A combination of caffeine and L-theanine, an amino acid commonly found in tea, provides evidence of acute changes in behavioural and neurophysiological measures related to selective attention in adolescents with ADHD. A recent experiment published in Nutritional Neuroscience indicates that a single dose of an L-theanine-caffeine combination enhances the ability of the brain to allocate cognitive resources and attenuate the progressive slowing of reaction time during the task and reduce false alarms in adolescents with ADHD. The findings suggest that this pairing might offer a supplementary nutritional approach for managing ADHD symptoms alongside standard treatments.
Attention-deficit hyperactivity disorder is a neurodevelopmental condition marked by persistent patterns of inattention, impulsivity, and hyperactivity. At the neurological level, the disorder involves a dysregulation of brain chemicals like dopamine and norepinephrine. These chemicals help manage motivation, executive functioning, and the ability to maintain focus. People with ADHD often experience specific difficulties with selective attention, which is the cognitive process of focusing on relevant information while suppressing distracting background stimuli.
To manage these symptoms, doctors frequently prescribe medications like methylphenidate. This medication works by increasing the available levels of dopamine and norepinephrine in the brain. At the same time, researchers have become increasingly interested in dietary compounds that might offer additional cognitive support. L-theanine is a non-protein amino acid found naturally in tea leaves that interacts with glutamatergic signalling in the brain, which is involved in processes such as learning and memory.
Caffeine is a well-known stimulant that blocks adenosine receptors, leading to an increase in the release of dopamine, noradrenaline, glutamate, and acetylcholine, which regulate attention, motivation, and executive functioning. Previous laboratory experiments suggest that high doses of L-theanine and caffeine taken together have a synergistic effect, enhancing attention and cognitive performance in healthy adults.
“One aspect of this study is why we chose to combine L-theanine with caffeine,” said Gayani Nawarathna, a lecturer in the Division of Physiology at the University of Peradeniya in Sri Lanka. “Caffeine can improve alertness and attention, but higher doses can also cause side effects, however, high doses of L-theanine are safe. So, combination of L-theanine with caffeine may provide caffeine sparing effect.”
Brain imaging research by Nawarathna and her colleagues also indicates the combination improves inhibitory control. The current study aimed to test whether this combination could improve selective attention in adolescents formally diagnosed with ADHD.
“Our study was motivated primarily by the limited evidence on whether the cognitive effects of the L-theanine-caffeine combination observed in healthy adults could also be seen in adolescents with ADHD,” Nawarathna explained. “Selective attention is particularly important in ADHD because difficulties in maintaining attention and suppressing distraction can affect academic performance and everyday functioning.”
“Although previous research had suggested that L-theanine and caffeine could improve attention in healthy adults, evidence specifically examining their effects in children with ADHD was very limited,” Nawarathna added. “We therefore wanted to investigate this question using not only behavioral measures, but also EEG measures of brain activity.”
The experiment included 21 adolescents between the ages of 10 and 19 who were actively taking methylphenidate, 18 of whom were male. The researchers used a double-blind, placebo-controlled crossover design, meaning each participant received all three treatments on different days. Neither the participants nor the researchers knew which treatment was given on a specific day.
The three treatments included a combination of L-theanine (2.5 milligrams per kilogram of body weight) and caffeine (2.0 milligrams per kilogram), a starch-based placebo, and the participant’s regular prescribed dose of methylphenidate. “We also included methylphenidate as a positive control, allowing us to put the effects of the combination into the context of an established ADHD treatment,” Nawarathna said.
The sessions were separated by a washout period of three to seven days to ensure complete elimination of L-theanine and caffeine from the blood. Before each session, participants had to sleep for at least six hours, abstain from tea, coffee or any food or drink containing caffeine for 24 hours, and delay their morning dose of methylphenidate. During each laboratory visit, participants completed a 12-minute computerized visual attention task, then they were given the pill (treatment). Afterward, they completed the exact same task again 50 minutes later.
The task required them to look at a screen displaying a series of triangles (the standard stimulus was an upward pointing triangle and the target stimulus was a downward pointing triangle). They had to press a button as quickly as possible when they saw a downward-pointing triangle, which appeared 20 percent of the time. They had to ignore the upward-pointing triangles, which appeared 80 percent of the time. While participants were doing the task, the researchers recorded their brain activity using electroencephalography, or EEG, via electrodes placed on the scalp.
