||This article needs more medical references for verification or relies too heavily on primary sources. (August 2014)|
|Onset of action||about 1 hour|
|Biological half-life||3 hours|
|Chemical and physical data|
|Molar mass||174.19766 g/mol|
|3D model (Jmol)||Interactive image|
|Melting point||174.20 °C (345.56 °F) |
|Boiling point||215 °C (419 °F) |
Theanine //, also known as L-γ-glutamylethylamide and N5-ethyl-L-glutamine, is an amino acid analogue of the proteinogenic amino acids L-glutamate and L-glutamine and is found primarily in particular plant and fungal species. It was discovered as a constituent of green tea in 1949 and in 1950 was isolated from gyokuro leaves, which have high theanine content. Appearance of the name "theanine" without a prefix is understood to imply the L-enantiomer, which is the form found in fresh teas and in some, but not all dietary supplements. The opposite D-enantiomer has far less studied pharmacologic properties, but is present in racemic chemical preparations, and substantially in some studied theanine supplements.
As an analogue of glutamate and glutamine, the theanine in common preparations is absorbed in the small intestine after oral ingestion; its hydrolysis is to L-glutamate and ethylamine occurs in both the intestine and liver. It can also cross the blood–brain barrier intact, and register pharmacological effects directly.
In Japan, L-theanine has been approved for use in all foods, including herb teas, soft drinks, desserts, etc., with some restrictions applying to infant foods. It provides a unique brothy or savory (umami) flavor to green tea infusions. As an ingredient, it is generally recognized as safe (GRAS) by the Food and Drug Administration (FDA), and is sold as a dietary supplement in the US. However, the German Federal Institute for Risk Assessment, an agency of their Federal Ministry of Food and Agriculture, has objected to the addition of isolated theanine to beverages.
The European Food Safety Authority EFSA advised negatively on health claims related to L-theanine and cognitive function, alleviation of psychological stress, maintenance of normal sleep, and reduction of menstrual discomfort. Therefore, health claims for L-theanine are prohibited in the European Union.
Structure and properties
The chemical name N5-ethyl-L-glutamine and other synonyms (see box) for theanine reflect its chemical structure. The name theanine, without prefix, is generally understood to imply the L- (S-) enantiomer, derived from the related proteinogenic L-amino acid glutamic acid. Theanine is an analog of this amino acid, and its primary amide, L-glutamine (also a proteinogenic amino acid). Theanine is a derivative of glutamine that is ethylated on the amide nitrogen (as the name N5-ethyl-L-glutamine describes), or alternatively, to the amide formed from ethylamine and L-glutamic acid at its γ- (5-) side chain carboxylic acid group (as the name γ-L-glutamylethylamide describes). Relative to theanine, the opposite (D-, R-) enantiomer is largely absent from the literature, except implicitly. While natural extracts that are not harshly treated are presumed to contain only the biosynthetic L- enantiomeric form, mishandled isolates and racemic chemical preparations of theanines necessarily contain both theanine and its D-enantiomer (and from racemic syntheses, in equal proportion), and studies have suggested that the D-isomer may actually predominate in some commercial supplement preparations. Amino acid racemization in aqueous media is a well-established chemical process promoted by elevated temperature and non-neutral pH values; prolonged heating of Camellia extracts—possible for oversteeped teas and in undisclosed commercial preparative processes—has been reported to result in increasing racemization of theanine to give increasing proportions of the nonnatural D-theanine, up to equal proportions of each enantiomer.
Discovery and distribution
Theanine is found primarily in plant and fungal species. It was discovered as a constituent of green tea (Camellia sinensis) in 1949 and in 1950, a laboratory in Kyoto successfully isolated it from gyokuro leaf, which has high theanine content. Theanine is substantially present in black, green, and white teas from Camellia sinensis in quantities of about 1% of the dry weight. The L-enantiomer is the form found in freshly prepared teas and some, but not all, human dietary supplements.
Digestion and metabolism
As a structural analog of glutamate and glutamine, the theanine in preparations (teas, pure supplements, etc.) is absorbed in the small intestine after oral ingestion; its hydrolysis to L-glutamate and ethylamine occur both in the intestine and liver. It can also cross the blood–brain barrier intact, and register pharmacological effects directly.
