Technically yes, if you'll drink eight or more cups of green tea a day. Realistically, no.
Watch l-theanine prices →Can you get it from food?
Green tea is essentially the only meaningful dietary source. A standard brewed cup carries somewhere between 5 and 60 mg depending on the leaf and how it's made, with ordinary sencha at the lower end — roughly 5 to 25 mg. To reach the 200 mg used in the trials you're looking at eight to forty cups of standard green tea. Matcha does better, at around 20 to 40 mg a serving, because you consume the whole leaf. Shade-grown gyokuro is the outlier at up to about 85 mg. There's a catch worth noting: those cups bring caffeine with them, and the trials tested theanine on its own.
| Food | To match the dose | |
|---|---|---|
| Standard green tea (sencha) | ~8–40 cups | Roughly 5–25 mg per cup |
| Matcha | ~5–10 serves | ~20–40 mg per serve; whole leaf consumed |
| Gyokuro (shade-grown) | ~2–3 cups | Up to ~85 mg per cup — the only realistic food route |
| Black tea | More again | Generally lower than green |
What the research actually shows
Unusually well-served by recent evidence. A systematic review and meta-analysis of 31 randomised trials (n=1,168) compared oral L-theanine against placebo in both healthy and clinical populations. A single 200 mg dose taken 30–60 minutes before testing significantly improved choice reaction time — a measure of attention. The same body of work found the effect on acute stress to be modest, and heavily influenced by studies at high risk of bias.
On certainty: The attention finding is the more robust of the two. The authors of a separate 2025 review titled their paper 'Promising, but Not Completely Conclusive', which is a fair summary of where this sits — worth knowing before anyone tells you it's settled.
The studies referenced
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Cognitive and affective effects of L-theanine: a systematic review and meta-analysis of 31 randomized trials
A single 200 mg dose taken 30–60 min before testing significantly improved choice reaction time. Acute stress reduction was modest and largely driven by high-risk-of-bias studies. -
Promising, but not completely conclusive — the effect of L-theanine on cognitive performance: a systematic review and meta-analysis of randomized placebo-controlled clinical trials
Concludes the cognitive evidence is promising but not conclusive. Included as the counterweight. -
Effects of tea (Camellia sinensis) or its bioactive compounds L-theanine or L-theanine plus caffeine on cognition, sleep and mood in healthy participants: a systematic review and meta-analysis of RCTs
Examines tea itself alongside isolated theanine and the theanine-plus-caffeine combination — directly relevant to whether drinking tea substitutes for supplementing.