The general picture that emerges from L-theanine research is an anti-stress effect, with the strongest hypothesis being that anti-glutamate receptor activity is the primary driver of this effect.
L-theanine supplementation has been shown to increase brain α-waves (8-10 Hz range) which are associated with reduced stress and anxiety. Beyond relaxation, increased α-waves are associated with selective attention mechanisms and mental alertness. These altered wave functions are said to be evidence that L-theanine has ‘relaxing and attention promoting’ properties. There’s also literature that report increased theta wave function which is associated with learning and memory.
Some preliminary human studies, typically using doses of L-theanine about 10 to 20 times higher than that in a cup of tea, have suggested possible benefits, such as the following:
In a Japanese study in the Journal of Physiological Anthropology, college students who took L-theanine experienced less anxiety and had smaller increases in blood pressure when under psychological or physical stress than when they took a placebo.
Similarly, another Japanese study in Pharmacology, Biochemistry, and Behavior found that graduate students who took L-theanine experienced less anxiety (as measured by a questionnaire and by a salivary marker for stress) when they were assigned stressful work in a pharmacy, compared to a placebo.
In a Canadian study in Alternative Medicine Review, L-theanine improved some aspects of sleep quality in boys with attention-deficit hyperactivity disorder (ADHD).
Study 4: L-theanine relieves positive, activation, and anxiety symptoms in patients with schizophrenia and schizoaffective disorder: an 8-week, randomized, double-blind, placebo-controlled, 2-center study.
In a preliminary Israeli study in the Journal of Clinical Psychiatry, L-theanine helped relieve anxiety symptoms and augment antipsychotic treatment in patients (ages 19 to 55) with schizophrenia.
Stressful condition increased the level of sAA that was essentially affected by individual trait anxiety. The low levels of pre-practice sAA and subjective stress in the L-theanine-group suggest that L-theanine intake suppressed initial stress response of students assigned for a long-term commitment of pharmacy practice.