Theanine is structurally similar to the excitatory neurotransmitter glutamate, and in accordance, binds to glutamate receptors, though with much lower affinity in comparison. Specifically, it binds to ionotropic glutamate receptors in the micromolar range, including the AMPA and kainate receptors and, to a lesser extent, the NMDA receptor. It acts as an antagonist of the former two sites and as an agonist of the latter site. Theanine also binds to group I mGluRs. In addition, it inhibits glutamine transporters and glutamate transporters, and thus blocks the reuptake of glutamine and glutamate. Lastly, theanine elicits umami taste, and this effect has been found to be a consequence of the fact that it directly binds to and activates the T1R1 + T1R3 heterodimer or umami (savory) taste receptor.
Theanine increases serotonin, dopamine, GABA, and glycine levels in various areas of the brain, as well as BDNF and NGF levels in certain brain areas. However, its effect on serotonin is still a matter of debate in the scientific community, with studies showing increases and decreases in brain serotonin levels using similar experimental protocols. It has also been found that injecting spontaneously hypertensive mice with theanine significantly lowered levels of 5-hydroxyindoles in the brain. Researchers also speculate that it may inhibit glutamate excitotoxicity.
Able to cross the blood–brain barrier, theanine has reported psychoactive properties. Theanine has been studied for its potential ability to reduce mental and physical stress, improve cognition, and boost mood and cognitive performance in a synergistic manner with caffeine.
A Natural Standard monograph that reviews current research on theanine reports that it is likely safe in doses of 200–250 mg up to a maximum daily dose of 1,200 mg. Theanine is used to help with anxiety, blood pressure control, mood, and cognition. Natural Standard rates the evidence to support the usage for anxiety, blood pressure control, and mood as “unclear or conflicting scientific evidence” and the evidence for cognition as “fair negative scientific evidence.” Many of the studies of theanine were done in combination with caffeine as found in tea. While the studies found that the combination had some effect on mood, the studies found that theanine alone had little effect. More sufficiently designed studies in humans are warranted to further our understanding of the effects of theanine.
Theanine is reported to promote alpha wave production in the brain. Early studies of theanine involved much larger doses than those found in a typical cup of tea. Researchers wonder whether drinking tea might have the same effects found in those studies. However, one recent study funded by Unilever found that smaller doses typical of those found in a cup of tea did induce changes in alpha waves as shown by EEG.
Studies on test rats have shown even repeated, extremely high doses of theanine cause little to no harmful psychological or physical effects. Theanine showed neuroprotective effects in one rat study.
Several beverage manufacturers are selling drinks containing theanine and are marketing them as drinks to help people focus and concentrate, while other manufacturers claim relaxing and tranquillizing properties.
In 2003, the German Federal Institute for Risk Assessment (Bundesinstitut für Risikobewertung, BfR) objected to the addition of isolated theanine to beverages. The institute stated the amount of theanine consumed by regular drinkers of tea or coffee is virtually impossible to determine. While it was estimated the quantity of green tea consumed by the average Japanese tea drinker per day contains about 20 mg of the substance, there are no studies measuring the amount of theanine being extracted by typical preparation methods, or the percentage lost by discarding the first infusion. Therefore, with the Japanese being exposed to possibly much less than 20 mg per day, and Europeans presumably even less, it was the opinion of the BfR that pharmacological reactions to drinks typically containing 50 mg of theanine per 500 milliliters could not be excluded—reactions such as impairment of psychomotor skills and amplification of the sedating effects of alcohol and hypnotics.
In 2006, a study found no consistent, statistically significant treatment-related adverse effects on behavior, morbidity, mortality, body weight, food consumption and efficiency, clinical chemistry, hematology, or urinalysis in rats fed high doses of theanine for 13 weeks. Large studies in humans have not been undertaken; however, several smaller-scale studies (less than 100 participants) have shown increased alpha wave generation and lowered anxiety, along with benefits to sleep quality in people with ADHD.
