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L-theanine, stress and sleep: what human trials show

A handful of small, short randomized trials, several funded by tea or ingredient companies, plus one publicly funded trial that was mostly null. Consistent direction for stress and self-rated sleep in healthy adults, not for anxiety disorders.

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Editorial section. Research summaries are not product claims and contain no product links or offers. A summary is never proof that any product works.

The question
What do randomized human trials show about L-theanine and stress responses, self-rated sleep and attention?
Study design
A systematic review of 9 randomized controlled trials of pure L-theanine supplements (Williams 2020); a 4-week double-blind crossover trial of 200 mg a day in 30 healthy adults (Hidese 2019); a lab stress study in 12 people (Kimura 2007); a 12-week parallel trial of about 100 mg a day in 50 analyzed adults aged 50 to 69 (Baba 2021); and an 8-week trial of 450 to 900 mg a day added to antidepressants in 46 people diagnosed with generalized anxiety disorder (Sarris 2019). [1][2][3][4][5][6]
What they found
The review concluded that 200 to 400 mg a day may help reduce stress and anxiety in people exposed to stressful conditions, and called for longer, larger trials. In the 4-week crossover, 200 mg a day lowered self-rated stress, anxiety and Pittsburgh Sleep Quality Index scores versus placebo, with shorter self-rated time to fall asleep, and improved verbal fluency. In the lab study, L-theanine blunted the heart rate and salivary IgA rise during a mental arithmetic stressor. In older adults, a single dose improved some attention and working memory scores, but 12 weeks of daily intake showed no effect on attention. In generalized anxiety disorder, L-theanine did not beat placebo for anxiety or insomnia severity; only a single item on sleep satisfaction favored it. [1][2][3][4][5][6]
Limitations
Samples were 12 to 50 people per trial, follow-up was at most 12 weeks, and most sleep outcomes were questionnaires, not lab sleep studies. Industry ties: Hidese 2019 was funded by Taiyo Kagaku, an L-theanine ingredient maker, and Baba 2021 was sponsored by ITO EN, a tea company that also supplied the test capsules. The one trial listing public funding (Australia's NHMRC) was Sarris 2019, and it was the most negative. Many outcomes were measured per trial, which raises the chance of a positive result by luck.
What this does not mean
These trials do not show that L-theanine treats anxiety, depression, insomnia or any disease, and the trial in people with an anxiety disorder found no benefit for anxiety. Our claim is structure/function only: supports relaxation without drowsiness. Memorial Sloan Kettering lists pregnancy and breastfeeding, sensitivity to green tea and use with sedatives as reasons to avoid it, and notes additive effects with the sedative midazolam in animals. If you take a sedative or any prescription drug, ask your clinician first. Not for children.
Sources
  1. Williams JL et al. Plant Foods Hum Nutr. 2020;75(1):12-23. doi.org/10.1007/s11130-019-00771-5
  2. Hidese S et al. Nutrients. 2019;11(10):2362. doi.org/10.3390/nu11102362
  3. Kimura K et al. Biol Psychol. 2007;74(1):39-45. doi.org/10.1016/j.biopsycho.2006.06.006
  4. Baba Y et al. J Med Food. 2021;24(4):333-341. doi.org/10.1089/jmf.2020.4803
  5. Sarris J et al. J Psychiatr Res. 2019;110:31-37. doi.org/10.1016/j.jpsychires.2018.12.014
  6. Memorial Sloan Kettering Cancer Center, About Herbs: L-theanine. www.mskcc.org/cancer-care/integrative-medicine/herbs/l-theanine

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