Story
L-Theanine
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In one pass The calm, unhurried feeling after a cup of tea is often credited to one amino acid in it: L-theanine. Not this — L-theanine = natural Xanax — Its main effect is easing the jitters caffeine causes; there is no strong evidence for it on its own as an insomnia treatment.
Educational content, not medical advice — consult a clinician.
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Chapter 1
What L-theanine is
The calm, unhurried feeling after a cup of tea is often credited to one amino acid in it: L-theanine. It is found almost only in the tea plant (Camellia sinensis), and the body does not use it to build protein, which is why it is called a non-protein amino acid.
Its shape resembles two of the brain's most basic signaling molecules: glutamate, which excites nerve cells, and (gamma-aminobutyric acid), which quiets them. Because it looks like an amino acid, it can ride a carrier in the blood-brain barrier that ferries large amino acids from the blood into the brain, called LAT1; that step comes from animal studies. In people who swallow it, the level in the blood peaks roughly 30-50 minutes later.
What matters for you: tea really does contain it, but a cup of tea holds more caffeine than theanine, so the feeling of drinking tea cannot be credited to theanine alone.
Its shape resembles two of the brain's most basic signaling molecules: glutamate, which excites nerve cells, and (gamma-aminobutyric acid), which quiets them. Because it looks like an amino acid, it can ride a carrier in the blood-brain barrier that ferries large amino acids from the blood into the brain, called LAT1; that step comes from animal studies. In people who swallow it, the level in the blood peaks roughly 30-50 minutes later.
What matters for you: tea really does contain it, but a cup of tea holds more caffeine than theanine, so the feeling of drinking tea cannot be credited to theanine alone.
Background · How much is in tea, and what is in a pill
How much is in teaTheanine makes up about 1-2% of the dry weight of tea leaves.Shade-grown teas such as matcha and gyokuro have the most. Sunlight makes the tea plant turn theanine into material for catechins; shading keeps more of it as theanine.Black and oolong teas contain it too.How much ends up in a brewed cup depends on the leaf and the brewing, and steeping time is the biggest variable. Either way, it is far less than the 100-200 mg that supplements and trials commonly use.
What is in a supplement
Theanine comes in two mirror-image forms, L and D, and the tea plant makes the L form. The trials in this story all used the L form. Chemically synthesized products may contain some of the D form, and there are few human data on whether the D form acts the same way. So check that the label says L-theanine, not just theanine.
One common standardized L-form ingredient is Suntheanine, made by the Japanese company Taiyo Kagaku. Employees of that company were among the authors of both Kobayashi 1998 and Hidese 2019, which this story cites, and the company supplied the tablets for the Hidese trial. That does not make the findings wrong, but it is worth knowing when you read those two studies.
Do not confuse it with tea polyphenols
The other tea compounds people often mention are catechins (including ). They are tea polyphenols, not theanine. Polyphenols bring astringency; theanine brings a savory, umami taste. Marketing copy often blends the two into one line, "green tea improves focus", but these are two different molecules, and they have been studied separately.
keenan-2011-theanine-per-cup
Chapter 2
What it does in the brain
What theanine does once it reaches the brain has been seen directly in people in only one way: changes in alpha waves on an EEG, the recording of the brain's electrical rhythms. The deeper receptor effects come from animal and cell studies.
Alpha waves, a rhythm of 8-12 cycles per second, are strongest when a person is awake, quiet, and resting with eyes closed. The first report linking theanine to alpha waves was Kobayashi 1998. Eight female university students took part: the 4 highest and the 4 lowest scorers on an anxiety questionnaire. In separate weeks they drank plain water, water with 50 mg of theanine, and water with 200 mg of theanine, and each session was recorded for 60 minutes. After 200 mg, alpha waves increased over the back of the head (the occipital and parietal regions), mainly in the second half hour, and more so in the high-anxiety group; the 50 mg dose gave no such clear rise.
This was a small study of 8 people, with a fixed order and plain water as the comparison, so it shows only that the effect exists. Its direction also depends on the task: in Kelly 2008, when people did an attention task after theanine plus caffeine, overall alpha power went down, not up. So "more alpha means relaxed focus" cannot be used as a universal fingerprint.