The combination of theanine and caffeine has been shown to promote faster simple reaction time, faster numeric working memory reaction time and improved sentence verification accuracy. Due to this, the combination has become popular over recent years as a nootropic supplement typically found in the form of a pill, drink, or gum.[medical citation needed]
Theanine may help the body's immune response to infection by boosting the disease-fighting capacity of gamma delta T cells. The study, published in 2003, included a four-week trial with 11 coffee drinkers and 10 tea drinkers, who consumed 600 milliliters of coffee or black tea daily. Blood sample analysis found the production of antibacterial proteins was up to five times higher in the tea drinkers, an indicator of a stronger immune response. L-Theanine may contain mast cell-stabilizing properties in an animal model.
Theanine has been reported to raise levels of brain serotonin, dopamine, and GABA, with possible improvement in specific memory and learning tasks.
In tea brewing
A research study in 2011 showed that regular brewed black tea contains significantly more L-theanine per cup when compared to commercially available green tea (24 mg versus 8 mg per cup). The study demonstrates that brewing time is the major factor in L-theanine extraction from tea brewing. Addition of sugar and small quantities of milk seem to make no significant difference.
Notes and references
- Scheid, L.; Ellinger, S.; Alteheld, B.; Herholz, H.; Ellinger, J.; Henn, T.; Helfrich, H.-P.; Stehle, P. (2012). "Kinetics of L-Theanine Uptake and Metabolism in Healthy Participants Are Comparable after Ingestion of L-Theanine via Capsules and Green Tea". Journal of Nutrition. 142 (12): 2091–2096. doi:10.3945/jn.112.166371. ISSN 0022-3166.
- "D-theanine | C7H14N2O3". ChemSpider.com. Retrieved 2015-05-21.
- "Theanine". Pubchem Compound. NCBI. Retrieved 21 February 2015.
- "Components of Gyokuro｜ IPPODO". Ippodo-tea.co.jp. Retrieved 2015-05-07.
- SAKATO Y. J. Agri. Chem. Soc. 1949, 23, 262-7
- MASON R. Altern. & Complementary Ther. 2001, 7, 91-5
- Q.V. Vuong, M.C. Bowyer & P.D. Roach (2011) J Sci Food Agric. L-Theanine: properties, synthesis and isolation from tea, 91(11):1931-1939, DOI 10.1002/jsfa.4373, see , accessed 17 August 2014.
- See also, e.g., M.J. Desai & D.W. Armstrong (2004) Analysis of derivatized and underivatized theanine enantiomers by high-performance liquid chromatography/atmospheric pressure ionization-mass spectrometry, Rapid Commun Mass Spectrom. 18(3):251-256, and references therein, see , accessed 17 August 2014.
- "How Gyokuro is Processed ｜ IPPODO". Ippodo-tea.co.jp. Retrieved 2015-05-07.
- Finger, Andreas; Kuhr, Susanne; Engelhardt, Ulrich (1992). "Chromatography of tea constituents". Journal of Chromatography. 624: 309–310. doi:10.1016/0021-9673(92)85685-M.
- Casimir, J.; Jadot, J.; Renard, M. (1960). "Séparation et caractérisation de la N-éthyl-γ-glutamine à partir de Xerocomus badius" [Separation and characterization of N-ethyl-γ-glutamine from Xerocomus badius]. Biochimica et Biophysica Acta (in French). 39 (3): 462–8. doi:10.1016/0006-3002(60)90199-2. PMID 13808157.
- S. Kurihara, T. Shibakusa & K.A.K. Tanaka (2013) Cystine and theanine: amino acids as oral immunomodulative nutrients, Springerplus 2:635, DOI 10.1186/2193-1801-2-635, PMID 24312747, see , accessed 17 August 2014.
- Yokogoshi, Hidehiko; Kobayashi, Miki; Mochizuki, Mikiko; Terashima, Takehiko (1998). "Effect of theanine, γ-glutamylethylamide, on brain monoamines and striatal dopamine release in conscious rats". Neurochemical Research. 23 (5): 667–73. doi:10.1023/A:1022490806093. PMID 9566605.
