This information came from a companies website, but many other sites have
the same information. So I suspect there's something to this.

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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 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.