Alpha waves, a rhythm of 8-12 cycles per second, are strongest when a person is awake, quiet, and resting with eyes closed. The first report linking theanine to alpha waves was Kobayashi 1998. Eight female university students took part: the 4 highest and the 4 lowest scorers on an anxiety questionnaire. In separate weeks they drank plain water, water with 50 mg of theanine, and water with 200 mg of theanine, and each session was recorded for 60 minutes. After 200 mg, alpha waves increased over the back of the head (the occipital and parietal regions), mainly in the second half hour, and more so in the high-anxiety group; the 50 mg dose gave no such clear rise.
This was a small study of 8 people, with a fixed order and plain water as the comparison, so it shows only that the effect exists. Its direction also depends on the task: in Kelly 2008, when people did an attention task after theanine plus caffeine, overall alpha power went down, not up. So "more alpha means relaxed focus" cannot be used as a universal fingerprint.
Mechanism · What it does at the receptors
Glutamate receptors: weak binding measured in rat neuronsGlutamate is the brain's main accelerator signal. It acts through several kinds of receptors called AMPA, kainate, and NMDA receptors. Kakuda 2002 tested theanine on neurons from the rat cerebral cortex using radioactively labeled binding assays; the study was originally trying to explain why theanine protects nerve cells in models of brain ischemia (brain tissue cut off from its blood supply):
Theanine bound all three receptor types (for the NMDA receptor, the test looked at its glycine site).Its binding was far weaker than glutamate's own, by a factor of 80 to 30,000.A binding assay shows only that it occupies the site; it cannot tell whether theanine blocks the receptor there or partly activates it.
Following this mechanism, theanine looks more like a light tap on the brake against excess glutamate signaling than a hard block of NMDA receptors like ketamine; that is also a common explanation for why it does not make people sleepy. But this step has not been measured directly in the human brain. After tea or a supplement, nobody has measured whether theanine in the human brain reaches a level that could actually occupy these receptors.
and other signaling molecules: animals only
In rat studies, theanine also changes the levels of several signaling molecules in the brain, such as dopamine measured by microdialysis in the striatum; a rise in GABA, the signaling molecule that quiets nerve cells, also comes from animal work. Whether serotonin (, a signaling molecule involved in mood and sleep) goes up or down differs between rat studies. Whether any of this happens in people is unclear, because human trials almost all measure behavior and questionnaires, not brain tissue.
It is not a sedative
Benzodiazepines (such as diazepam) and alcohol both directly enhance the GABA-A receptor, the brain's main brake receptor, so at high enough doses they pull a person all the way down into drowsiness. Theanine does not act directly on this receptor, which is why the usual description is relaxed but not dulled. antidepressants are something else again: they block the recycling of serotonin back into nerve endings rather than activating a receptor, and theanine does not work that way either.
Put together: theanine acts only weakly on the glutamate system, alpha-wave changes have been measured only in small samples, and the usual description of its effect is mild and not sedating. On its own it usually falls short of an anti-anxiety drug; the evidence is in the later chapter on whether it helps anxiety or sleep when taken alone.
Chapter 3
Taking it with caffeine
The use of theanine with the most evidence is not taking it alone but taking it with caffeine.
In a few small single-dose crossover trials (the same people took different combinations on different days), the pair beat placebo on the speed and accuracy of attention tasks, and in one trial people also rated themselves as having less headache and fatigue; compared with caffeine alone, the advantage was small and inconsistent. The mechanism suggests why: caffeine blocks adenosine, the signal that makes you sleepy, and wakes you up, but it also pushes up heart rate and shakiness; theanine does not act on the adenosine pathway and may pull excess excitation back down a little. The less tense half, however, was not specifically measured in these trials; they mainly measured attention and self-rated alertness and fatigue.
The ratio used most often in the trials is caffeine to theanine 1:2 (50 mg with 100 mg). It came from practice; no trial has compared ratios head to head. A cup of tea contains more caffeine than theanine, so its theanine falls far short of that dose.