- Nathan, Pradeep; Lu, Kristy; Gray, M.; Oliver, C. (2006). "The Neuropharmacology of L-Theanine(N-Ethyl-L-Glutamine)". Journal of Herbal Pharmacotherapy. 6 (2): 21–30. doi:10.1300/J157v06n02_02. PMID 17182482.
- Kakuda T, Nozawa A, Sugimoto A, Niino H (2002). "Inhibition by theanine of binding of [3H]AMPA, [3H]kainate, and [3H]MDL 105,519 to glutamate receptors". Biosci. Biotechnol. Biochem. 66 (12): 2683–6. doi:10.1271/bbb.66.2683. PMID 12596867.
- Kakuda T (2011). "Neuroprotective effects of theanine and its preventive effects on cognitive dysfunction". Pharmacol. Res. 64 (2): 162–8. doi:10.1016/j.phrs.2011.03.010. PMID 21477654.
- Kakuda T (2002). "Neuroprotective effects of the green tea components theanine and catechins". Biol. Pharm. Bull. 25 (12): 1513–8. doi:10.1248/bpb.25.1513. PMID 12499631.
- Wakabayashi C, Numakawa T, Ninomiya M, Chiba S, Kunugi H (2012). "Behavioral and molecular evidence for psychotropic effects in L-theanine". Psychopharmacology (Berl.). 219 (4): 1099–109. doi:10.1007/s00213-011-2440-z. PMID 21861094.
- Nagasawa K, Aoki H, Yasuda E, Nagai K, Shimohama S, Fujimoto S (2004). "Possible involvement of group I mGluRs in neuroprotective effect of theanine". Biochem. Biophys. Res. Commun. 320 (1): 116–22. doi:10.1016/j.bbrc.2004.05.143. PMID 15207710.
- Sugiyama T, Sadzuka Y, Tanaka K, Sonobe T (2001). "Inhibition of glutamate transporter by theanine enhances the therapeutic efficacy of doxorubicin". Toxicol. Lett. 121 (2): 89–96. doi:10.1016/s0378-4274(01)00317-4. PMID 11325559.
- Sugiyama T, Sadzuka Y (2003). "Theanine and glutamate transporter inhibitors enhance the antitumor efficacy of chemotherapeutic agents". Biochim. Biophys. Acta. 1653 (2): 47–59. doi:10.1016/s0304-419x(03)00031-3. PMID 14643924.
- Narukawa M, Toda Y, Nakagita T, Hayashi Y, Misaka T (2014). "L-Theanine elicits umami taste via the T1R1 + T1R3 umami taste receptor". Amino Acids. 46 (6): 1583–7. doi:10.1007/s00726-014-1713-3. PMID 24633359.
- Yamada T, Terashima T, Wada K, Ueda S, Ito M, Okubo T, Juneja LR, Yokogoshi H (2007). "Theanine, r-glutamylethylamide, increases neurotransmission concentrations and neurotrophin mRNA levels in the brain during lactation". Life Sci. 81 (16): 1247–55. doi:10.1016/j.lfs.2007.08.023. PMID 17904164.
- Yokogoshi H, Kobayashi M, Mochizuki M, Terashima T (1998). "Effect of theanine, r-glutamylethylamide, on brain monoamines and striatal dopamine release in conscious rats". Neurochem. Res. 23 (5): 667–73. doi:10.1023/A:1022490806093. PMID 9566605.
- Yokogoshi, Hidehiko; Mochizuki, Mikiko; Saitoh, Kotomi (1998). "Theanine-induced Reduction of Brain Serotonin Concentration in Rats". Bioscience, Biotechnology, and Biochemistry. 62 (4): 816–7. doi:10.1271/bbb.62.816. PMID 9614715.
- Yokogoshi, Hidehiko; Kato, Yukiko; Sagesaka, Yuko M.; Takihara-Matsuura, Takanobu; Kakuda, Takami; Takeuchi, Naokazu (1995). "Reduction Effect of Theanine on Blood Pressure and Brain 5-Hydroxyindoles in Spontaneously Hypertensive Rats". Bioscience, Biotechnology, and Biochemistry. 59 (4): 615–8. doi:10.1271/bbb.59.615. PMID 7539642.
