Theanine consumption, stress and anxiety in human clinical trials: a systematic review

Background/Aims: Theanine (L-THE) is a non-proteinaceous amino acid predominately found in green tea that has been associated with a number of health benefits including improvements in mood, cognition and reduction of anxiety-like symptoms. However, the majority of these studies have been performed in animal models of psychological stress while human trials are relatively scarce. The main aim of this study was to review the current literature on the effect of L-THE intake, in the form of nutritional supplements, on stress and anxiety levels in humans. Methods: The systematic review was conducted following the preferred reporting items for systematic reviews and meta-analyses (PRISMA) guidelines on four major electronic databases (PubMed, Cochrane Library, Scopus and Web of Science). Only L-THE supplement studies that were published in peer-review journals in English, and conducted on humans in randomised controlled trials (RCTs) were included. Results: Five RCTs with L-THE (200-400 mg/day; up to 8weeks) in a total of 104 (M=77; F=27) participants met the inclusion criteria. Four studies included participants who had no pre-existing mental illnesses and one study used participants with existing mental conditions. All studies were placebo-controlled but one study also compared L-THE against a commonly used anti-anxiety pharmaceutical (alprazolam). The findings in four studies indicated significant improvements in reducing stress and anxiety (p<0.05). However, the results of the study comparing the L-THE and alprazolam did not show a significant anxiolytic effects with either treatments. Conclusions: L-THE supplementation can assist in reducing acute stress and anxiety in people experiencing stressful situations.