Abstract
This randomized, placebo-controlled, crossover, and double-blind trial aimed to examine the possible effects of four weeks L-theanine administration on stress-related symptoms and cognitive functions in healthy adults. Participants were 30 individuals (nine men and 21 women; age: 48.3 ± 11.9 years) who had no major psychiatric illness. L-theanine (200 mg/day) or placebo tablets were randomly and blindly assigned for four-week administration. For stress-related symptoms, Self-rating Depression Scale, State-Trait Anxiety Inventory-trait, and Pittsburgh Sleep Quality Index (PSQI) scores decreased after L-theanine administration (p = 0.019, 0.006, and 0.013, respectively). The PSQI subscale scores for sleep latency, sleep disturbance, and use of sleep medication reduced after L-theanine administration, compared to the placebo administration (all p < 0.05). For cognitive functions, verbal fluency and executive function scores improved after L-theanine administration (p = 0.001 and 0.031, respectively). Stratified analyses revealed that scores for verbal fluency (p = 0.002), especially letter fluency (p = 0.002), increased after L-theanine administration, compared to the placebo administration, in individuals who were sub-grouped into the lower half by the median split based on the mean pretreatment scores. Our findings suggest that L-theanine has the potential to promote mental health in the general population with stress-related ailments and cognitive impairments.
Highlights
L-theanine (γ-glutamylethylamide) is a unique non-protein amino acid found in green tea (Camellia sinensis), a widely consumed beverage associated with human health [1,2,3]
We found that improvement in the Brief Assessment of Cognition in Schizophrenia (BACS) verbal fluency (p = 0.002), especially letter fluency (p = 0.002), score was significantly greater during the L-theanine period than the placebo period among those individuals who were sub-grouped into the lower half by the median split (n = 15)
The effects on stress-related symptoms were broad among the symptom indices presented in the study, a comparison to the placebo administration somewhat limits the efficacy of L-theanine administration for some sleep disturbance measurements
Summary
L-theanine (γ-glutamylethylamide) is a unique non-protein amino acid found in green tea (Camellia sinensis), a widely consumed beverage associated with human health [1,2,3]. As the structure of L-theanine resembles that of L-glutamic acid, its mechanism of action may be potentially mediated through glutamate receptors [4], a possibility supported by its partial co-agonistic effect on the. Given that L-theanine is a phytochemical ingested in daily life, it has the potential to become a nutraceutical ingredient that mitigates and prevents stress-related psychic confusion in modern society [7,8,9,10]. And centrally administered L-theanine has been reported to act on the brain [12,13], to modulate monoamine levels in the central nervous system [14,15,16,17], and glutamate and glycine neurotransmissions [18,19]. L-theanine has been reported to influence hippocampal synaptic plasticity, improving performance in recognition tasks in stressed young rats [20,21]
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