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

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Synopsis

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

L-theanine is an amino acid predominantly found in tea leaves, especially green tea. It is known for its calming effects without causing drowsiness. L-theanine does not directly stimulate or support the sympathetic nervous system (SNS)—which governs the "fight or flight" response—but rather modulates its activity by promoting relaxation and reducing stress. Several human and animal studies have shown that L-theanine supplementation can decrease physiological markers of stress, such as heart rate and blood pressure, often linked to sympathetic activation. For example, a 2016 randomized controlled trial published in Pharmacology, Biochemistry, and Behavior found that L-theanine attenuates the sympathetic response to stress, as measured by reduced heart rate and salivary alpha-amylase activity. Other research indicates L-theanine increases alpha brain wave activity, associated with a relaxed, alert mental state, suggesting it may help counterbalance excessive SNS activation. While L-theanine is not used to "stimulate" the SNS, there is moderate scientific evidence supporting its use to modulate SNS overactivity, particularly in stress-related scenarios. Traditional usage does not focus on the SNS specifically, as its calming effects have only been characterized in recent decades. Overall, scientific evidence supports the use of L-theanine for stress modulation involving the sympathetic nervous system, though more high-quality, large-scale studies are needed.

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