Using tea (or coffee) as a daily stimulant is ubiquitous but its effects on brain activity is still poorly understood

Tea is one of the most popular drinks (after water) globally, providing a much needed early morning – or anytime of the day – wake up call to billions of brains around the world.

For decades, scientists have been interested in discovering the wellness benefits of tea and coffee on people’s mood, cognition and general health through studying their two key ingredients with psychotropic properties – L-theanine and caffeine (read about other psychotropics on the brain here).   Studies have highlighted the beneficial effects of caffeine at moderate doses on cognition such as increased feelings of alertness, faster reaction times and improved sustained attention.  In contrast, theanine has a slightly different effect and appears to help increase subjective feelings of relaxation and reduce stress. The mix of both theanine and caffeine, as found in tea, is proposed to generate a much desired state of “relaxed wakefulness” – and may help to explain its popularity.

But what actually happens inside your brain when you drink a cup of tea? How does the EEG pattern observed reflect this subjective assessment?

Tea on the brain.

Several EEG studies have attempted to explore the effect of tea’s active ingredients on the brain, but the results are far from clear cut.

They have typically taken two approaches – recording the ongoing EEG whilst the person is sitting at rest; or whilst performing an attentionally demanding task – in both instances asking the participant to ingest the “active” drink beforehand. The alpha frequency band of the EEG has been of key interest, a frequency band closely associated with attentional mechanisms in the brain.

For example, a study from the University of Oxford, by Anna Nobre and colleagues using 35 participants used EEG explored the effect of a theanine tea infusion on the brain whilst people sat at rest with their eyes open. The theanine was offered at levels which were comparable to those within a standard cup of tea, and showed that there was a greater gradual increase in alpha activity in the theanine condition compared to a matched control condition with the effect beginning from the occipital and parietal regions and encompassing the entire brain in little over an hour. Interesting, however, is their assertion that L-theanine was effective only in participants  classified as highly anxious using a manifest anxiety scale.  Such results suggest that the effects are specific to individual characteristics and not necessarily universal.

Focusing your attention

A series of studies by John Foxe and colleagues in New York however, had somewhat contrasting results, when using EEG in the context of attention-tasks to compare the effects of theanine against those of caffeine as well as a mixture of theanine and caffeine combined, as found in a typical cup of tea.  Although in an initial study (n=15) using a cued spatial attention task they found attention-specific alpha increases related to ingesting theanine, a follow up study (n=16)  failed to replicate this finding and instead they found that tonic alpha – in other words alpha occurring at background level not specifically evoked by the attentional task – was lowest in the combined caffeine-theanine condition compared to the theanine-only and caffeine-only conditions, as well as the placebo condition.  Attention performance improvements were also greatest in the caffeine-theanine-combined condition.

In a more recent study from the same lab they explored the effects of caffeine and theanine alone and combined on sustained attention using SART – or Sustained Attention to Response Task (n=21, within-subject design). They found that the decrease in performance over time, typically observed in this kind of task as an increase in  error rate, was lessened after the ingestion of caffeine alone, theanine alone or the combination of both. In addition, there were reaction time improvements in the caffeine-only condition. Analyzing the ongoing EEG they reported no difference in alpha activity between the theanine-only and the placebo condition but observed decreases in tonic (background) alpha in both the caffeine only and the combined caffeine-theanine condition as in their earlier study, especially during the 1st hour of the task. These results provisionally suggest that theanine on its own isn’t sufficient to alter ongoing alpha activity or attentional performance when people are required to focus their attention.

Confounding factors.

The contrasting results may arise from a number of confounds.

One of these is dosage. Although many studies try to match the theanine dosage  to a “standard cup of tea”, this depends on mode of preparation and type of tea and, according to some estimates, can range from 24.2 +/- 5.7 mg (for black tea) and 7.9 +/- 3.8 mg (for green tea). Selecting the most appropriate dosage is therefore not straightforward and can partly explain conflicting results across different experimental studies.

Second is the recruitment of participants.  If indeed the effects depend on the predisposition of the person to anxiety , and perhaps other unknown aspects of one’s state of mind, then small sample sizes could yield rather different results depending on who participated making large samples sizes and more extensive controls extremely important (read Human Brains and the Control Issue).

Another consideration is the tea and coffee drinking habits of the participants and the decision to ask people to refrain from drinking tea and coffee in the 24/48 hour period prior to the experimental study. Although the screening that took place in the SART study was more comprehensive than previous studies, they choose to prioritize participants who did not drink coffee or tea, initiating a debate about whether it is better to include tea and coffee drinkers who may be more used to their active substances but who might exhibit a “withdrawal effect” from having to abstain immediate prior to the experient, or individuals who never drink coffee or tea and so aren’t so accustomed, at a physiological level, to their active ingredients – both of which could lead to bias in results.

Understanding the way our dietary choices influence our brain response is critical for being able to make optimal choices to maximize our brain’s operating potential. But such an endeavor requires careful consideration of all the experimental variables, confounds and participant factors to ensure methodological consistency across different studies and research groups.