Theanine (or gamma-glutamylethylamide, 5-N-ethyl-glutamine) is a unique, non-proteinic amino acid contained almost exclusively in tea plants  (especially in green tea leaves ) with the exception of Xerocomus badius (a kind of mushroom). L-theanine is a substance responsible for feeling of relaxation after green tea ingestion and its exotic taste . It has been historically used as a relaxing agent. Suntheanine TM is a trade name for L-theanine.
Typical used and recommended dose is between 100-200 mg. Studies which reported anti-anxiety effect used doses up to 400 mg.
Relaxing Effects of Theanine
Based on mental condition there are four types of brain waves (alpha, beta, gamma and theta). Increase in alpha wave activity is considered to be an index of relaxation. Kobayashi et al.  and Higashiyama et al.  found that 200 mg (but not 50 mg) led to increased generation of alpha wave activity (on the occipital and parietal regions of the brain surface) in human volunteers 40 minutes after supplementation. Which signified a relaxation without causing drowsiness . However, a more recent study by Gomez-Ramirez and colleagues  noted a decrease in alpha activity following 250 mg L-theanine. This reduction in alpha power was only seen while subjects engaged in a very demanding cognitive task. Despite this, attention-related alpha effects were significantly greater for the theanine group. Findings by Kristy Lu et al.  suggested L-theanine may have some relaxing effects under resting conditions.
Theanine and Caffeine Interaction
EEG evaluation (in animals) has shown that L-theanine is able to inhibit the stimulation produced by caffeine . However, a more recent and well-controlled studies in humans reported further increased alert ratings [7,8] and reduced tired ratings  with combination of caffeine and L-theanine (but with much lower doses than previously mentioned study). Haskell et al.  also reported that the combination of L-theanine and caffeine leads to faster simple reaction times and numeric working memory reaction time, improved sentence verification accuracy, increased alertness and decreased tired ratings compared to placebo while caffeine alone did not.
It has also been reported that 97 mg of L-theanine in combination with 40 mg of caffeine helps to focus attention during a demanding cognitive task; no significant effects on other cognitive tasks, such as visual search, choice reaction times, or mental rotation .
Theanine as Anti-fatigue Nutrient
Animal study by Li et al.  concluded that theanine has the effect of delaying exercise-induced fatigue. Results revealed that possible mechanisms are enhanced dopamine levels and inhibition of synthesis or secretion of 5-HT in brain.
Effects of Theanine on Anxiety
The first human study  to evaluate anxiolytic effects of L-theanine in experimentally induced anxiety reported that 200 mg L-theanine has no significant anxiolytic effects measured by model of anticipatory anxiety. Same was noted by Higashiyama and colleagues . However, a higher dose of L-theanine (400 mg/daily) was able to reduce anxiety symptoms in schizophrenia and schizoaffective disorder patients .
It appears that potential benefit of L-theanine to soothe anxiety is minor at best.
Memory and Learning
Animal studies have reported improved memory and learning abilities . In persons with mild cognitive impairmen, a theanine solution called LGNC-07 (360mg of green tea extract and 60mg theanine) administered three times daily for 16 weeks led to improvements in memory by marginally increasing delayed recognition in the Rey–Kim memory test .
It has been shown that administration of theanine has a significant effect on the release or reduction of neurotransmitters like dopamine and serotonin, which are known to be closely related to memory and learning ability [4,9].
Other Theanine Benefits
Theanine also acts to lower blood pressure and regulates brain and nerve functions .
Theanine Side Effects and Toxicity
There are no safety concerns despite wide use. Side effects reported in human studies include headache and dizziness. In a toxicity test where rats were fed with 4,000 mg/kg bodyweight (the highest dose tested) for 13 weeks no observable adverse effects were reported . Food and Drug Administration has granted GRAS (generally recognized as safe) status to L-theanine.
There is no information regarding safety and efficacy in pregnancy and lactation.
(Other common names: Gamma-glutamylethylamide, L-Théanine, N-Éthyl-L-Glutamine, Teanina, Théanine, 2-Amino-4- (ethylcarbamoyl) Butyric Acid, 5-N-ethylglutamine, γ-L-glutamylethylamide)
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- Kobayashi, K., et al. “Effects of L-theanine on the release of alpha-brain waves in human volunteers.” Nippon Nogeikagaku Kaishi 72.2 (1998): 153-157.
- Gomez-Ramirez, Manuel, et al. “The deployment of intersensory selective attention: a high-density electrical mapping study of the effects of theanine.” Clinical neuropharmacology 30.1 (2007): 25.
- Juneja, Lekh Raj, et al. “L-theanine—a unique amino acid of green tea and its relaxation effect in humans.” Trends in Food Science & Technology 10.6 (1999): 199-204.
- Suzuki, Yasuo, Noriyuki Miyoshi, and Mamoru Isemura. “Health-promoting effects of green tea.” Proceedings of the Japan Academy. Series B, Physical and Biological Sciences 88.3 (2012): 88.
- Kakuda, Takami, et al. “Inhibiting effects of theanine on caffeine stimulation evaluated by EEG in the rat.” Bioscience, biotechnology, and biochemistry 64.2 (2000): 287-293.
- Haskell, Crystal F., et al. “The effects of L-theanine, caffeine and their combination on cognition and mood.” Biological psychology 77.2 (2008): 113-122.
- Owen, Gail, et al. “The combined effects of L-theanine and caffeine on cognitive performance and mood.” Nutritional neuroscience 11.4 (2008): 193-198.
- LI, Min, Xin-nan SHEN, and Guo-ying YAO. “Effect of theanine on delaying exercise-induced fatigue and its mechanism [J].” Acta Nutrimenta Sinica 4 (2005): 019.
- Giesbrecht, Timo, et al. “The combination of L-theanine and caffeine improves cognitive performance and increases subjective alertness.” Nutritional neuroscience 13.6 (2010): 283-290.
- Borzelleca, J. F., D. Peters, and W. Hall. “A 13-week dietary toxicity and toxicokinetic study with L-theanine in rats.” Food and chemical toxicology 44.7 (2006): 1158-1166.
- Lu, Kristy, et al. “The acute effects of L-theanine in comparison with alprazolam on anticipatory anxiety in humans.” Human Psychopharmacology-Clinical and Experimental 19.7 (2004): 457-466.
- Higashiyama, Akiko, et al. “Effects of l-theanine on attention and reaction time response.” Journal of Functional Foods 3.3 (2011): 171-178.
- Ritsner, Michael S., et al. “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.” The Journal of clinical psychiatry 72 (2011): 34-42.
- Park, Sang-Ki, et al. “A combination of green tea extract and l-theanine improves memory and attention in subjects with mild cognitive impairment: a double-blind placebo-controlled study.” Journal of medicinal food 14.4 (2011): 334-343.