In a few small single-dose crossover trials (the same people took different combinations on different days), the pair beat placebo on the speed and accuracy of attention tasks, and in one trial people also rated themselves as having less headache and fatigue; compared with caffeine alone, the advantage was small and inconsistent. The mechanism suggests why: caffeine blocks adenosine, the signal that makes you sleepy, and wakes you up, but it also pushes up heart rate and shakiness; theanine does not act on the adenosine pathway and may pull excess excitation back down a little. The less tense half, however, was not specifically measured in these trials; they mainly measured attention and self-rated alertness and fatigue.
The ratio used most often in the trials is caffeine to theanine 1:2 (50 mg with 100 mg). It came from practice; no trial has compared ratios head to head. A cup of tea contains more caffeine than theanine, so its theanine falls far short of that dose.
Evidence · What each trial actually measured
Behind the phrase a few small trials are the studies below. All tested a single dose over a few hours in healthy adults; the fuller trial-by-trial detail is in the Caffeine + L-Theanine story.Owen 2008 (Nutritional Neuroscience)
27 healthy volunteers in a crossover design: on separate days each took 50 mg of caffeine, 50 mg of caffeine plus 100 mg of theanine, or a placebo.The combination: at 60 minutes, both the speed and the accuracy of attention switching improved, and people were less easily thrown by distracting information in a memory task.This trial had no theanine-alone group, so it cannot answer whether the combination beats each ingredient on its own.
Kelly 2008 (The Journal of Nutrition)
4 test days: placebo, 100 mg of theanine, 50 mg of caffeine, or both together.The task was a cued visual-attention test, with brain waves recorded at the same time.The combination raised both hit rate and discriminability (d′, the ability to tell targets from distractors) compared with placebo; caffeine alone raised only discriminability; theanine alone showed no measurable effect.With the combination, overall alpha power was actually lower; the authors read this as attention being directed more fully at the visual task.
Haskell 2008 (Biological Psychology)
A randomized, placebo-controlled, double-blind, balanced crossover with four conditions: 250 mg of theanine, 150 mg of caffeine, both together, and placebo. This is not the 50 mg plus 100 mg dose.Caffeine alone: faster reactions on a digit-vigilance task, more accurate rapid visual information processing (RVIP), and a smaller rise in mental fatigue.The combination: besides RVIP accuracy and fatigue, faster simple reaction time and faster numeric working-memory reactions, and more accurate sentence checking; people rated themselves as having less headache, feeling less tired, and more alert.Theanine alone: self-rated headache actually rose, and people got fewer answers right when counting down by 7 (serial sevens).
Dietz 2017 (Current Pharmaceutical Design)
A narrative review that describes 49 human intervention studies side by side; it is not a and did not pool effect sizes.Its conclusion: the combination particularly improved attention switching and alertness, but to a lesser extent than caffeine alone, and theanine reduced the arousal caffeine causes.
Read together: only Kelly and Haskell, which included a theanine-alone arm, let the claim that the combination beats each alone be tested, and it held for only some tasks. The evidence is a handful of small single-dose trials pointing the same way, with modest effects. All of them looked only at the few hours after one dose, so they cannot tell you what daily use over the long term does.
In practice · When to take it and in what ratio
Why it might work: a mechanistic argumentCaffeine raises alertness mainly by blocking the adenosine A1 and A2A receptors. The cost is that excitation rises across the body: a faster heart rate, shaky hands, restlessness. Theanine leaves adenosine alone. Following the mechanism, it lightly brakes the glutamate system and may shave off part of this noise while leaving attention intact. The ideal is not 1 + 1 = 2 but keeping caffeine's upside while softening its downside. Read it carefully, though: the trials measured the attention half; the less tense half rests mainly on how people feel and on reasoning.
When to take it
The two reach their peak in the blood at similar times after swallowing: roughly 30-60 minutes for caffeine and 30-50 minutes for theanine. The trials gave both at once, which is also the simplest approach. Some people drink coffee first and add theanine once the jitters show up; that approach has not been compared in trials.
