L-Theanine

Calmer under stress

L-theanine (γ-glutamylethylamide) is a non-protein amino acid found almost only in Camellia sinensis tea leaves, where it's a large part of the savory, umami taste. It crosses the blood–brain barrier via the large neutral amino acid transporter. It's structurally similar to glutamate, weakly blocks some glutamate receptors, and raises GABA, dopamine and serotonin in rodent brains. In human EEG studies it increases alpha-wave activity at rest — the relaxed-but-alert state.³ (Animal for the neurotransmitter effects; small human EEG studies)

In a small crossover trial (12 people), 200 mg of theanine taken before a mental-arithmetic stress task blunted the rise in heart rate and in salivary immunoglobulin A compared with placebo.¹ In a 4-week crossover RCT in 30 healthy Japanese adults, 200 mg/day lowered depression, trait-anxiety and sleep-quality (PSQI) scores and improved verbal fluency and executive function.² (Moderate — small but consistent trials)

Several of the early trials, including the Kimura study, have co-authors from Taiyo Kagaku, which makes the branded Suntheanine. The 2019 Hidese trial was run by Japan's National Center of Neurology and Psychiatry. Theanine doesn't have an obvious funding-driven result pattern, but note the funder where it applies.

Smoother focus with caffeine

Tea has always paired theanine with caffeine, and the combination seems to work better than either alone. In a crossover study, 100 mg theanine plus 50 mg caffeine improved attention-switching accuracy and reduced susceptibility to distraction compared with placebo, and did better than caffeine alone on some measures.⁴ Theanine seems to take the jitteriness and blood-pressure bump off caffeine without cancelling its alertness. (Moderate — several acute crossover trials, several of them Unilever-funded, the company behind Lipton tea)

The effect per cup of tea is small: brewed black or green tea has roughly 10–25 mg theanine against 30–60 mg caffeine. That's why the trials used isolated theanine. Matcha and shade-grown teas have more. See Assam and Pu-erh for the rest of what tea brings.

Better sleep quality

In a 6-week RCT in 98 boys aged 8–12 with ADHD, 400 mg/day theanine increased sleep percentage and sleep efficiency on wrist actigraphy compared with placebo. Time to fall asleep didn't change.⁵ In the Hidese trial in adults, sleep latency, sleep disturbance and use of sleep medication all dropped.² Theanine isn't a sedative; it seems to help people sleep more soundly rather than knock them out. (Moderate — small trials; the ADHD trial was Taiyo-funded)

How to take it

• Calm / focus: 100–200 mg. Effects show up within 30–60 minutes and last a few hours.
• With coffee: about 2:1 theanine to caffeine is the ratio most trials used (e.g. 200 mg with a 100 mg-caffeine coffee).
• Sleep: 200–400 mg in the hour before bed.
• Form: plain L-theanine. Suntheanine is the branded, pure L-form used in most trials. Cheap synthetic versions can be a D/L mix; the D form is less active.
• Pairs with: magnesium and chamomile for an evening routine, by different mechanisms.

Safety