Evidence supporting the use of: L-theanine
For the body system: Parasympathetic Nervous System

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Synopsis

Source of validity: Scientific
Rating (out of 5): 3

L-theanine, an amino acid primarily found in green tea, has been studied for its effects on the nervous system, particularly regarding its potential to promote relaxation and support the parasympathetic nervous system (PNS). The PNS is responsible for the "rest and digest" functions in the body, counterbalancing the "fight or flight" responses of the sympathetic nervous system. Several clinical and preclinical studies have demonstrated that L-theanine can increase alpha brain wave activity, which is associated with a state of relaxed alertness. Additionally, L-theanine has been shown to reduce stress and anxiety in both human and animal studies, possibly by modulating levels of neurotransmitters such as GABA, dopamine, and serotonin. Some research indicates that L-theanine may reduce heart rate and salivary immunoglobulin A responses to acute stress, suggesting a shift toward parasympathetic dominance. However, while these findings suggest a supportive role for L-theanine in promoting relaxation and mitigating stress, direct evidence of L-theanine specifically enhancing PNS activity (such as measured changes in vagal tone or other markers) is less robust. The current body of evidence, though promising and consistent with its traditional use for relaxation, is not yet strong enough to warrant the highest rating. Therefore, the use of L-theanine to support the parasympathetic nervous system is scientifically plausible, with moderate evidence supporting its efficacy.

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