Effects of L-Theanine, a Constituent of Tea, on Cognitive Functions and Attention

Tea (Camellia sinensis) is a historically and economically important beverage for Sri Lanka; and is traditionally believed to have relaxing or calming effects. Some evidence attributes these effects to a constituent of tea leaves; L-theanine, a non-protein-forming amino acid structurally similar to glutamic acid. Over the years, research has found L-theanine to also have effects on various cognitive functions, including attention. This review focuses on the research conducted thus far on the acute effects of L-theanine on neurobehavioral, neurophysiological, and functional neuroimaging indices, cognition and attention. Despite the heterogeneity of testing paradigms across studies, experimental evidence increasingly suggests that L-theanine could improve visual selective attention. Contrary to traditional belief, the amount of L-theanine in a cup of tea (4.5-22.5mg) seems to cause no significant effects; and attentional improvement is more prominent with much higher doses (100-500mg). This makes it worth exploring the possibility of enriching tea with L-theanine, while preserving the unique flavour of the product. L-theanine has an additive effect with caffeine in enhancing attention, but unlike caffeine, L-theanine can be ingested in high doses with no risk of adverse effects, tolerance or dependence. The evidence on attentional effects of L-theanine is almost entirely based on healthy samples who have undergone laboratory-based attentional tasks that employed abstract stimuli.  When translating these laboratory findings to real-life scenarios, incorporating more real-life-based stimulus paradigms (i.e., simulated driving scenarios) is warranted. The clinical relevance of L-theanine research could also be improved by exploring the effects of L-theanine in cognitively compromised groups of individuals.