L-Theanine Improves Attention and Shows Potential Antidepressant Effects, Meta-Analysis Finds

A systematic review and meta-analysis of 31 randomized controlled trials (total n = 1,168) compared oral L-theanine with placebo in both healthy volunteers and clinical samples. The primary focus was the acute impact of a single dose on stress in healthy adults; secondary outcomes included repeated-dose stress, anxiety, depressive symptoms, fatigue, and cognitive performance. The analysis identified a robust short-term benefit on attention and a possible antidepressant effect, while safety was confirmed.

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L-theanine is an amino acid naturally occurring in green tea. Prior research has suggested it may alleviate affective symptoms and boost cognitive function, yet its clinical relevance has remained uncertain. This meta-analysis was undertaken to clarify the magnitude and reliability of these purported benefits across a broad set of trials.

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• Attention: A single 200 mg dose taken 30–60 minutes before testing improved choice reaction time (Standardised Mean Difference [SMD] = 0.51; 95 % CI 0.25–0.77).

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• Acute Stress: Reduction was modest (SMD = 0.31) and driven largely by studies judged at high risk of bias.

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• Depressive Symptoms: Excluding one outlier, a single dose lowered depressive scores in healthy participants (SMD = 0.69; 95 % CI 0.13–1.25) with low heterogeneity.

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• Anxiety: Effects were inconsistent and non-significant, except for one trial on psychotic anxiety (SMD = 0.54; 400 mg/day for 8 weeks).

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• Fatigue: No significant change was observed.

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• Safety: No serious adverse events were reported; dropout rates did not differ from placebo.

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The findings suggest that L-theanine is a safe supplement that can enhance attentional performance in the short term for healthy adults. The observed antidepressant signal, though preliminary, points to a potential role in mood regulation that warrants further investigation. If confirmed, L-theanine could offer a low-risk option for cognitive and affective support, complementing existing therapeutic strategies.

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• Risk of Bias: The modest stress reduction appears contingent on studies with high methodological risk, limiting confidence in that outcome.

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• Anxiety Inconsistency: Divergent results across trials, with only a single study showing benefit, highlight uncertainty regarding anxiolytic properties.

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• Clinical Populations: Most data derive from healthy volunteers; evidence for efficacy in clinical groups remains sparse.

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• Outlier Influence: The depressive-symptom benefit emerged only after removing an outlier, indicating sensitivity to individual study effects.

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• “Conclusions L-theanine is safe and shows a robust short-term benefit on attention in healthy adults, with a potential antidepressant effect warranting confirmation in high-quality trials, especially in clinical populations.” — Study Authors

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• “31) and largely influenced by studies with a high risk of bias.” — Study Authors

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• “No serious adverse events were reported.” — Study Authors

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The authors call for high-quality, double-blind trials—particularly in clinical populations—to verify the antidepressant potential and to clarify the anxiolytic profile of L-theanine. Future research should also address methodological limitations that currently obscure the true magnitude of stress-reduction effects.

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