The Science of L-Theanine, A Dietary Supplement that Relieves Stress, Improves Focus, and Enhances Sleep

Do you sometimes feel stressed out or anxious? Perhaps you occasionally struggle to focus at work? Or are there times when, despite your best efforts, a good night’s sleep eludes you? If your answer to any of these is yes, you might find today’s blog helpful. In it, we’re diving into the science of L-theanine, a well-studied substance in tea that has an impressively large array of beneficial effects, hence why we include it in Energise and Energise & Rebuild versions of Long Range Fuel.

This blog gets nerdy in places, so I’ve summarised the important stuff below, including how you can use this special substance to sharpen your mind, unwind, and sleep better.


Key Takeaways

  • L-theanine seems to exert its effects in part by reducing glutamatergic signaling and increasing dopaminergic, serotonergic, noradrenergic, and GABAergic signaling in certain brain regions.
  • Consuming 200 to 400 mg L-theanine per day (as 1 or 2 doses) consistently reduces stress and anxiety responses in people exposed to stressful conditions. Take the first dose about 1 h before the stressful event.
  • Consuming 200 to 900 mg L-theanine per day (as 1 or 2 doses) may improve subjective sleep quality and reduce night-time awakenings. Take the final dose about 1 h before bedtime.
  • Consuming 100 to 200 mg L-theanine per day (as 1 or 2 doses) helps people stay calm and attentive during cognitive tasks. Take the first dose about 1 h before the cognitive task.
  • L-theanine and caffeine have some complementary effects on alertness and the ability to deliberately switch attention during tasks.
  • L-theanine appears to support immune function and detoxification by acting as a precursor for glutathione, a key antioxidant.
  • L-theanine might be good for other aspects of health, including cardiovascular function.
  • L-theanine is perfectly safe and well tolerated.


What is L-theanine?

L-theanine (2-amino-4-(ethylcarbamoyl)butyric acid / γ-glutamyl-L-ethylamide γ-glutamylethylamide / N-ethyl-L-glutamine) is a water-soluble amino acid that makes up nearly 50% of amino acids in tea. There’s typically 25 to 60 mg L-theanine in 200 ml green tea - more than black or oolong tea because fermentation of these teas reduces L-theanine content – and L-theanine contributes to the slightly sweet and strong umami taste of tea.

If you find that drinking coffee sometimes makes you feel jittery but drinking tea is a more calming experience, could this be due to the L-theanine in tea?  

Before we get to that, let’s explore what L-theanine does in our bodies.


L-theanine’s main mechanisms of action in the brain

Nobody truly understands all the pathways by which ingredients such as L-theanine affect us, and much of our knowledge of these mechanisms comes from studies of non-human animals… so bear in mind that humans are a bit different from rodents! Nonetheless, let’s explore what studies of other animals reveal about how L-theanine might affect us apes.

After ingestion, L-theanine is transported through the intestinal brush border by neutral amino acid systems. In humans, this occurs perhaps 10 to 24 mins after consumption, with L-theanine then reaching its maximal concentration in the blood 30 to 50 mins after ingestion.

In the blood, L-theanine eventually reaches the blood-brain barrier and crosses it via the amino acid transport system L. Now in the brain, L-theanine influences glutaminergic and glutamatergic signalling because it’s structurally like L-glutamine and L-glutamate. Specifically, L-theanine seems to inhibit neuronal glutamine uptake, meaning less glutamine can be converted to glutamate by neuronal mitochondria. L-theanine therefore reduces synaptic glutamate levels. This is significant, for glutamate is the most abundant “excitatory” neuromodulator in your brain, meaning that glutamate generally increases the likelihood that an electrical signal will be passed down the neuron it is acting on. While in many instances this type of excitation is helpful, excessive glutamate release from neurons leads to rapid calcium influx to cells, ultimately causing neurons to undergo programmed cell death. 

Not ideal.

L-theanine affects other brain chemicals too. In mice, for instance, L-theanine stimulates (“agonises”) certain receptors of some brain cells, leading to release of GABA. This might help protect the brain against damage when blood flow is compromised. Given the key roles of GABA in relaxation and sleep, it seems plausible that this effect on GABA might contribute to the stress-relieving, sleep-enhancing effects of L-theanine too.

L-theanine also seems to influence signaling of the monoamines serotonin, dopamine, and noradrenaline. To produce depression-like behaviour in rats, scientists sometimes impose chronic, unpredictable stressors. Under such conditions, L-theanine has antidepressant-like effects that coincide with increases in monoamine signaling in brain regions including the hippocampus (which is particularly important to learning and memory), nucleus accumbens (which is key to processing rewarding stimuli such as food and sex), and prefrontal cortex (which is central to goal-directed behaviour). So, these effects on monoamines might contribute to any effects of L-theanine intake on cognition.  

