, and NMDA receptors.^[20] Its effect on serotonin is still a matter of debate in the scientific community, with studies showing increases and decreases in brain serotonin levels using similar experimental protocols.^[5][21] It has also been found that injecting spontaneously hypertensive mice with theanine significantly lowered levels of 5-hydroxyindoles in the brain.^[22] Researchers also speculate it may inhibit glutamic acid excitotoxicity.^[20]Theanine also promotes alpha wave production in the brain.^[10]

Studies on test rats have shown even repeated, extremely high doses of theanine cause little to no harmful psychological or physical effects.^[23]Theanine showed neuroprotective effects in one rat study.^[24]

A placebo-controlled trial has shown adding theanine to ongoing antipsychotic medication is helpful in reducing some symptoms of schizophrenia.^[7]

Several beverage manufacturers are selling drinks containing theanine and are marketing them as drinks to help people focus and concentrate,^[8] while other manufacturers claim relaxing and tranquillizing properties.^[7]