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Kimura K, Ozeki M, Juneja LR, Ohira H. L-Theanine reduces psychological and physiological stress responses. Bio Pscyhol. 2007; 74: 39-45.
Acute stress increases extracellular brain levels of the neurotransmitter glutamate. Glutamate may also promote activity of the sympathetic nervous system. The sympathetic nervous system increases heart rate and blood pressure during stress. L-Theanine is one of the major amino acids in green tea (Camellia sinensis). L-Theanine can cross the blood brain barrier and may influence secretion and function of neurotransmitters. L-Theanine can bind to glutamate receptors and block binding of L-glutamic acid. The authors hypothesized that L-Theanine might be able to reduce stress-induced excitation of peripheral sympathetic activity.
Twelve healthy male undergraduate students (mean age 21 years) participated in this double-blind, placebo-controlled, parallel study with a counterbalanced design. The study was conducted in Japan. On 4 separate occasions, participants began the trial with a 20 minute rest period, followed by a mental arithmetic task for 20 minutes (which was previously shown to produce stress), and then 2 rest periods of 10 minutes each. Saliva samples were collected and psychological measures made at the end of each period. There were 4 test conditions (1) consuming 200 mg of L-Theanine (Suntheanine, Taiyo Kagaku Inc, Japan) in 100 mL of water before the initial rest period, (2) consuming L-Theanine immediately before the mental task, (3) consuming water (placebo) before the initial rest period, or (4) no treatment.
Anxiety scores were higher during the mental task period, and the levels of anxiety were significantly higher in the placebo group than in the other groups (P-value not reported). Also, the perception of stress in the task period was higher under the placebo condition than under the other conditions. There were no differences between the 2 L-Theanine groups. Heart rate was also elevated during the mental task. Heart rate was significantly lower in the L-Theanine groups than in the placebo group (P-value not reported). There was no significant difference in heart rate between the L-Theanine groups. The heart rate variability and elevation of salivary secretory immunoglobulin A was significantly higher during the mental task under placebo conditions than the other conditions (p-value not reported). The study was well-blinded; none of the participants could tell the water (placebo) from the water containing L-Theanine.
The authors conclude that acute stress responses elicited by the mental task were reduced by L-Theanine, as evidenced by both the subjective perception of stress and reduced physiological stress. The authors state that the reduction of acute stress response was mediated by changes in the amount of neurotransmitters or L-Theanine working as an antagonist against glutamic acid receptors.
The study suggests that L-Theanine can buffer sympathetic activity during an acute stressor; however, the study had several limitations. The limitations were (1) only male participants, (2) participants were relatively young, (3) only a few stress parameters were evaluated, and (4) changes in neurotransmitters were not directly evaluated. Further studies are needed that address these limitations.
—Heather S. Oliff, PhD
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