Theanine is an amino acid found in high concentrations in green tea. Theanine can pass through the brain-blood barrier and may play an agonist or an antagonist of some receptors. Research has shown that L-theanine supplementation does provide some relaxing effects possibly by increasing levels of GABA and serotonin.60 A small study showed that administration of 200 mg of L-theanine increased alpha brain wave activity and induced a sense of relaxation.61 L-theanine is also known to block the binding of L-glutamic acid to glutamate receptors in the brain. A double-blind placebo-controlled study showed that L-theanine intake resulted in a reduction in the heart rate and salivary immunoglobulin A responses to an acute stress task compared to the placebo group, likely attributable to an attenuation of sympathetic nervous activation suggesting anti-stress activity of theanine.62 Using animal models, L-theanine has been shown to cause dopamine release from dopaminergic neurons and may inhibit excitatory neurotransmission and cause inhibitory neurotransmission via glycine receptors, suggesting a possible mechanism for its anxiolytic activity.63
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