Does L-Theanine Really Boost Cognitive Function? | Scientific Studies and Evidence

Herbal medicine has been around for as long as humans have hunted and gathered; all medicine is derived from plants. In that vein, it’s important not to neglect the wisdom of centuries of folklore. But it’s equally important to make sure that we research the health claims of certain plants and their compounds for actual effect.

Green tea is full of antioxidants and other beneficial substances, but it can’t fix everything. In this article, I’ll examine one of Green Tea’s foundational compounds, L-Theanine. We’ll look at the scientific literature to find any evidence that actually meets one of its biggest claims: improving cognitive function.

What you need to know

  • L-Theanine is an amino acid, found predominantly in Green Tea.
  • L-Theanine has been proven to help with mood, by calming and focusing the brain.
  • Following research into mood, scientists have also studied its effects on cognitive function, including attention, memory, and verbal fluency.
  • Most of the benefits of L-Theanine are increased by combining it with caffeine, either in Green Tea or in supplements.

Foundational Literature

L-Theanine and Green Tea are nearly inextricably linked; nearly every study of L-Theanine mentions Green Tea, and even a chemical analysis of L-Theanine’s mechanisms can’t fail to mention the tea it comes from.

Most of this literature takes it for granted that Green Tea is full of helpful antioxidants, and even takes it as a given that it helps with mood. This article, however, will focus on the evidence from clinical trials that it can help with cognitive function, either as an ingredient in Nootropic supplements, or by itself.

Clinical Trials of L-Theanine

Rarely do I see such rigorous, transparent data presented for any substance in the Nootropics field. Even the phrase “cognitive function” can mean different things to different people. That’s why I’ve pulled only studies that provide not only realistic measures, but also quantifiable data (reaction times, correct answers on memory tests, etc.), rather than trials that use subjective data (participants “feel” smarter).

Japanese Research

Not surprisingly, much of the literature around L-Theanine comes from cultures that regularly or traditionally used Green Tea for therapeutic or cultural reasons. In Japan, researchers just last year were able to conduct a randomized, double-blind, placebo-controlled study of L-Theanine, using established metrics of Cognitive Function.

The study, which you can read here, studied not only the long-term (12 weeks) but also single-use applications of L-Theanine. Scientists found that the L-Theanine group had significantly better test results in reaction time to tasks, increased correct answers, and fewer omitted answers–all after a single use of L-Theanine.

Scientists in this study used both the Cognitrax and Mini-Mental State exam to determine results, both reliable and repeatable tests. Interestingly, there was no change in the Mini-Mental State numbers between the test groups, while the Cognitrax results were much better with the test group. This indicates that L-Theanine is beneficial for healthy, unimpaired individuals and their cognitive function, rather than for people with degenerative brain illnesses.

Another group of Japanese researchers used mood and cognitive function as outcomes; they, like most other teams, found that L-Theanine improved mood and even sleep quality. They also found better cognitive function, specifically in what they called “verbal fluency.” Interestingly, they found that L-Theanine had the greatest effect on people who scored lower on the pre-trial test–essentially, L-Theanine has the greatest cognitive benefit for people who need it most.

Finally, we have a third study which found that people with bouts of high-anxiety not only get calming effects from L-Theanine, but that those calming effects have a “pronounced” effect on their attention and reaction times. These data, then, combine with the other studies to cover nearly every subgroup of people who may be looking for cognitive benefits from L-Theanine or Green Tea.

Mechanisms of L-Theanine

While clinical and trial data is important–even vital–to the establishment of a substance’s efficacy, it’s not the only bar that researchers need to clear. After a correlation is established (Thing A leads to Effect B), the real hard work of trying to find out how and why gets underway.


A team of Korean researchers was able to isolate in their laboratory several neurons from human brains. During experimentation, they applied L-Theanine directly to the brain cells and found that it actively prevented several neurotoxins from attacking the cells. L-Theanine was so protective of brain cells that researchers concluded it may be helpful in the treatment of Parkinson’s Disease.

Beta-Amyloid Reduction

I’ve written about Beta-Amyloids before (also written as β-Amyloid, and Aβ). It is a natural substance that builds up in mammal brains over time–and then becomes deadly to healthy brain cells. A team of researchers using laboratory animals found that application of L-Theanine stopped the growth of harmful Aβ proteins, prevented healthy protein damage, and had a positive effect on the retention of memory-related brain cells.

Brain Waves

So far we’ve seen the before and after, so to speak. Clinical research has shown that L-Theanine can improve cognitive function, and laboratory research has shown the chemical reactions of L-Theanine with brain tissue. Now we’ll look at the “during,” where scientists were able to actually see the effect of L-Theanine in action.

Scientists not only had participants in a placebo controlled experiment perform cognitive tasks–which the L-Theanine group excelled at compared to the control group–but during the tests, researchers conducted eye and brain scans.

They found that after L-Theanine use, participants had greater cerebral activity in four different areas of the brain–including greater theta waves. All four brain areas, and theta waves specifically, indicate greater memory and attention function.

L-Theanine and Caffeine

Caffeine is a regularly used substance in nearly every culture around the globe. From drip coffee to espresso, from tea to chocolate, caffeine fuels the world. It’s an open secret that it can help focus the mind, especially where there is fatigue or multi-tasking. Often, though, people complain of the jitters, nervousness, or even anxiety when drinking coffee. That’s what has made low-caffeine Green Tea so popular–it not only energizes, but somehow, also calms.

To study the effects of Green Tea’s predominant chemical, L-Theanine, and how it interacts with caffeine, requires rigorous, disciplined methodology. In both of the studies I’ll quote below, researchers had to conduct several rounds of blinded study:

  • participants taking a placebo
  • participants taking only L-Theanine
  • participants taking only caffeine
  • participants taking L-Theanine and caffeine together

Scientists then needed to compare the results of cognitive function tests to determine if there’s a difference between any of the four models. 

In the first study, researchers found that combining L-Theanine and caffeine had better results than either of them separately, specifically on mental tasks requiring speed, accuracy, and attention-switching. They found their results were noticeable even after only 60 minutes from application.

The second study looked at the statistical significance of this effect, and found that combining caffeine and L-Theanine was more effective over the individual substances than those substances were effective over placebo. Put another way, people taking either caffeine or L-Theanine saw a boost; but people taking both saw a boost over the individual groups, and their boost over individual treatment was greater than the individual treatment over none at all.


In scientific research, the Holy Grail is to find causation. It’s one thing to find out that apples fall from trees; it’s another thing entirely to find out why. In this case, researchers from around the globe have bent toward the tasks of not only establishing that L-Theanine can improve cognitive function, but also figuring out how. From protecting neurons in the brain to stimulating brain waves, it appears that L-Theanine can indeed improve memory, recall, accuracy of mental tasks, and attention–especially when multiple tasks are conducted.

These data have not been contradicted in any literature I could find, and all the data established that L-Theanine is safe in every dose measured. Not only that, but later research has established that the efficacy of L-Theanine is increased when combined with one of its natural partners, caffeine. All of these data held constant whether the L-Theanine came from Green Tea or in a supplement form.

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About the Author

Sam is a passionate health and fitness enthusiast who has been interested in supplements, fitness, and wellness for over 10 years. He is the founder of Great Green Wall - the health and wellness brand and has completed multiple fitness certificates, including personal training and nutrition certifications. Sam has been working as a personal trainer for the past three years and is dedicated to helping his clients achieve their fitness goals and lead healthier lifestyles. He believes that a healthy lifestyle is crucial to a happy and fulfilling life and is committed to sharing his knowledge and passion with others.

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