Why 1:2 comes up so often
The trials most often used 50 mg of caffeine with 100 mg of theanine; Haskell 2008 used 150 mg of caffeine with 250 mg of theanine, which is not exactly 1:2 either.A common claim online is that at 1:1 the theanine cannot hold back caffeine's tension, and at 1:4 (four times as much theanine as caffeine) it dampens alertness too. These are users' impressions and mechanistic guesses; no trial has compared ratios head to head.Some people scale up to 100 mg of caffeine with 200 mg of theanine. Push the caffeine higher and its own side effects (palpitations, shakiness, poorer sleep) become more and more noticeable.
Compared with just drinking more tea
A cup of tea contains more caffeine than theanine, so tea gives you a natural combination, but its theanine is far below the 100 mg used in the trials. Tea is often felt to be gentler than coffee; less caffeine is one reason, and theanine may be another. To reach the trial dose of theanine, one cup of tea is not enough, which is one of the few places where a supplement actually fills a gap.
Chapter 4
Alone, does it help anxiety or sleep?
Taken alone, theanine shows a small signal for anxiety and sleep, but the evidence is thin and far weaker than the marketing suggests.
The most-cited trial is Hidese 2019: 30 adults with no major psychiatric illness took 200 mg of theanine every night before bed for 4 weeks and a placebo for another 4 weeks, in a crossover design. During the theanine weeks, scores on depression, anxiety, and sleep questionnaires all fell. But compared with the placebo weeks, only three small parts of the sleep questionnaire differed statistically (how long it took to fall asleep, how often sleep was disturbed, and use of sleep medication), and each only just crossed the line.
What this means for you: it is not a sleeping pill, and it is not an anti-anxiety drug. If your mind simply will not settle at bedtime, it may help a little; if you have an anxiety disorder or insomnia, it cannot replace proper treatment.
The most-cited trial is Hidese 2019: 30 adults with no major psychiatric illness took 200 mg of theanine every night before bed for 4 weeks and a placebo for another 4 weeks, in a crossover design. During the theanine weeks, scores on depression, anxiety, and sleep questionnaires all fell. But compared with the placebo weeks, only three small parts of the sleep questionnaire differed statistically (how long it took to fall asleep, how often sleep was disturbed, and use of sleep medication), and each only just crossed the line.
What this means for you: it is not a sleeping pill, and it is not an anti-anxiety drug. If your mind simply will not settle at bedtime, it may help a little; if you have an anxiety disorder or insomnia, it cannot replace proper treatment.
Evidence · What the 30-person trial found
How Hidese 2019 (Nutrients) was done30 adults with no major psychiatric illness.A randomized, double-blind, placebo-controlled crossover trial: everyone took 4 weeks of theanine (200 mg every night before bed) and 4 weeks of placebo, in random order, with a break in between.The theanine and placebo tablets were supplied by the manufacturer, Taiyo Kagaku, and two of the authors were employees of that company.
What it found
Within the theanine weeks, scores on a depression scale (SDS), a trait-anxiety scale (STAI-T, which measures how anxiety-prone a person generally is), and the Pittsburgh Sleep Quality Index (PSQI) all fell from where they started.But the placebo weeks have to be read alongside. Comparing the two, only three PSQI sub-scores (time to fall asleep, sleep disturbance, and use of sleep medication) improved clearly more, and each sat right at the edge of statistical significance. The depression scale and the PSQI total showed only a trend in that direction, and the anxiety scale showed no difference.On cognitive tests, compared with placebo, only the half of participants with poorer starting scores improved in verbal fluency; the Stroop test and the other tests showed no difference.
The honest reading: in adults without psychiatric illness who simply feel some stress, 200 mg a night for 4 weeks produced small improvements on a few sleep-questionnaire items. That is not evidence that it treats anxiety: the sample was small, the trial was run at a single center, and no larger trial has replicated it.
Other trials of theanine alone
Most other trials of theanine alone are also small and short, with doses ranging from 200 to 400 mg, and they show no clear relationship between dose and effect.The Dietz 2017 review concluded that theanine alone, from 200 mg upward, improved self-rated relaxation, tension, and calmness. Those are self-rated feelings, not clinical outcomes for an anxiety disorder.