Okay, with that neurochemistry out the way, let’s turn to what we know about how L-theanine intake affects human health and behaviour.


Effects of L-theanine on stress and anxiety

Last year, scientists in Australia systematically reviewed research on how L-theanine supplementation affects stress and anxiety. Six studies assessed acute effects of consumption of 200 mg L-theanine (that’s the amount in a 100 g Energise or Energise & Rebuild Long Range Fuel), and 3 studies had participants take between 400 and 900 mg L-theanine each day for between 2.5 and 8 weeks. The studies were all placebo controlled, and most of them assessed both feelings of stress and anxiety, as well as physiological correlates of these states, such as heart rate, heart rate variability, blood pressure, cortisol (a hormone involved in the stress response), and salivary alpha amylase (an enzyme secreted in response to stress hormones).

The results?

The scientists found that 200 to 400 mg L-theanine per day (as 1 or 2 doses) consistently reduces stress and anxiety responses in people exposed to stressful conditions. Importantly, in the 8-week long studies, side effects in the L-theanine groups did not differ from the placebo groups, showing that L-theanine seems to be perfectly safe and well tolerated, even when taken for months.

So, if you know you have a stressful event coming up and want to try L-theanine, I’d take the amount shown to be effective by this analysis. Take the first dose about 1 h before the event, although I’ll add that the timing of L-theanine intake hasn’t been well studied.


Effects of L-theanine on sleep

As stress and rumination can ruin sleep, you might expect that L-theanine intake can support healthy sleep. Sure enough, this seems to be the case.

One of the studies included in the review above found that 450 to 900 mg L-theanine per day for 8 weeks improved how satisfied people were with their sleep in adults with generalised anxiety disorder.

L-theanine might be helpful in other clinical populations too, for 250 mg L-theanine per day for 8 weeks improved sleep quality in schizophrenia patients.

What’s more, 100 mg L-theanine twice daily for 6 weeks reduced overnight awakenings in 8- to 12-year-old boys with ADHD.

But what about if you’re healthy? 

A study of healthy middle-aged Japanese adults found that 200 mg L-theanine per day for 4 weeks reduced time taken to fall asleep, minimised sleep disturbances, and reduced reliance on other sleep aids. Interestingly, participants’ feelings of depression and anxiety declined, and measures of verbal fluency (which tests how many words of a given type you can list in a limited time) and executive function (which influences performance in goal-directed tasks) improved.

Based on these studies, if your sleep quality is poor, try 200 to 900 mg L-theanine per day (as 1 or 2 doses), with the final dose about 1 h before bedtime.


Effects of L-theanine on cognition and mood

Of all the effects of L-theanine, cognition has been particularly well studied. Several research groups have reported that consuming 50 to 250 mg L-theanine changes patterns of brain electrical activity (measured via electrodes on the scalp) in ways that would be expected to support the ability to maintain attention during cognitive tasks. Since these studies were done, many more have explored the effects of L-theanine on brain function.

While now a bit dated, a study published in 2014 analysed all placebo-controlled trials of the effects of L-theanine alone and in combination with caffeine on cognition and mood. This study was a so-called “meta analysis”, meaning that the researchers numerically combined the results of multiple studies in an attempt to uncover what the whole body of research shows. While there weren’t enough data to meta analyse the effects of L-theanine in isolation, the studies included seemed to show the following: 

  • L-theanine might not affect baseline anxiety much, but it does seem to reduce anxiety arising in response to stressors.
  • L-theanine seems to increase calmness in the first 2 h after intake.
  • L-theanine goes well with caffeine: combining the two can provide a statistically small-moderate boost in alertness and ability to deliberately switch attention.

I won’t go into all the relevant studies, but since this analysis came out, these findings have generally been backed up by newer research. For example, scientists found that 160 mg caffeine and 200 mg L-theanine additively enhance attention in a vision task, an effect recapitulated by more recent research. Furthermore, the combination of caffeine (2 mg per kg bodyweight) and L-theanine (2.5 mg per kg bodyweight) improved overall cognitive function in children with ADHD, suggesting this combo might be a useful alternative to the often-problematic drugs commonly used to treat ADHD.

It’s also worth bearing in mind that several studies have shown that L-theanine alone improves some aspects of cognition, including reaction time and working memory. So, if you’re avoiding stimulants for whatever reason, you might still find L-theanine helpful at work.

Please note that not all studies have found caffeine and L-theanine have additive effects on cognition. For example, after people were exposed to negative film clips and photos, 200 mg caffeine and 200 mg L-theanine had opposite effects on certain aspects of attention. Other research reported that 50 mg L-theanine countered the effects of 75 mg L-theanine on attention and mood.

