L-Theanine
L-theanine is an amino acid found almost only in tea leaves, where it accounts for much of the savory taste of green tea. In small human studies it shifts the resting EEG toward alpha activity and blunts the response to a stressful task, and paired with caffeine it improves attention-switching more than caffeine alone. The effects are mild, and it does not sedate.
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What It Does
Theanine is absorbed in the small intestine and crosses the blood-brain barrier on the large neutral amino acid transporter. Its structure resembles glutamate. In rodent studies it binds weakly to glutamate receptors, inhibits glutamine uptake, and raises brain GABA, but those studies used doses and measures that do not translate directly to a 200 mg capsule. In people, the best-defined effect is on the EEG: 50 mg raised alpha-band activity at rest over the following hour or two (Nobre 2008). Alpha activity is associated with relaxed wakefulness, not drowsiness.
Under stress, 200 mg lowered heart rate and salivary immunoglobulin A responses to a mental arithmetic task in a small crossover study (Kimura 2007). In a four-week trial of 200 mg daily in 30 healthy adults, theanine improved some measures of sleep quality and verbal fluency compared with placebo (Hidese 2019). These are small studies with modest effects.
The caffeine pairing has the most consistent data. With 100 mg theanine and 50 mg caffeine, participants switched between tasks faster and more accurately and were less distracted (Owen 2008). A higher-dose pairing, 250 mg theanine and 150 mg caffeine, improved reaction time and alertness (Haskell 2008), and the combination at about 97 mg and 40 mg, roughly the amount in two cups of tea, improved attention-switching accuracy (Giesbrecht 2010). Caffeine does most of the work for alertness; theanine appears to sharpen focus and may soften the edge. Trials have run for weeks at most and none has reported tolerance or withdrawal, though none was designed to look for them.
Forms and Bioavailability
The standard supplemental form.
| Form | Absorption | Typical dose | Cost |
|---|---|---|---|
| L-TheanineSuntheanine, gamma-glutamylethylamide | HighAbsorbed in the small intestine through amino acid transporters and crosses the blood-brain barrier by the large neutral amino acid transporter. Plasma levels peak within about an hour. | 100-400 mg daily | Low |
There is one supplemental form. Suntheanine is produced by an enzymatic process and is the pure L-isomer. Some generic theanine is chemically synthesized and can contain D-theanine, which does not share L-theanine's evidence; the panel should say L-theanine. Tea itself supplies theanine, but in variable amounts that depend on the leaf and the brew.
Typical Dosing
Trials used 50-400 mg; 200 mg is the most common single dose. Caffeine pairings used about 100-250 mg theanine with 40-150 mg caffeine.
Food sources: green tea, black tea, oolong tea.
What to Look For
Look for 100-200 mg of L-theanine per capsule. For focus with coffee, a dose about twice the caffeine dose matches the trials: 200 mg theanine with a 100 mg cup of coffee, for example. The Caffeine Clearance calculator can help time the caffeine side of that pairing so it does not carry into the night.
For evening use, theanine will not knock anyone out. It may make it easier to settle, and it combines safely with good sleep habits. People taking sedatives or blood pressure medication should start low.
Red flags: "calm" or "sleep" blends that hide the theanine dose inside a proprietary mix, and anxiety treatment claims. Green flags: L-theanine or Suntheanine named, milligrams per capsule stated, and nothing else in the capsule unless you want it there.
L-Theanine
Inexpensive and well tolerated. Expect a quiet shift in focus and tension, not a sedative effect.
What to look for: L-theanine or Suntheanine, 100-200 mg per capsule, as a single ingredient.
Find on iHerb Affiliate linkProducts on Titrate
2,247 of the 214,745 labels in the NIH Dietary Supplement Label Database list L-theanine as an active ingredient (1,375 on market). 449 are sold as L-theanine products by name.
Top on-market labels, ranked by label completeness and simplicity:
- Magnesium Glycinate with L-Theanine 350 mg
- L-Theanine 200 mg
- L-Theanine 200 mg
- L-Theanine 200 mg
- Kyolic Nattokinase, Suntheanine
L-Theanine does not have its own search category yet. Each product above links to its full label in the NIH database.
Related Tools
References
Kimura K, Ozeki M, Juneja LR, Ohira H. L-Theanine reduces psychological and physiological stress responses. Biol Psychol. 2007;74(1):39-45.
Nobre AC, Rao A, Owen GN. L-theanine, a natural constituent in tea, and its effect on mental state. Asia Pac J Clin Nutr. 2008;17 Suppl 1:167-168.
Owen GN, Parnell H, De Bruin EA, Rycroft JA. The combined effects of L-theanine and caffeine on cognitive performance and mood. Nutr Neurosci. 2008;11(4):193-198.
Haskell CF, Kennedy DO, Milne AL, Wesnes KA, Scholey AB. The effects of L-theanine, caffeine and their combination on cognition and mood. Biol Psychol. 2008;77(2):113-122.
Giesbrecht T, Rycroft JA, Rowson MJ, De Bruin EA. The combination of L-theanine and caffeine improves cognitive performance and increases subjective alertness. Nutr Neurosci. 2010;13(6):283-290.
Hidese S, Ogawa S, Ota M, et al. Effects of L-theanine administration on stress-related symptoms and cognitive functions in healthy adults: a randomized controlled trial. Nutrients. 2019;11(10):2362.
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