Why it cannot be compared directly with anti-anxiety drugs
The common online claim that it is a natural anti-anxiety drug does not hold up. Anti-anxiety drugs and antidepressants were shown to beat placebo in patients diagnosed with anxiety disorders. None of the theanine studies cited in this story was done in patients with an anxiety disorder, and none compared theanine with an anti-anxiety drug. With different scales and different populations, the scores cannot simply be subtracted. What can be said is that the evidence for theanine is nowhere near strong enough to replace medication, and anyone already in treatment should not switch to it on their own.
Clinical · Which kind of poor sleep it might help
A few clarifications about sleepTheanine is not a sleeping pill. It does not send the body a night has fallen signal the way melatonin does, and it does not pull consciousness down the way benzodiazepines do. In Hidese 2019, what differed from placebo were three self-rated items: time to fall asleep, sleep disturbance, and use of sleep medication. The trial did not use (an overnight recording of brain waves, breathing, and more), so it cannot say whether theanine changes the balance of deep sleep and dreaming sleep.
If your problem is the body is tired but the mind will not stop, it may help a little. If you keep waking early, wake in the night and cannot get back to sleep, or snore and gasp for breath at night and feel very sleepy in the daytime (common signs of sleep apnea), it basically cannot help; those need a doctor to find the cause.
Why the solo effect is hard to detect but the caffeine combination is easier
One explanation: in solo trials, theanine has to push an ordinary alert person a little further toward relaxed, and that person is already far from over-excited, so there is little room to move. In combination trials, caffeine first pushes people toward the excited side, so theanine has further to pull them back, and the change is easier to measure. This explanation is plausible but has not been tested directly. Another explanation is just as plausible: much of the benefit seen with the combination was caffeine's to begin with. In Kelly 2008 and Haskell 2008, theanine alone showed no benefit.
Chapter 5
Who should try it
Theanine is a cheap adjunct that looks safe so far: weak and thinly supported on its own, and linked to better attention in a few small trials when combined with caffeine. It is not a treatment, and it is not the natural smart drug of the marketing.
The best fit is wanting caffeine's alertness without its palpitations and shaky hands, or, when you feel like one more afternoon coffee, switching to a small dose of caffeine plus theanine; less caffeine also means less effect on that night's sleep. The worst use is substituting it for anti-anxiety medication, sleep medication, or medication for attention-deficit/hyperactivity disorder (ADHD). If you already take these and want to switch or cut back, ask your doctor first.
The best fit is wanting caffeine's alertness without its palpitations and shaky hands, or, when you feel like one more afternoon coffee, switching to a small dose of caffeine plus theanine; less caffeine also means less effect on that night's sleep. The worst use is substituting it for anti-anxiety medication, sleep medication, or medication for attention-deficit/hyperactivity disorder (ADHD). If you already take these and want to switch or cut back, ask your doctor first.
In practice · Who might try it, and who should not bother
Overall, theanine's place is this: a cheap adjunct that looks safe so far, with limited effects on its own and small-scale evidence for attention when combined with caffeine. Below, the situations where it fits and where it does not.Worth a try
You fear caffeine's shaky hands and racing heart but want to keep the focus: this is its best fit. Take 50 mg of caffeine with 100 mg of theanine together; 50 mg of caffeine is about one strong cup of tea or half a cup of black coffee, plus one 100 mg theanine capsule. The trials measured small improvements on attention tasks; alert but not jittery is a common impression that has not been specifically measured.Long stretches of mental work such as exams, coding, or writing: people who need 2-4 hours of sustained attention and do not want one big coffee can try the small-dose combination. Whether two small doses feel smoother than one large one has not been compared in trials.A mind that will not settle at bedtime, short of an insomnia disorder: take 200 mg before bed (the dose and timing in the Hidese trial) for 2-4 weeks and see whether you fall asleep faster. If you have ongoing insomnia, early waking, or sleep apnea, do not pin your hopes on it.When you want one more coffee in the afternoon: switch to a small-dose combination (for example 50 mg of caffeine with 100 mg of theanine). With less total caffeine, the effect on falling asleep that night should also be smaller.