- Gomez-Ramirez, Manuel; Higgins, Beth A.; Rycroft, Jane A.; Owen, Gail N.; Mahoney, Jeannette; Shpaner, Marina; Foxe, John J. (2007). "The Deployment of Intersensory Selective Attention". Clinical Neuropharmacology. 30 (1): 25–38. doi:10.1097/01.WNF.0000240940.13876.17. PMID 17272967.
- Kimura, Kenta; Ozeki, Makoto; Juneja, Lekh Raj; Ohira, Hideki (2007). "L-Theanine reduces psychological and physiological stress responses". Biological Psychology. 74 (1): 39–45. doi:10.1016/j.biopsycho.2006.06.006. PMID 16930802.
- Park, Sang-Ki; Jung, In-Chul; Lee, Won Kyung; Lee, Young Sun; Park, Hyoung Kook; Go, Hyo Jin; Kim, Kiseong; Lim, Nam Kyoo; et al. (2011). "A Combination of Green Tea Extract andl-Theanine Improves Memory and Attention in Subjects with Mild Cognitive Impairment: A Double-Blind Placebo-Controlled Study". Journal of Medicinal Food. 14 (4): 334–43. doi:10.1089/jmf.2009.1374. PMID 21303262.
- Haskell, Crystal F.; Kennedy, David O.; Milne, Anthea L.; Wesnes, Keith A.; Scholey, Andrew B. (2008). "The effects of l-theanine, caffeine and their combination on cognition and mood". Biological Psychology. 77 (2): 113–22. doi:10.1016/j.biopsycho.2007.09.008. PMID 18006208.
- Raloff, Janet (September 29, 2007). "Distracted? Tea might help your focus". Science News. 172 (13): 206. doi:10.1002/scin.2007.5591721319.
John J. Foxe of the Nathan S. Kline Institute for Psychiatric Research in Orangeburg, N.Y., and his colleagues recruited 16 people for tests of attentiveness on four days. Before testing, each individual drank a glass of water. On 3 days, the drink was spiked with 100 milligrams of theanine, 60 mg of caffeine, or both. The theanine dose was equivalent to that in 4 to 5 cups of tea, and the caffeine translated to about 2.5 cups of tea. In the difficult tests, participants watched a computer screen and pressed a button when a designated shape appeared on the side of a busy visual field to which an arrow had previously pointed. Participants' accuracy differed little between days when they got water alone or with only one additive. Accuracy improved dramatically, however, on the day that they got the theanine-caffeine combination. The attention benefit lasted throughout the 3 hours of testing.
- Owen, Gail N.; Parnell, Holly; De Bruin, Eveline A.; Rycroft, Jane A. (2008). "The combined effects of L-theanine and caffeine on cognitive performance and mood". Nutritional Neuroscience. 11 (4): 193–8. doi:10.1179/147683008X301513. PMID 18681988.
- Einöther, Suzanne J.L.; Martens, Vanessa E.G.; Rycroft, Jane A.; De Bruin, Eveline A. (2010). "L-Theanine and caffeine improve task switching but not intersensory attention or subjective alertness". Appetite. 54 (2): 406–9. doi:10.1016/j.appet.2010.01.003. PMID 20079786.
- Giesbrecht, T.; Rycroft, J.A.; Rowson, M.J.; De Bruin, E.A. (2010). "The combination of L-theanine and caffeine improves cognitive performance and increases subjective alertness". Nutritional Neuroscience. 13 (6): 283–90. doi:10.1179/147683010X12611460764840. PMID 21040626.
- Kelly, Simon P.; Gomez-Ramirez, Manuel; Montesi, Jennifer L.; Foxe, John J. (2008). "L-theanine and caffeine in combination affect human cognition as evidenced by oscillatory alpha-band activity and attention task performance". The Journal of Nutrition. 138 (8): 1572S–1577S. PMID 18641209.
