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L-theanine, also known as N-ethyl-L-glutamine, is a major non-protein, water soluble  amino acid commonly found in green tea [2; 3].
Mechanism and interactions
Once consumed, L-theanine rapidly interacts with the intestinal tract, where it is absorbed, as well as being transported into the brain [4;5]. Within 5 hours L-theanine reaches its maximum level, where it then lasts for 24 hours – having exited the kidneys, liver and brain .
Benefits and Effects
L-theanine has anti-tumour, anti-diabetes, anti-hypertension and anti-stress effects [7; 8]. Additionally, L-theanine prevents nerve cell damage and protects the liver from damage .
L-theanine and major depressive disorder (MDD)
In a more recent study assessing the effect of L-theanine on patients with depression, L-theanine – in a 250mg/day dose administered over 8 weeks – showed a beneficial outcome in depressive symptoms: anxiety, sleep disturbance and cognitive impairment in all patients .
L-theanine and ADHD
One study found that L-theanine in studying ADHD, given in a 400mg dose to 8-12-year-old boys, was effective in improving sleep quality . Researchers of this study suggested the importance of considering a 400mg, safe dose in treating childhood ADHD .
L-theanine effects on sleep
L-theanine was found to improve sleep quality in patients suffering from schizophrenia . An investigation into ADHD-related sleep disorders found that L-theanine did not produce a significant change in total sleep time; however, it did significantly increase sleep efficiency along with a reduction in nocturnal activity .
L-theanine and blood pressure
Animal work has found that L-theanine can reduce blood pressure in hypersensitive rats . This has largely been attributed to the way L-theanine interacts with the autonomic nervous syste, (ANS), where it carries out anti-stress activities .
Typical doses of between 50-200mg daily is most commonly reported . Studies in rats have demonstrated that up to 4 g/kg per day – considered a high L-theanine dose – had a non-toxic outcome on food intake, body weight and morbidity . Other studies have reported that a high dose of 400mg daily L-theanine is safe, tolerable and effective . In addition, a 200-400mg dose of L-theanine reported a decrease in anxiety and stress in healthy participants .
Results of acute and toxicity have shown safety even if a high dose is consumed . Toxicological descriptions have also reported the absence of negative side effects in animal studies investigating L-theanine .
Research has consistently used L-theanine and caffeine in low, moderate and high doses. In this manner, it has been shown that 100mg L-theanine stacked with 50mg caffeine has positive outcomes on attention .
L-theanine is approved as generally regarded as safe (GRAS) by the Food and Drug Administration (FDA) . In addition, the FDA has suggested that the daily amount of L-theanine should not exceed 1200mg.