L-theanine: Benefits, uses and side effects

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  • L-theanine is an amino acid that you can obtain from tea. It’s found to have a beneficial impact on mental health, sleep and cognition
  • One of the most well-known benefits of L-theanine is its ability to help promote relaxation, making it a recommended supplement for managing anxiety. Its anxiolytic effect also makes it an ideal sleep agent
  • Read this to discover what L-theanine can do for your health and what to keep in mind before taking this supplement

L-theanine is a compound that may help promote relaxation and improve sleep quality.1 Read on to learn more about the benefits of L-theanine to your well-being, its side effects and what to keep in mind before adding this supplement to your repertoire.

What is L-theanine?

L-theanine is a non-protein amino acid that can cross the blood-brain barrier.2 It is one of the two isomers of theanine — the other one is D-theanine, but L-theanine is the form that is present in supplements and foods. According to the Asian Pacific Journal of Tropical Biomedicine, L-theanine is an odorless and slightly sweet crystalline that is transparent, colorless and water-soluble in nature, with a bioavailability of 47% to 54%. L-theanine also provides various biological activities, including:3,4

  • Antidepressant
  • Immune-boosting
  • Hepatoprotective
  • Anti-fatigue

Moreover, L-theanine is a nonessential amino acid that’s not naturally produced by the human body.5,6 It’s found to have a structural similarity to glutamate,7 the principal excitatory neurotransmitter in the brain, allowing it to interact with glutamate receptors and help inhibit glutamate toxicity.8,9

Natural sources of L-theanine

L-theanine naturally occurs in the Camellia sinensis plant, which is used for making green and black tea. According to Alternative & Complementary Therapies, Japanese researchers discovered the presence of this compound in tea leaves in 1949, and it was found to constitute around 1% to 2% of the dry weight of tea leaves. L-theanine is responsible for giving tea its characteristic umami taste.10

The amount of L-theanine is different for every type of tea, as it’s affected by environmental factors such as climate, rainfall and the exposure of leaves to sunlight. High-grade matcha green tea contains the highest percentage of this compound, as it’s made from tea leaves that are cultivated under shade. According to a study published in the journal Nutrients, protecting the tea leaves from sunlight allows them to retain most of their amino acid content, especially L-theanine.11

Gyokuro and sencha green tea also contain fairly high amounts of this compound, with 26 to 46 milligrams per cup. Non-green teas that have high levels of L-theanine include Ceylon, Darjeeling and Earl Grey teas. Aside from tea leaves, L-theanine is also found in edible bay boletes mushroom (Xerocomus badius).12

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The benefits of L-theanine for anxiety

One of the most well-known benefits of L-theanine is its ability to help promote relaxation, making it a recommended supplement for managing anxiety. Studies have shown that L-theanine exhibits its anti-anxiety effect through different mechanisms of action.

L-theanine has been found to help stimulate the production of alpha brain waves, which are associated with the state of “wakeful relaxation” and mental alertness.13 According to a study published in the journal Basic and Clinical Neuroscience, increased levels of alpha brain waves may help relieve the symptoms of generalized anxiety disorder (GAD) and improve the social, occupational and psychological functioning of individuals with this condition.14

L-theanine may also help increase levels of the neurotransmitters gamma-aminobutyric acid (GABA), serotonin and dopamine. According to a study published in the journal Ancient Science, low GABA levels are associated with high levels of anxiety, while a deficiency in serotonin may increase the risk for depression. Low dopamine levels, on the other hand, have been associated with generalized social phobia.15

By helping boost the levels of these brain chemicals, L-theanine may help induce relaxation, ease anxiety and promote the proper regulation of mood, emotions, concentration, appetite, energy and cognitive skills.16,17,1819 An animal study also shows that L-theanine may have antidepressant-like effects, thanks to its ability to increase serotonin and dopamine.20

How L-theanine improves sleep

The anxiolytic effect of L-theanine makes it an ideal sleep agent. A study published in the Journal of the American College of Nutrition shows that the administration of L-theanine before going to bed may help support sleep quality without drowsiness because it works through anxiolysis, not sedation. This mechanism of action makes it a safe alternative to sedatives that are often linked to addiction and adverse side effects.21

Another study published in the journal Pharmaceutical Biology22 also suggests that L-theanine may help decrease sleep latency, or the time it takes for you to fall asleep, and increase rapid eye movement (REM) sleep, a highly restorative stage of sleep that is necessary for memory consolidation.23

Since L-theanine is often consumed together with caffeine through tea, there have been several studies that discuss its effectiveness in the presence of caffeine. One example is the study published in journal Pharmacology, Biochemistry and Behavior, which shows that L-theanine may help counteract caffeine-induced sleep disturbance, particularly reductions in slow-wave sleep (SWS) or deep sleep.24

6 other benefits of L-theanine

In addition to its anti-anxiety and sleep-promoting effects, L-theanine may also be used to help:

Reduce stress responses — A study published in the journal Biological Psychology suggests that oral intake of L-theanine may help reduce psychological and physiological responses by attenuating the sympathetic nervous activation, which is responsible for the flight-or-fight response.25

Improve attention and reaction time — According to a study published in the Journal of Functional Foods, L-theanine may help improve visual attentional performance and reaction time response in individuals with a tendency to have high anxiety levels.26

Maintain a healthy weight — A study published in the journal In Vivo shows that l-theanine provides an anti-obesity effect by helping inhibit body weight increase and fat accumulation.27

Boost immune system — A study published in the Journal of the International Society of Sports Nutrition evaluated the effects of L-theanine supplementation on the immune system of athletes exposed to overtraining. The results show that L-theanine may help balance the levels of Th1/Th2, which are T helper cells that can cause immune impairment when they’re imbalanced.28

Inhibit high blood pressure levels — According to a study published in the Journal of Physiological Anthropology, L-theanine may help inhibit blood pressure increases in high-stress-response adults who performed mental tasks.29

Reduce the risk for stroke — A meta-analysis published in the journal Stroke suggests that L-theanine, derived through daily intake of green tea, may help reduce the risk of stroke through its antagonistic action against glutamate,30 which can cause neuronal cell death by acting as a neurotoxin.31

L-theanine studies support its neuroprotective effects

Many studies about L-theanine are focused on its protective effects on the brain. For instance, in a 2013 study published in the journal Phytotherapy Research,32 the neuroprotective effect of L-theanine on rat model of stroke was evaluated.