Poor fits: do not waste your money
As an anti-anxiety drug: the solo evidence comes only from people without psychiatric illness, on a few questionnaire items; it has not been shown to work in patients with an anxiety disorder. Using it in place of proper treatment leads to disappointment and delays care.As a natural stimulant: theanine alone does not wake you up. In Kelly 2008, theanine alone showed no measurable effect, and in Haskell 2008, self-rated headache actually rose with theanine alone. Without caffeine, it will not make you more awake.As a treatment for attention-deficit/hyperactivity disorder (ADHD): no trial has shown that it can replace ADHD medication, so do not swap it for the plan your doctor prescribed.As a sleeping pill: at most it helps a little with how long it takes to fall asleep; a real sleep disorder needs its cause found first.If you already use benzodiazepines, Z-drug sleeping pills (the zolpidem class), or heavy drinking to get to sleep: theanine cannot replace them and cannot help you cut down safely. If you want to reduce them, talk to your doctor first.
Safety · Dose, products, and known risks
DoseHuman trials mostly used 100-400 mg a day:
100 mg: the usual dose combined with 50 mg of caffeine (Owen 2008, Kelly 2008).200 mg: the solo dose in Hidese 2019, taken once each night before bed.250 mg: the single dose in Haskell 2008.400 mg: used in some solo trials; the existing small trials do not show it working better than 200 mg.
Products
Clinical studies used the L form of theanine, much of it a standardized ingredient from one Japanese manufacturer (sold as Suntheanine). When choosing a product, check that the label says L-theanine; there are few data on what the D form in synthetic D/L mixtures does in people.
Safety
In Hidese 2019, 4 weeks of nightly use produced no apparent adverse events.But with a single 250 mg dose in Haskell 2008, self-rated headache actually rose; it is not entirely without effects.Human data on drug interactions are scarce (for example, whether it affects CYP450, the liver enzymes that break down many drugs). If you take blood-pressure medication, sedatives or sleeping pills, or psychiatric drugs, ask your doctor or pharmacist first.Human data on long-term continuous use are scarce, so whether tolerance or dependence develops is uncertain.Pregnancy and breastfeeding: there are not enough data; the cautious choice is to skip separate supplements and get it only from tea.People with low blood pressure should be cautious with large doses: animal studies and a few human studies suggest it may lower blood pressure slightly, which is negligible for most people.
Putting it together
Its most valuable role is as a buffer against tension when combined with caffeine; the attention half has trial evidence, while less tension itself rests mostly on how people feel.Its solo effect is small; do not use it in place of anti-anxiety medication, sleeping pills, or ADHD medication.The main road to cognitive health is still the same: 7-9 hours of sleep, regular exercise, a Mediterranean-style diet, and staying socially engaged and learning. Theanine is at most a small tool beside that road.
References · 8
- Kakuda, T., Nozawa, A., Sugimoto, A., & Niino, H. (2002). Inhibition by theanine of binding of [3H]AMPA, [3H]kainate, and [3H]MDL 105,519 to glutamate receptors. Bioscience, Biotechnology, and Biochemistry, 66(12), 2683–2686. Rat cortical neurons: theanine bound the AMPA, kainate and NMDA-glycine sites, but far more weakly than glutamate; the abstract gives its IC50 as 80- to 30,000-fold less than that of L-glutamic acid, which in context means weaker binding (abstract, PMID 12596867). 10.1271/bbb.66.2683
- Scheid, L., Ellinger, S., Alteheld, B., Herholz, H., Ellinger, J., Henn, T., Helfrich, H.-P., & Stehle, P. (2012). Kinetics of L-theanine uptake and metabolism in healthy participants are comparable after ingestion of L-theanine via capsules and green tea. The Journal of Nutrition, 142(12), 2091-2096. Randomized crossover pharmacokinetic study: 12 participants took 100 mg L-theanine as capsules or as green tea (3 more took 50 and 200 mg capsules). Plasma L-theanine peaked 0.8 h after intake with both (24.3 and 26.5 µmol/L), AUC rose with dose, and theanine appeared to be hydrolyzed to ethylamine and glutamic acid; capsule and tea kinetics were comparable (abstract, PMID 23096008). 10.3945/jn.112.166371
