L-theanine is a non-protein amino acid mainly found naturally in the green tea plant (Camellia sinensis). L-theanine is the predominant amino acid in green tea and makes up 50% of the total free amino acids in the plant. The amino acid constitutes between 1% and 2% of the dry weight of green tea leaves. L-theanine is considered the main component responsible for the taste of green tea, which in Japanese is called umami. L-theanine is marketed in Japan as a nutritional supplement for mood modulation.

L-theanine is a derivative of L-glutamic acid. It is a water-soluble solid substance with the molecular formula C7H14O3 N and a molecular weight of 160.19 daltons. L-theanine is also known as gamma-ethylamino-L-glutamic acid, gamma-glutamylethylamide, r-glutamylethylamide, L-glutamic acid gamma-ethylamide and L-N-ethylglutamine. L-glutamic acid is a powerful brain stimulant, but theanine has the opposite neurological effect. Most people report that theanine relaxes without causing drowsiness. In animal experiments it also protects brain cells from death by glutamic-acid poisoning. We often encounter glutamic acid as MSG, the flavor enhancer that is routinely used in Asian foods and blamed for a variety of bad reactions to these foods.

L-theanine is a unique free form amino acid found only in the tea plant and in the mushrooms Xerocomus badius and certain species of genus Camellia, C. japonica and C. sasanqua. L-theanine is a relaxant that increases alpha-waves producing mental and physical relaxation decreasing stress and anxiety, without inducing drowsiness.

Theanine is the major amino acid in tea, comprising about half the total amino-acid content. It is a unique amino acid found only in tea and in a few mushrooms. Green tea contains a much higher concentration of theanine than other teas. A heavy green tea drinker (six to eight cups daily) will ingest between 200 to 400 milligrams of L-theanine daily. Theanine is a unique amino acid found in the leaves of green tea (Camellia sinensis). Theanine is quite different from the polyphenol/ and catechin antioxidants for which green tea is typically consumed. In fact, through the natural production of polyphenols, the tea plant converts theanine into catechins. This means that tea leaves harvested during one part of the growing season may be high in catechins (good for antioxidant benefits), while leaves harvested during another time of year may be higher in theanine (good for anti-stress and cortisol-controlling effects). Three to four cups of green tea are expected to contain 100-200 mg of theanine.

All teas have some caffeine, even decaffeinated teas, but even regular green tea has only about one-third the caffeine of black tea. Because caffeine is chemically an "alkaloid" while theanine is an amino acid, the decaffeination process does not remove theanine. In addition, the ability of theanine to block caffeine's stimulation is much greater than caffeine's ability to stimulate, so 80 milligrams of theanine in a cup of green tea more than offsets the 10 milligrams of caffeine.

The unique aspect of theanine is that it acts as a non-sedating relaxant to help increase the brain's production of alpha-waves (those associated with "relaxed alertness"). This makes theanine extremely effective for combating tension, stress, and anxiety-without inducing drowsiness. By increasing the brain's output of alpha waves, theanine is thought to control anxiety, increase mental focus, improve concentration, and promote creativity.

Theanine increases brain levels of gamma amino butyric acid, a calming neurotransmitter, while caffeine decreases it. Theanine also affects levels of both serotonin and L-dopamine neurotransmitters in the brain. Serotonin is one of the major mood-enhancing neurotransmitters, and dopamine is our "reward" neurotransmitter. By shutting down the "worry" mode and increasing the ability to concentrate and focus one's thoughts, as measured by increased generation of brain alpha waves, some believe that theanine even makes learning easier.