Green tea has a reputation for making people feel calm and alert at the same time. That combination is strange when you think about it. Most things that make you alert also make you jittery, and most things that calm you down also make you sleepy. Tea somehow does neither.
The usual explanation is L-theanine, an amino acid that shows up almost nowhere in nature except the tea plant. It's worth understanding what it does, partly because it's the active ingredient in a growing number of drinks, and partly because the claims made about it have gotten well ahead of what anyone has actually demonstrated.
What it is
L-theanine is a non-protein amino acid found in Camellia sinensis, the plant that produces white, green, oolong, and black tea. It's also present in a couple of other Camellia species and in the bay bolete mushroom, but for practical purposes, tea is the source.
It makes up a surprising share of what's in a tea leaf. Depending on cultivar, season, and growing conditions, L-theanine accounts for somewhere between 0.9 and 3.1 percent of the dry weight of the leaves, and it can represent up to half of the total amino acid content. A standard cup of brewed tea delivers somewhere in the range of 25mg.
Structurally it resembles glutamate, one of the brain's main excitatory neurotransmitters. That resemblance matters, because it's part of how L-theanine appears to work. It binds to glutamate receptor subtypes, though with relatively low affinity, which is a polite way of saying it interacts with the system without hitting it hard.
It also crosses the blood-brain barrier. In mammals, plasma concentrations peak somewhere between 30 and 120 minutes after ingestion, and doses between roughly 25mg and 100mg produce dose-dependent increases in plasma theanine.
What the research shows
The most cited work on the calm-focus effect involves alpha brain waves. Alpha oscillations are associated with a relaxed but wakeful state, the kind of mental gear you're in when you're absorbed in something without being tense about it.
A 2016 study published in Nutrients by David White and colleagues at Swinburne University tested an L-theanine-based nutrient drink on 34 healthy adults between 18 and 40, in a double-blind, placebo-controlled, crossover design. The researchers looked at mood responses to a cognitive stressor, and used magnetoencephalography to track resting-state alpha activity. The design is solid: crossover means every participant served as their own control, and double-blind placebo control is the standard you want to see.
A broader review published in Frontiers in Nutrition in 2022 by Ming-Yue Li and colleagues catalogued the range of effects attributed to L-theanine across the literature, covering antioxidant, anti-inflammatory, and neuroprotective activity among others. Reviews like that are useful for mapping the territory. They're less useful for telling you whether any single effect is reliable in humans at realistic doses.
Where the evidence gets thin
Here's the part most articles about L-theanine skip.
A recent review in the scientific literature took a deliberately skeptical look at the supplement and concluded, in the absence of well-designed and carefully controlled human clinical trials, that caution is warranted around pharmacologic doses, and that the science does not yet match the hype behind the ingredient.
That's a fair reading. A lot of what gets claimed for L-theanine comes from animal work, cell studies, or small human trials that haven't been replicated at scale. The calm-focus effect has the most support behind it. The longer list of benefits that shows up on supplement labels has considerably less.
There's also a dose question nobody has settled cleanly. Studies have used everything from 25mg to 400mg, and the amounts you'll find in commercial products vary just as widely. Doses in the 100mg to 200mg range are the ones most often studied for relaxation and attention effects, but "most often studied" isn't the same as "established optimal."
Why people take it with caffeine, and why some don't
A lot of the interest in L-theanine comes from combination studies, where it's paired with caffeine. The theory is that caffeine supplies the alertness while L-theanine takes the edge off, producing focus without the jitters. Green tea does this naturally, since it contains both.
That combination has its own body of research and it's genuinely interesting. But it's a different question from what L-theanine does on its own, and the two get conflated constantly. If you're taking L-theanine specifically to avoid caffeine, the combination studies don't tell you much about your situation.
On its own, without caffeine, the effect is subtle. Nobody who takes 100mg of L-theanine reports feeling like they took a stimulant, because they didn't. What people describe is closer to the edge coming off. Whether that reads as meaningful depends a great deal on what your baseline was.
Is it safe?
L-theanine occurs naturally in a beverage that billions of people drink daily, which is a reasonable starting point for thinking about safety. It's generally well tolerated in the amounts found in tea and in commercial products, and it isn't a stimulant, so there's no tolerance-and-withdrawal cycle attached to it.
The caution flagged in the literature applies to high pharmacologic doses taken as supplements, not to the amounts you'd get from tea or from a drink containing 100mg. If you're on medication, particularly anything affecting blood pressure or the central nervous system, that's a conversation for your doctor rather than a blog post.
What this means practically
If you want the effect from tea, green tea gives you roughly 25mg per cup along with caffeine. If you want it without caffeine, you're looking at a supplement or a functional drink, and then the number on the label is the whole question.
That's the part worth paying attention to. Plenty of products list L-theanine without listing how much, which usually means the amount wouldn't sell the product on its own. A "proprietary blend" that names five trendy ingredients and no doses is telling you something.
SÜTHE uses 100mg per can, printed on the can, in the range that's been studied for relaxed focus. That's the reason we print it. You can't evaluate an ingredient you can't measure, and the whole argument for functional water falls apart if the functional part is a rounding error.
Want the full breakdown of what's in every can and why? See what's inside.
Sources referenced
Li, M.-Y., Liu, H.-Y., Wu, D.-T., et al. (2022). L-Theanine: A Unique Functional Amino Acid in Tea (Camellia sinensis L.) With Multiple Health Benefits and Food Applications. Frontiers in Nutrition, 9:853846.
White, D. J., de Klerk, S., Woods, W., Gondalia, S., Noonan, C., & Scholey, A. B. (2016). Anti-Stress, Behavioural and Magnetoencephalography Effects of an l-Theanine-Based Nutrient Drink. Nutrients, 8(1):53.
Williams, J., Kellett, J., Roach, P. D., et al. (2016). l-Theanine as a Functional Food Additive: Its Role in Disease Prevention and Health Promotion. Beverages, 2(2):13.
This article is for general information and isn't medical advice. Talk to a healthcare provider about your own situation.