Looking for science backed information about theanine? What is it? What’s it for and does it really do all that? Join MNB as we take an evidence based look at nutraceuticals.
What is Theanine?
Scientists have discovered that the molecule L-Theanine seems to have a relaxing effect on humans and can help us to concentrate better. Some fans of the tea derived nutrient say that it might bolster cardiovascular health, aid weight loss and boost our immune system. So what is theanine, and does it really do all that?
The only natural source of theanine is the tea plant, Camellia sinensis. Theanine is an amino acid that is, as far as we know, only made by tea plants. When we think of amino acids, usually it’s because they are the building blocks from which proteins are assembled. This molecule is a bit unusual; it’s an amino acid, but it’s not one that gets incorporated into proteins. Instead its job is to help plants carry nitrogen from their roots to their leaves. They consume this nitrogen to build new cells and grow.
Plants incorporate nitrogen into theanine to turn it into a form they can move around their body. This is a rather interesting characteristic, as the molecular structure that allows thiamine to be picked up and transported by proteins in a tea plant also makes it biologically useful in animal cells.
Researchers have used mass spectrometry and other approaches to map where it’s made and which parts of the plant contain the highest concentrations. So far it looks like it is produced in the roots and then moves to the leaves and stems. It’s most abundant in young leaves and shoots, and tea plants relocate it from mature parts to growing parts as needed.
Can I get theanine from drinking tea?
Yes! Unlike a lot of the other nutraceuticals, we can directly absorb theanine through our intestine. Theanine is a water-soluble chemical that our cells recognize as useful and absorb into themselves through amino acid transport channels. Scientists have found that the amino acid passes into our blood stream, travelling round the body and can cross through the blood-brain barrier and act on the brain.
You don’t need to buy expensive supplements, simply drinking tea made from young shoots will give you plenty.
What is Theanine good for?
While there are all sorts of claims made about theanine’s potential health properties, at medical news bulletin we are all about evidence. Rather than listing off a bunch of lab-based findings that may or not prove true, we concentrate on what clinical trials tell us.
Overall it looks like theanine has a small effect on improving cognition, reducing stress and a calming effect. There is also some evidence that it’s helpful for people who find it hard to stay asleep. Most reported effects seem to arise from theanine’s purported relaxing effect on the brain – blood pressure, heart rate, etc.
- Relaxation: Clinical trials have shown that theanine does have a relaxing effect. Scientists found that it can lower your heart rate in a stressful situation, it increases alpha brain waves and people report better subjective descriptors of relaxation than they do after taking the benzodiazepam alprazolam. A 2025 meta-analysis concluded there was some evidence that theanine alone could be beneficial for cognitive and mood outcomes.
- Sleep: A systematic review of the research performed on theanine and sleep in 2025 concluded that there is some evidence that theanine helps people to stay asleep. Academics reviewed 18 randomized controlled trials that tested whether the molecule improves subjective sleep onset latency, daytime tiredness and overall sleep quality.
Immune health: Japanese doctors say that they saw an increase in immune response to the flu shot in seniors when they were given l-theanine together with l-cysteine. Another study found that L-theanine combined with catechin reduced rates of flu in health care workers. - Cancer: A clinical trial found that theanine helped reduce diarrhoea as a side effect from chemotherapy for colorectal cancer.
Sources
Kimura K, Ozeki M, Juneja LR, Ohira H. L-Theanine reduces psychological and physiological stress responses. Biol Psychol. 2007;74(1):39-45. doi:10.1016/j.biopsycho.2006.06.006
Lv X, Chen C, Liang Y, et al. The Emerging Role of Citrulline and Theanine in Health and Disease: A Comprehensive Review. Nutrients. 2025;17(21):3496. doi:10.3390/nu17213496
Lu K, Gray MA, Oliver C, et al. The acute effects of L-theanine in comparison with alprazolam on anticipatory anxiety in humans. Hum Psychopharmacol. 2004;19(7):457-465. doi:10.1002/hup.611
Miyagawa K, Hayashi Y, Kurihara S, Maeda A. Co-administration of l-cystine and l-theanine enhances efficacy of influenza vaccination in elderly persons: nutritional status-dependent immunogenicity. Geriatr Gerontol Int. 2008;8(4):243-250. doi:10.1111/j.1447-0594.2008.00483.x
Nobre AC, Rao A, Owen GN. L-theanine, a natural constituent in tea, and its effect on mental state. Asia Pac J Clin Nutr. 2008;17 Suppl 1:167-168.
Payne ER, Aceves-Martins M, Dubost J, Greyling A, de Roos B. 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 Randomized Controlled Trials. Nutr Rev. 2025;83(10):1873-1891. doi:10.1093/nutrit/nuaf054
Wang Q, Yu J, Lin W, et al. L-Theanine Metabolism in Tea Plants: Biological Functions and Stress Tolerance Mechanisms. Plants. 2025;14(3):492. doi:10.3390/plants14030492