Overall, my interpretation of this literature is that the optimal combination of caffeine and L-theanine isn’t yet clear and may well depend on factors including task, bodyweight, clinical conditions, recent sleep, and more. Nevertheless, I think 100 to 200 mg L-theanine per day (as 1 or 2 doses) is a reasonable starting point if you want to be calm and focused, with the first dose about 1 h before the cognitive task. You should check out our blog on caffeine for more on the best dose of caffeine to use.


Some other plausible effects of L-theanine: countering high blood pressure, bolstering immune function, and more

While most research on L-theanine has focused on the brain, this amino acid actually supports a wide range of bodily systems. Let’s take a whistle-stop tour through some of these actions.

Beginning with the cardiovascular system, L-theanine seems to minimise blood pressure and heart rate spikes following exposure to certain stressors, as I touched on above. I won’t go into details here, but studies of non-human animals have started to tease out some of the mechanisms underlying these effects (e.g., enhanced endothelial nitric oxide production), as well as other cardioprotective properties that are relevant to disease processes such as atherosclerosis.

Moving to the immune system, L-theanine is metabolised to glutamic acid (mainly in the liver), and by acting as a glutamic acid donor, L-theanine supports production of glutathione, a “master” antioxidant involved in recycling other antioxidants (e.g., vitamin C), repairing DNA, and more. As combining L-theanine supplementation with cysteine supplementation is thought to additively boost glutathione synthesis, several studies of humans have combined L-theanine (70 to 280 mg per day) with cysteine (175 to 700 mg per day). Other studies have explored L-theanine’s isolated actions on immune function at doses of 150 to 200 mg per day. Overall, these studies haven’t shown dramatic effects on immune function, but some of the more intriguing results include decreased incidence of the common cold and reduced exercise-induced changes in immune system parameters. Participants in both of these studies took 280 mg L-theanine and 700 mg cysteine per day.

Given L-theanine’s possible immune-strengthening activities, it’s no surprise that many studies of mice and rats have found that L-theanine protects these animals against the harmful effects of a range of toxicants, including ethanol, carbon tetrachloride, polychlorinated biphenyls, aluminium chloride, cadmium, and more. It’s also intuitive that L-theanine might help protect the brain against insults, and studies of other animals have revealed that L-theanine counters various facets of neurodegeneration in these creatures.

Staying on the subject of dangerous chemicals but reverting to human beings, an intriguing study found that lacing the cigarette filters of smokers with tea reduced cigarette consumption by 57% over 2 months and helped 32% of smokers quit smoking. While very speculative, some people believe this might be through L-theanine in the tea blocking (“antagonising”) nicotine acetylcholine receptors in the brain.

Finally, going by studies of rodents, there could be other valuable effects of L-theanine on musculoskeletal health (e.g., anti-arthritis actions), metabolism (e.g., white adipose tissue browning and associated mitigation of diet-induced obesity), gastrointestinal health (e.g., increased short-chain fatty acid production), respiratory health (e.g., reduced lung inflammation and mucus production in asthma), genitourinary health (e.g., improved bladder function), and more.


Which L-theanine is best?

Chemically, L-theanine is L-theanine, regardless of its source. L-theanine can be isolated from tea leaves, but this approach yields little L-theanine and is expensive. Another option is to chemically synthesise L-theanine. However, synthesis generates some toxic waste chemicals, and some synthetic theanine contains a mix of L-theanine and D-theanine (tea only contains L-theanine), which isn’t ideal because these two forms compete for absorption and are metabolised slightly differently. Then there is enzymatic synthesis, which in our view is the best of the bunch for it produces pure L-theanine and does so efficiently.

So, if price is not a consideration, you could use L-theanine extracted from tea. But this option comes with a premium price tag that doesn't correspond to better results.


Can I take too much L-theanine?

L-theanine is metabolised to ethylamine and glutamic acid and is then excreted in urine, with a very small amount temporarily being temporarily retained in red blood cells. It therefore has little potential to accumulate in your body.

To my knowledge, there’s no evidence that L-theanine is toxic in humans, even if there are reports of liver injury following tea intake... please remember that tea contains lots of bioactive constituents!

The US Food and Drug Administration recommends that total daily L-theanine intake be no higher than 1,200 mg, but this is not based on toxicity studies. To my knowledge, the highest-dose study of L-theanine had people take 450 to 900 mg L-theanine per day for 8 weeks, and the participants didn’t experience any different side effects from the placebo group. Perhaps the highest-dose study of non-human animals had rats eat 4 g L-theanine per kg bodyweight for 13 weeks, and the rats experienced zero side effects, going by the comprehensive measures assessed.

Basically, L-theanine is very safe stuff!



L-theanine is not magic. But it sure can be helpful, hence why we include it in Energise and Energise & Rebuild Long Range Fuel.

Hopefully this blog contains about as much relevant info as you could want to know about L-theanine, but if you still have any questions, do get in touch via Instagram!