The results show that administering L-theanine three and 12 hours after reperfusion (a type of tissue damage that occurs when blood supply returns to the tissue after a period of lack of oxygen33) helped “substantially reduced the size of brain infarct” or tissue death due to inadequate blood supply.34 The researchers also stated that repeated injections of L-theanine during reperfusion “prevented brain injury caused by glutamate receptor agonists.”

Another study published in the journal Nutrition Research found that L-theanine, when combined with luteolin, helps reduce neuro inflammation and improve insulin signaling and norepinephrine metabolism. The researchers concluded that the combination of these chemicals may be a useful therapeutic option for reducing the risk or delaying the progression of memory dysfunction, which is associated with Alzheimer’s disease.35

A 2019 meta-analysis published in the journal Neurochemistry International further supports L-theanine’s neuroprotective effects, suggesting that it has the potential to help inhibit the pathophysiological changes linked to Parkinson's disease because of its antioxidant and anti-inflammatory properties. Its ability to help improve motor behavioral abnormalities, increase dopamine levels and reduce neurodegeneration due to glutamate excitotoxicity also contribute to its brain-boosting benefits.36

L-theanine side effects you should be aware of

L-theanine is classified as generally recognized as safe (GRAS) by the FDA. It’s also unlikely to cause adverse effects for most people, although WebMD notes that this supplement may cause mild side effects like sleepiness or headaches. There is not enough evidence to guarantee the safety of L-theanine during pregnancy or when breastfeeding, so it would be wise to avoid this supplement if you’re pregnant or lactating.

Individuals with low blood pressure levels are also advised to practice precaution when taking L-theanine, as it could cause blood pressure levels to drop too low. If you’re currently taking other medications and would like to add L-theanine to your routine, make sure you talk to your physician first to guarantee that it will not interact with your current medicines.37,38,39

How to get optimal amounts of L-theanine at night

While drinking tea may be an excellent way to top up on L-theanine during daytime, this method may not be as beneficial at night if you’re planning to use this amino acid to significantly boost your REM sleep. A cup of tea may not have enough L-theanine to have this effect. Drinking excessive amounts of tea is also out of the question, as it could increase your nighttime trips to the bathroom and further interrupt your sleep.

With that said, the best way to obtain L-theanine at night is by taking a pure, active L-theanine supplement. But be careful when buying L-theanine supplement, as some brands may have an inactive form of this amino acid, which blocks its effectiveness. Be sure to choose a brand that provides its active form.

Frequently asked questions (FAQs)

Q: What does L-theanine do?

A: L-theanine may provide a variety of health benefits, including relief from stress and anxiety, better sleep, improved cognitive function, reduced risk of weight gain, lower blood pressure levels, healthier immune system and reduced risk for stroke.40,41,42,43

Q: Is L-theanine safe?

A: The FDA classifies L-theanine as generally recognized as safe.44 Most people are also unlikely to experience side effects when taking this supplement. However, individuals with low blood pressure should observe caution when taking L-theanine, as it may cause blood pressure levels to become even lower.

Pregnant and breastfeeding women are also advised to avoid this supplement, since there’s not enough information to guarantee its safety during pregnancy and breastfeeding.45

Q: What is L-theanine used for?

A: L-theanine is commonly used to help promote relaxation, making it an ideal natural remedy for the management of anxiety and stress. It’s also used to help stimulate sleep and enhance memory, attention and focus.46

Q: Does L-theanine work?

A: Several studies have shown the effectiveness of L-theanine through different mechanisms of action, including boosting GABA, dopamine and serotonin levels, enhancing alpha brain waves and inhibiting glutamate excitotoxicity.47,48

Q: How long does L-theanine take to work?

A: The effects of L-theanine may be felt within 30 to 40 minutes after oral ingestion.49

Q: How long do the effects of L-theanine last?

A: The calming effect of L-theanine may last up to eight to 10 hours.50

Q: Is L-theanine good for sleep?

A: Studies show that L-theanine may be used as a safe natural sleep aid due to its anxiolytic property, which allows it to promote sleep quality without drowsiness. It’s also found to help decrease sleep latency and improve vital REM sleep.51,52

Q: Can you drink alcohol while taking L-theanine?

A: A study published in the journal Food and Chemical Toxicology shows that taking L-theanine along with alcohol may help lower the risk for alcoholic liver injury, as it helps enhance hepatocyte antioxidant abilities.53

However, this doesn’t mean that you should rely on L-theanine to attenuate the negative effects of alcohol. I recommend you to avoid drinking alcohol altogether, as it can increase your risk of various health consequences, including DNA damage, cancer, systemic inflammation and cardiovascular diseases, not to mention that it also impairs your decision-making abilities and motor function, making you more prone to accidents.

Q: Does L-theanine affect the liver?

A: L-theanine has beneficial effects on the liver. Studies show that L-theanine exhibits hepatoprotective property by helping decrease the biomarkers of hepatic impairment.54,55

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