High Doses of Theanine Improve Visual Stimulus Discrimination in a Dose‐dependent Manner
Authors: Kahathuduwa Chanaka , Weerasinghe Vajira , Dassanayake Tharaka , Amarakoon Tissa , Binks Martin 
Published: November 2023

Theanine is an amino acid found naturally in tea. High doses of theanine, caffeine and their combination improve visual attention and processing. Therefore, we aimed to examine 1) the effects of theanine, caffeine and their combination (TC); 2) the possible interaction of theanine and caffeine and 3) the dose‐response relationship of theanine in improving visual stimulus discrimination (VSD).This study represents an archival analysis of data derived from a primary study in which 200mg of theanine, 160mg of caffeine, TC and distilled water (placebo) were administered orally at least 1 week apart to 20 healthy males (21.9±.7 years) using a crossover design. Recognition visual reaction time was measured before (RVRTB) and 55 minutes after (RVRTA) each administration, using a computer‐based test (SermionTM). In the task, subjects responded to 10 randomly occurring white‐flashes (target‐stimuli) by pressing a button, while ignoring random red‐flashes. Simple visual reaction time (SVRTA) was also measured immediately before RVRTA. In this task, subjects indicated the presence of 10 random white‐flashes (without red‐flash distractor). Post‐dose motor evoked potential latency (MEP; Medtronic KeypointTM, Denmark) after transcranial magnetic stimulation (Magstim 200TM, UK) and P100 visual evoked potential latency (P100; RMSEMG EPMK‐II, India) were recorded sequentially at 35 minutes of substance administration. RVRTA was regressed vs. SVRTA, RVRTB, P100, MEP and substance (model 1). To examine the synergistic effect, RVRTA of theanine, caffeine TC and placebo were regressed vs. respective SVRTA, RVRTB, P100, MEP, 2 dummy coded variables for theanine and caffeine and their interaction (model 2). In another 5‐way crossover, 12.5, 25, 50, 100 and 200mg of theanine were administered at least 3 days apart to 5 subjects to assess RVRTB, RVRTA and SVRTA. RVRTA of this study was regressed vs. RVRTB, SVRTA and dose (model 3). All procedures were in accord with the Helsinki Declaration and were approved by the IRB at Faculty of Medicine, Peradeniya.Model 1 (R2=.41, p<.001) suggested that when RVRTB, SVRTA, P100 and MEP were constant, compared to the placebo, theanine and TC significantly reduced RVRTA by 25.86ms (t=−2.3, p=.025) and 35.84ms (t=−3.1, p=.002) respectively. Caffeine had a trend of improvement (β=22.5ms, t=−1.9, p=.051). Model 2 (R2=.35, p<.001) revealed a significant effect for theanine (p=.030) and a trend for caffeine (p=.070) but not for the interaction (p=.46). Model 3 (R2=.50, p=.002) indicated that when controlled for SVRTA and RVRTB, increasing theanine dose by 1mg reduced RVRTA by 0.28ms (t=−3.02, p=.007).Thus, when controlled for visual and motor conduction and simple stimulus processing latencies, theanine and TC seem to improve VSD. Effects of theanine and caffeine appear to be additive rather than synergistic. Improvement of VSD by theanine seems to be dose‐dependent.Support or Funding InformationNational Research Council, Sri Lanka (Grant 09‐32), International Research Center, University of Peradeniya, Sri Lanka.