L-Theanine Explained: The Tea Amino Acid, Doses & Research
L-theanine — direct answers
L-theanine is a non-protein amino acid found almost uniquely in tea (Camellia sinensis), first isolated from green tea leaves in Japan in 1949. A cup of tea provides roughly 8–25mg depending on type and brewing; supplements typically provide 100–200mg. Human studies frequently pair it with caffeine. Notable studies include Nobre et al. 2008 (Asia Pacific Journal of Clinical Nutrition, on alpha brain waves), Hidese et al. 2019 (Nutrients, randomised placebo-controlled), and Juneja et al. 1999 (Trends in Food Science & Technology). No EU/UK health claims are authorised for L-theanine. Vitality Supplements uses 100mg of L-theanine in Cortisol Balance, described factually.
L-theanine — the amino acid that lives in tea.
Where it comes from, how much a cup of tea actually contains, what the human research measured and what a 100mg dose means — referenced to primary sources.
L-theanine is a non-protein amino acid found almost uniquely in tea (Camellia sinensis), first isolated from green tea leaves in 1949. A cup of tea contains roughly 8–25mg; supplements typically provide 100–200mg — several cups’ worth in one dose. Human studies, several of them randomised and placebo-controlled, have measured its effects on things like alpha-band brain activity. No EU/UK health claims are authorised for L-theanine, so it should be described factually with the research linked.
- L-theanine is a non-protein amino acid found almost uniquely in one plant: tea (Camellia sinensis)
- It was first isolated from green tea leaves in Japan in 1949 and contributes to tea’s umami character
- A cup of tea provides roughly 8–25mg; the studied supplement range is typically 100–200mg
- Human research includes EEG studies of alpha-band activity and randomised placebo-controlled trials — linked below
- No EU/UK health claims are authorised for L-theanine; claims on labels for it are not from the official register
An amino acid almost nothing else makes.
Most amino acids are everywhere — the twenty protein-building ones occur in essentially every organism. L-theanine is different. It is a non-protein amino acid, meaning it is not used to build proteins, and in the human diet it comes from essentially one source: the tea plant, Camellia sinensis. Outside tea, it has been found in only a handful of organisms, most famously the bay bolete mushroom.
It was first isolated from green tea leaves by Japanese researchers in 1949, and it is part of what gives brewed tea its savoury, umami character — theanine is chemically related to glutamate, the classic umami molecule.
That natural pairing — theanine plus caffeine in the same leaf — is exactly why so much of the modern research studies the two together, and why the supplement form exists at all: to provide the theanine at a defined dose, with or without the caffeine.
What a cup contains — and what a capsule does.
Brewed tea delivers a modest, variable amount of L-theanine: roughly 8–25mg per cup, depending on the tea type, leaf quality and brewing time, with green and white teas at the higher end. The doses used in human studies are higher and fixed — typically 100–200mg per dose, several cups’ worth, delivered without variability.
As with every ingredient in Cortisol Balance, the amount is printed on the supplement-facts panel and verified by independent third-party testing — no “proprietary calm blend” arithmetic.
The human research, linked directly.
L-theanine is one of the more-studied supplement ingredients, including EEG work and randomised placebo-controlled trials. We link the papers rather than paraphrase their conclusions; read them and draw your own conclusions.
- Nobre AC, Rao A, Owen GN (2008). L-theanine, a natural constituent in tea, and its effect on mental state. Asia Pacific Journal of Clinical Nutrition 17(S1):167–168. EEG study measuring alpha-band brain activity after a 50mg dose. PubMed →
- 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. 200mg daily, four weeks, randomised, placebo-controlled crossover. PubMed →
- Juneja LR, Chu D-C, Okubo T, Nagato Y, Yokogoshi H (1999). L-theanine — a unique amino acid of green tea and its relaxation effect in humans. Trends in Food Science & Technology 10(6–7):199–204. The much-cited early review of theanine’s occurrence, chemistry and human data.
What we do not say: no EU/UK health claim is authorised for L-theanine. These studies concern specific doses in specific populations under trial conditions; findings do not transfer automatically to any individual, and Vitality has no involvement in this research.
Safety and who should avoid it.
L-theanine has been consumed in tea for millennia and, at studied supplement doses, published trials have generally reported it as well tolerated over study durations. The standard cautions apply:
- Pregnant or breastfeeding: consult a doctor before using any food supplement; Cortisol Balance is not for use in pregnancy or breastfeeding because of its other botanical ingredients.
- On medication or under medical supervision: speak to your doctor or pharmacist before use.
- Under 18: not for use.
- Discontinue and consult a doctor if any adverse reaction occurs; food supplements must not replace a varied, balanced diet.
Frequently asked questions.
A non-protein amino acid found almost uniquely in tea (Camellia sinensis), first isolated from green tea leaves in 1949. It contributes to tea’s umami taste and is chemically related to glutamate.
Roughly 8–25mg depending on the tea type, leaf grade and brewing time, with green and white teas generally at the higher end. Supplement doses of 100–200mg equate to several cups’ worth at a fixed, known quantity.
We make no such claim — no EU/UK health claim is authorised for L-theanine. Human studies, including EEG work on alpha-band activity and randomised placebo-controlled trials, are linked in this guide. Read them directly.
Study doses typically range from 50mg (Nobre 2008) to 200mg daily (Hidese 2019). Cortisol Balance provides 100mg per serving as one of seven ingredients — within the studied range. Follow the label; do not exceed the stated dose.
No. They are different molecules that happen to co-occur in tea leaves. Much of the research studies them together because that is how tea delivers them; Cortisol Balance contains L-theanine and no caffeine.
Anyone under 18, and anyone on medication or with a medical condition without speaking to a doctor first. Cortisol Balance as a whole is not for use during pregnancy or breastfeeding.
Cortisol Balance contains 100mg of L-theanine per serving as part of a 7-ingredient, 925mg complex — alongside 300mg KSM-66® Ashwagandha, standardised rhodiola, phosphatidylserine, magnesium glycinate, vitamin C and vitamin B5, every one individually dosed.
The tea amino acid, at a studied dose.
Cortisol Balance provides 100mg of L-theanine alongside KSM-66® Ashwagandha, standardised rhodiola and four more individually dosed ingredients — 925mg per serving, every amount on the label, independently third-party tested.
View Cortisol Balance →About Vitality Supplements
Vitality Supplements is a UK supplement brand built on full-dose transparency: every ingredient named and individually dosed, third-party tested at an ISO/IEC 17025 accredited laboratory, and every guide referenced to primary literature. Our guides are educational; they describe compounds and research factually and are not medical advice. Food supplements must not replace a varied, balanced diet and healthy lifestyle.
Reviewed & updated: August 2026 · Vitality Supplements, London