Synopsis

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

Theanine, an amino acid commonly found in tea leaves, has been studied primarily for its effects on stress, relaxation, and cognitive function. Its relevance to the sympathetic nervous system (SNS)—the branch of the autonomic nervous system responsible for the 'fight or flight' response—stems from its ability to modulate stress responses. Several studies have shown that theanine can promote relaxation without sedation, likely by influencing neurotransmitter levels such as GABA, dopamine, and serotonin.

Research using physiological markers, such as heart rate and salivary alpha-amylase activity, indicates that theanine may blunt sympathetic nervous system activation during stressful tasks. For example, randomized controlled trials have reported that theanine supplementation can reduce increases in heart rate and stress-related markers during acute stress. Functional MRI studies also suggest that theanine reduces neural markers of stress reactivity. However, while theanine appears to help regulate or dampen excessive SNS activity, it does not directly “support” or enhance SNS function in the sense of stimulation; rather, it helps restore balance between the SNS and the parasympathetic nervous system during stress.

Overall, while there is moderate scientific evidence supporting theanine’s beneficial effects on stress-related SNS overactivity, the evidence is not robust enough to warrant a high rating. Its primary benefit is in promoting relaxation and mitigating excessive sympathetic activity rather than directly supporting or enhancing SNS function.

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