- Kobayashi, K., Nagato, Y., Aoi, N., Juneja, L. R., Kim, M., Yamamoto, T., & Sugimoto, S. (1998). Effects of L-theanine on the release of α-brain waves in human volunteers. Nippon Nōgeikagaku Kaishi (Journal of the Agricultural Chemical Society of Japan), 72(2), 153–157. J-STAGE abstract (Japanese): 8 female students chosen with the Manifest Anxiety Scale, 4 high-anxiety and 4 low-anxiety; after 200 mg L-theanine in 100 mL water, alpha waves appeared over the occipital and parietal regions from about 40 min and were thought to last at least 2 h; the relaxation effect was greater in the high-anxiety group; no drowsiness was reported. The abstract describes no placebo condition and reports only the 200 mg dose. Six of the seven authors are from the Taiyo Kagaku Co. research institute (abstract and author affiliations, J-STAGE). 10.1271/nogeikagaku1924.72.153
- Kelly, S. P., Gomez-Ramirez, M., Montesi, J. L., & Foxe, J. J. (2008). L-theanine and caffeine in combination affect human cognition as evidenced by oscillatory alpha-band activity and attention task performance. The Journal of Nutrition, 138(8), 1572S–1577S. Note: despite the id, published in The Journal of Nutrition. Four test days: placebo, 100 mg L-theanine, 50 mg caffeine, or both. The combination raised hit rate and target discriminability (d') vs placebo; caffeine alone raised d' but not hit rate; no effects were detected for L-theanine alone. EEG showed LOWER overall tonic alpha power with the combination, not more phasic alpha biasing (abstract, PMID 18641209). 10.1093/jn/138.8.1572S
- Haskell, C. F., Kennedy, D. O., Milne, A. L., Wesnes, K. A., & Scholey, A. B. (2008). The effects of L-theanine, caffeine and their combination on cognition and mood. Biological Psychology, 77(2), 113–122. 10.1016/j.biopsycho.2007.09.008
- Owen, G. N., Parnell, H., De Bruin, E. A., & Rycroft, J. A. (2008). The combined effects of L-theanine and caffeine on cognitive performance and mood. Nutritional Neuroscience, 11(4), 193–198. Note: despite the id, published in Nutritional Neuroscience. 27 healthy volunteers; 50 mg caffeine with or without 100 mg L-theanine vs placebo, with a 7-day washout - there was no L-theanine-alone arm. Caffeine alone improved subjective alertness at 60 min and attention-switching accuracy at 90 min; the combination improved attention-switching speed and accuracy at 60 min and reduced susceptibility to distraction in the memory task at 60 and 90 min (abstract, PMID 18681988). 10.1179/147683008X301513
- Dietz, C., & Dekker, M. (2017). Effect of green tea phytochemicals on mood and cognition. Current Pharmaceutical Design, 23(19), 2876–2905. A narrative review of 49 human intervention studies, not a pooled analysis despite the id. Caffeine improved demanding long-duration tasks and self-rated alertness from as little as 40 mg; L-theanine alone improved self-reported relaxation, tension and calmness from 200 mg; the combination improved attention switching and alertness, but less than caffeine alone; EGCG was inconclusive (abstract, PMID 28056735). 10.2174/1381612823666170105151800
- Hidese, S., Ogawa, S., Ota, M., Ishida, I., Yasukawa, Z., Ozeki, M., & Kunugi, H. (2019). Effects of L-theanine administration on stress-related symptoms and cognitive functions in healthy adults: A randomized controlled trial. Nutrients, 11(10), 2362. Randomized, placebo-controlled, double-blind crossover: 30 adults (9 men, 21 women; mean age 48.3) without major psychiatric illness, L-theanine 200 mg/day or placebo for 4 weeks each, taken before sleep each night. Depression, trait anxiety (STAI-T) and PSQI scores fell after L-theanine, but these are before-after changes; against placebo only the PSQI sleep-latency, sleep-disturbance and sleep-medication subscales (all p < 0.05) and, in the lower-performing half, verbal (letter) fluency differed. The authors note that only about 20% of the scores changed vs placebo. Two authors are employees of Taiyo Kagaku, which supplied the tablets (abstract and COI, PMID 31623400; full text, PMC6836118). 10.3390/nu11102362