Theanine

Structure and properties

The chemical name N⁵-ethyl-L-glutamine and other synonyms (see box) for theanine reflect its chemical structure. The name theanine, without prefix, is generally understood to imply the L- (S-) enantiomer, derived from the related proteinogenic L-amino acid glutamic acid. Theanine is an analog of this amino acid, and its primary amide, L-glutamine (also a proteinogenic amino acid). Theanine is a derivative of glutamine that is ethylated on the amide nitrogen (as the name N⁵-ethyl-L-glutamine describes), or alternatively, to the amide formed from ethylamine and L-glutamic acid at its γ- (5-) side chain carboxylic acid group (as the name γ-L-glutamylethylamide describes). Relative to theanine, the opposite (D-, R-) enantiomer is largely absent from the literature, except implicitly. While natural extracts that are not harshly treated are presumed to contain only the biosynthetic L- enantiomeric form, mishandled isolates and racemic chemical preparations of theanines necessarily contain both theanine and its D-enantiomer (and from racemic syntheses, in equal proportion), and studies have suggested that the D-isomer may actually predominate in some commercial supplement preparations. Amino acid racemization in aqueous media is a well-established chemical process promoted by elevated temperature and non-neutral pH values; prolonged heating of Camellia extracts—possible for oversteeped teas and in undisclosed commercial preparative processes—has been reported to result in increasing racemization of theanine to give increasing proportions of the nonnatural D-theanine, up to equal proportions of each enantiomer.

Discovery and distribution

Theanine is found primarily in plant and fungal species. It was discovered as a constituent of green tea (Camellia sinensis) in 1949 and in 1950, a laboratory in Kyoto successfully isolated it from gyokuro leaf, which has high theanine content. Theanine is substantially present in black, green, and white teas from Camellia sinensis in quantities of about 1% of the dry weight. The L-enantiomer is the form found in freshly prepared teas and some, but not all, human dietary supplements.

Digestion and metabolism

As a structural analog of glutamate and glutamine, the theanine in preparations (teas, pure supplements, etc.) is absorbed in the small intestine after oral ingestion; its hydrolysis to L-glutamate and ethylamine occur both in the intestine and liver. It can also cross the blood–brain barrier intact, and register pharmacological effects directly.

Pharmacodynamics

Theanine is structurally similar to the excitatory neurotransmitter glutamate, and in accordance, binds to glutamate receptors, though with much lower affinity in comparison. Specifically, it binds to ionotropic glutamate receptors in the micromolar range, including the AMPA and kainate receptors and, to a lesser extent, the NMDA receptor. It acts as an antagonist of the former two sites and as an agonist of the latter site. Theanine also binds to group I mGluRs. In addition, it inhibits glutamine transporters and glutamate transporters, and thus blocks the reuptake of glutamine and glutamate. Lastly, theanine elicits umami taste, and this effect has been found to be a consequence of the fact that it directly binds to and activates the T1R1 + T1R3 heterodimer or umami (savory) taste receptor.

Theanine increases serotonin, dopamine, GABA, and glycine levels in various areas of the brain, as well as BDNF and NGF levels in certain brain areas. However, 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. It has also been found that injecting spontaneously hypertensive mice with theanine significantly lowered levels of 5-hydroxyindoles in the brain. Researchers also speculate that it may inhibit glutamate excitotoxicity.

Effects

Able to cross the blood–brain barrier, theanine has reported psychoactive properties. Theanine has been studied for its potential ability to reduce mental and physical stress, improve cognition, and boost mood and cognitive performance in a synergistic manner with caffeine.

A Natural Standard monograph that reviews current research on theanine reports that it is likely safe in doses of 200–250 mg up to a maximum daily dose of 1,200 mg. Theanine is used to help with anxiety, blood pressure control, mood, and cognition. Natural Standard rates the evidence to support the usage for anxiety, blood pressure control, and mood as “unclear or conflicting scientific evidence” and the evidence for cognition as “fair negative scientific evidence.” Many of the studies of theanine were done in combination with caffeine as found in tea. While the studies found that the combination had some effect on mood, the studies found that theanine alone had little effect. More sufficiently designed studies in humans are warranted to further our understanding of the effects of theanine.

Theanine is reported to promote alpha wave production in the brain. Early studies of theanine involved much larger doses than those found in a typical cup of tea. Researchers wonder whether drinking tea might have the same effects found in those studies. However, one recent study funded 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.

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

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

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

Supplement use

In 2003, the German Federal Institute for Risk Assessment (Bundesinstitut für Risikobewertung, BfR) objected to the addition of isolated theanine to beverages. The institute stated the amount of theanine consumed by regular drinkers of tea or coffee is virtually impossible to determine. While it was estimated the quantity of green tea consumed by the average Japanese tea drinker per day contains about 20 mg of the substance, there are no studies measuring the amount of theanine being extracted by typical preparation methods, or the percentage lost by discarding the first infusion. Therefore, with the Japanese being exposed to possibly much less than 20 mg per day, and Europeans presumably even less, it was the opinion of the BfR that pharmacological reactions to drinks typically containing 50 mg of theanine per 500 milliliters could not be excluded—reactions such as impairment of psychomotor skills and amplification of the sedating effects of alcohol and hypnotics.

In 2006, a study found no consistent, statistically significant treatment-related adverse effects on behavior, morbidity, mortality, body weight, food consumption and efficiency, clinical chemistry, hematology, or urinalysis in rats fed high doses of theanine for 13 weeks. Large studies in humans have not been undertaken; however, several smaller-scale studies (fewer than 100 participants) have shown increased alpha wave generation and lowered anxiety, along with benefits to sleep quality in people with ADHD.

The combination of theanine and caffeine has been shown to promote faster simple reaction time, faster numeric working memory reaction time and improved sentence verification accuracy. Due to this, the combination has become popular over recent years as a nootropic supplement typically found in the form of a pill, drink, or gum.

Theanine may help the body's immune response to infection by boosting the disease-fighting capacity of gamma delta T cells. The study, published in 2003, 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 the production of antibacterial proteins was up to five times higher in the tea drinkers, an indicator of a stronger immune response. L-Theanine may contain mast cell-stabilizing properties in an animal model.

Theanine has been reported to raise levels of brain serotonin, dopamine, and GABA, with possible improvement in specific memory and learning tasks.

In tea brewing

A research study in 2011 showed that regular brewed black tea contains significantly more L-theanine per cup when compared to commercially available green tea (24 mg versus 8 mg per cup). The study demonstrates that brewing time is the major factor in L-theanine extraction from tea brewing. Addition of sugar and small quantities of milk seem to make no significant difference.