l-theanine: From tea leaf to trending supplement – does the science match the hype for brain health and relaxation?

• l -Theanine is a unique non-protein amino acid found abundantly in tea leaves. • Interest in its potential use as a dietary supplement is growing. • The evidence supporting many popular health claims remains limited. • Caution is warranted regarding the purported health benefits of l -theanine. l -Theanine is a unique non-protein amino acid found abundantly in tea leaves. Interest in its potential use as a dietary supplement has surged recently, especially claims related to promoting relaxation and cognitive enhancement. This review surveys the chemistry, metabolism, and purported biological activities of l -theanine. It is well absorbed from the intestine and can cross the blood-brain barrier. Some studies suggest l -theanine may increase alpha waves in the brain associated with relaxation and selective attention, reduce stress and anxiety, and improve sleep quality, though findings are often inconsistent. Potential neuroprotective and anti-seizure effects have also been reported in animal models. When combined with caffeine, l -theanine may improve cognitive performance, alertness and focus. However, the evidence supporting many health claims remains limited, especially the lack of rigorous human clinical trials. While l -theanine exhibits a good safety profile based on toxicology studies, caution is warranted regarding the purported health benefits, until stronger scientific substantiation emerges. Overall, the mechanisms of action and therapeutic potential of l -theanine require further investigation, given the current interest and increasing popularity of this nutraceutical supplement marketed for brain health and relaxation. In the absence of well-designed and carefully controlled human clinical trials, we would urge caution in the use of l -theanine supplements at pharmacologic doses by the wider population, and believe that the science does not yet match the hype behind this trending supplement for brain health and relaxation. l -theanine is naturally found in tea leaves. It is well absorbed in the intestine and can cross the blood brain barrier. Its putative biological activities include relaxation and cognitive enhancement. The rationale is that l -theanine increases alpha brain waves, which is associated with relaxation and selective attention. However, human research is scant and more of it is needed to fully understand its mechanisms of action and potential side effects.