Promising, but Not Completely Conclusive-The Effect of l-Theanine on Cognitive Performance Based on the Systematic...

Background: Green tea (Camellia sinensis) has been consumed for centuries, and its beneficial effects on human health have been studied in recent decades. l-theanine, an active ingredient in green tea, has been used to improve cognition and mood. Although the effects of l-theanine on cognition have been investigated in clinical trials that have reported various results, these studies have not yet been critically evaluated in meta-analyses. Objectives: Our objective was to systematically evaluate the efficacy of l-theanine on cognitive functions compared to a placebo, in a meta-analysis based on randomized controlled trials (RCTs). Methods: PubMed, the Cochrane Central Register of Controlled Trials, Embase and Web of Science were searched for relevant studies until 31 July 2024 and registered in PROSPERO (registration number: CRD42024575122). Placebo-controlled clinical trials investigating the efficacy of l-theanine in healthy adults were included. Conference abstracts, study protocols and reports of non-RCTs were excluded. For risk of bias assessment, the Cochrane Risk of Bias Tool (version 2.0) was used. A random effects model was applied to conduct the meta-analysis. Mean differences (MD) with 95% confidence intervals (CIs) were calculated. Results: Based on the included five RCTs involving 148 healthy adults, l-theanine had a dose-dependent effect on cognitive function based on rapid visual information processing and recognition visual reaction time (MD: -15.20 ms; 95%-CI [-28.99; -1.41]). The effects of l-theanine were non-significant on reaction time to a simple stimulus (MD: -0.46 ms; 95% [CI: -15.65; 14.73]) and in the Stroop test (MD: -37.38 ms; 95%-CI [-86.39; 11.62]). Conclusions: The beneficial effects of l-theanine on cognitive performance could not be confirmed by all test methods. The contradictory results could be explained by the fact that l-theanine only affects certain cognitive domains, but also by the low number of trials and the heterogeneity of the test preparations. Further trials using standardized products with larger sample sizes are required for the accurate assessment of efficacy.