Abstract
L-theanine is a naturally occurring, non-proteinogenic amino acid found in the tea plant
Camellia sinensis
and some mushrooms. The widespread consumption of tea, together with its long history of traditional use for supporting various aspects of wellbeing, has led to significant scientific interest in the effects of L-theanine. Much of this research has focused on L-theanine's ability to modulate the central nervous system and related functions. This has also led to its inclusion in dietary supplements for its purported effects on human health and performance. In this mini-review, we summarize the latest evidence on L-theanine and its effects on cognition, mood and sleep based on systematic reviews and meta-analyses of human randomized controlled trials. We also examine evidence relating to its safety, consider limitations of the existing scientific literature and highlight key areas for future research. Current evidence in healthy adults suggests that supplementation with 100–400mg of L-theanine can produce small but statistically significant improvements in aspects of cognitive performance, particularly on visual attention tasks and when combined with caffeine. It has also been reported to reduce both perceived and physiological markers of stress and anxiety under stress-inducing conditions at doses of 200–400mg/day, without causing sedation. Additionally, trials report that supplementation with 200–450mg/day of L-theanine can support sleep quality, with little effect on total sleep duration. Supplementation with L-theanine has generally been reported to be well-tolerated in clinical trials. Further high-quality studies are warranted to establish optimal dosing strategies, clarify underlying mechanisms and evaluate potential interactions with other bioactive compounds.