“In particular, we measured P3b amplitude, interpreted as reflecting the allocation of neural resources to the task, and P3b latency, which is associated with the speed of stimulus evaluation,” Nawarathna said.
Behavioral data from 20 participants and EEG data from 18 participants were included in the final analysis. “The main message is that a single dose of a high-dose L-theanine-caffeine combination produced several changes associated with selective attention in adolescents with ADHD,” Nawarathna told PsyPost. “Compared with placebo, the combination reduced false alarms, suggesting better suppression of distracting or irrelevant information.” Methylphenidate also reduced false alarms. Overall accuracy in identifying the correct targets was very high across all sessions, with participants correctly hitting the button for nearly 68 or 69 out of 70 targets regardless of the treatment. Any variations in correct hits between the treatments were not statistically significant.
When looking at the speed of the behavioral responses, only methylphenidate improved the average reaction time compared to the placebo, speeding up responses by about 44 milliseconds. “The combination did not significantly improve average reaction time in the same way that methylphenidate did,” Nawarathna explained. “However, when we examined performance across the approximately 12-minute task, we found that L-theanine-caffeine substantially reduced the progressive slowing of responses that occurred as the task continued.”
Under the placebo condition, reaction times slowed down by about 72 milliseconds from the beginning to the end of the post-dose task, which points to a decline in attention. Both methylphenidate and the L-theanine-caffeine combination attenuated this progressive slowing. “In other words, the combination appeared to help maintain relatively stable attention over time,” Nawarathna added.
The EEG data provided evidence of how the brain achieved these attentional improvements. “It also produced changes in the P3b EEG response consistent with faster stimulus evaluation and greater allocation of neural resources to the task,” Nawarathna said. Compared to the placebo, the L-theanine and caffeine combination decreased the latency of the P3b event-related potential by approximately 30 milliseconds, meaning the brain evaluated the targets more quickly. The P3b event-related potential changes associated with the L-theanine-caffeine combination were not significantly different from those observed with methylphenidate.
“I would strongly emphasize that this study does not demonstrate that L-theanine-caffeine is a replacement for methylphenidate or other established ADHD treatments, and it should not be interpreted as a recommendation for adolescents with ADHD to self-administer high doses of caffeine or L-theanine,” Nawarathna highlighted.
“Our experimental paradigm and interpretation of our findings have certain limitations,” Nawarathna noted. “First, the task yielded high accuracy rates, displaying ceiling effects across treatment conditions. Thus, if future studies aim to explore the potential effects of task accuracy, more demanding attentional tasks have to be employed.” A ceiling effect occurs when an assessment is too easy, causing almost all participants to achieve a near-perfect score.
“Second, the present sample is powered enough only to reveal a large effect size (Cohen’s d = 0.7, power = 0.8, alpha = 0.05) in outcome measures; thus, future investigations should have larger samples if the objective is to extract more subtle treatment effects,” Nawarathna explained. Effect size and statistical power refer to the mathematical ability of a study to detect meaningful differences between groups. Because this sample size was small, researchers could only identify major neurological and behavioral shifts.
“Third, this study focuses only on acute effects of single-dose intervention,” Nawarathna added. “Whether the attentional benefits hold over long-term L-theanine-caffeine supplementation is confounded by other factors, including the potential for tolerance and treatment compliance.”
“Finally, the present study and the functional neuroimaging studies conducted hitherto tap into specific cognitive functions of the participants tested under controlled laboratory environments,” Nawarathna said. “Whether the attentional improvement we observed translates into their classroom and other real-life scenarios is yet to be determined.”
Future studies are needed to determine the longer-term safety, tolerability, and potential effectiveness of L-theanine-caffeine supplementation. “We need to find out whether the improvements seen in brain activity lead to better academic performance and everyday functioning (whether they translate into meaningful real-world outcomes) before it can be recommended for regular use as a nutraceutical supplement.”
The study, “Effects of L-theanine-caffeine combination on selective attention among adolescents with attention-deficit hyperactivity disorder: a double-blind, placebo-controlled, crossover study,” was authored by Gayani S. Nawarathna, Dewasmika I. Ariyasinghe, Nilantha Balasooriya, Anshu Fernando, and Tharaka L. Dassanayake.
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