- "Theanine Monograph". Natural Standard. Archived from the original on December 24, 2014. Retrieved 30 October 2014.[dead link]
- Roan, Shari (May 17, 2009). "L-theanine: New drinks promise focus, but more research attention needed". Chicago Tribune.
- 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 Suppl 1: 167–8. PMID 18296328.
- Borzelleca, J.F.; Peters, D.; Hall, W. (2006). "A 13-week dietary toxicity and toxicokinetic study with l-theanine in rats". Food and Chemical Toxicology. 44 (7): 1158–66. doi:10.1016/j.fct.2006.03.014. PMID 16759779.
- Egashira, Nobuaki; Ishigami, Noriko; Pu, Fengling; Mishima, Kenichi; Iwasaki, Katsunori; Orito, Kensuke; Oishi, Ryozo; Fujiwara, Michihiro (2008). "Theanine prevents memory impairment induced by repeated cerebral ischemia in rats". Phytotherapy Research. 22 (1): 65–8. doi:10.1002/ptr.2261. PMID 17705146.
- Ritsner, Michael S.; Miodownik, Chanoch; Ratner, Yael; Shleifer, Tatyana; Mar, Maria; Pintov, Leonid; Lerner, Vladimir (2011). "L-Theanine Relieves Positive, Activation, and Anxiety Symptoms in Patients With Schizophrenia and Schizoaffective Disorder". The Journal of Clinical Psychiatry. 72 (01): 34–42. doi:10.4088/JCP.09m05324gre. ISSN 0160-6689. PMID 21208586.
- Di, X.; Yan, J.; Zhao, Y.; Chang, Y.; Zhao, B. (2012). "L-theanine inhibits nicotine-induced dependence via regulation of the nicotine acetylcholine receptor-dopamine reward pathway". Science China Life Sciences. 55 (12): 1064–1074. doi:10.1007/s11427-012-4401-0. PMID 23233221.
- "Getränke mit isoliertem L-Theanin" [Beverages with isolated L-theanine] (PDF) (in German). Bundesinstitut für Risikobewertung. August 2003.
- Robin B. Kanarek; Harris R. Lieberman (6 October 2011). Diet, Brain, Behavior: Practical Implications. CRC Press. pp. 239–. ISBN 978-1-4398-2156-5.
- Lyon, Michael R.; Kapoor, Mahendra P.; Juneja, Lekh R. (2011). "The effects of L-theanine (Suntheanine®) on objective sleep quality in boys with attention deficit hyperactivity disorder (ADHD): a randomized, double-blind, placebo-controlled clinical trial" (PDF). Alternative Medicine Review. 16 (4): 348–54. PMID 22214254.
- Kobayashi, Kanari; Nagato, Yukiko; Aoi, Nobuyuki; Juneja, Lekh Raj; Kim, Mujo; Yamamoto, Takehiko; Sugimoto, Sukeo (1998). "L-テアニンのヒトの脳波に及ぼす影響" [Effects of L-Theanine on the Release of α-Brain Waves in Human Volunteers]. Journal of the Agricultural Chemical Society of Japan (in Japanese). 72 (2): 153–7. doi:10.1271/nogeikagaku1924.72.153.
- Bryan, Janet (2008). "Psychological effects of dietary components of tea: Caffeine and L-theanine". Nutrition Reviews. 66 (2): 82–90. doi:10.1111/j.1753-4887.2007.00011.x. PMID 18254874.
- Kelly, Simon P.; Gomez-Ramirez, Manuel; Montesi, Jennifer L.; Foxe, John J. (2008). "L-Theanine and Caffeine in Combination Affect Human Cognition as Evidenced by Oscillatory alpha-Band Activity and Attention Task Performance". The Journal of Nutrition. 138 (8): 1572S–1577S. PMID 18641209.
- Kamath, Arati B.; Wang, Lisheng; Das, Hiranmoy; Li, Lin; Reinhold, Vernon N.; Bukowski, Jack F. (2003). "Antigens in tea-beverage prime human Vγ2Vδ2 T cells in vitro and in vivo for memory and nonmemory antibacterial cytokine responses". Proceedings of the National Academy of Sciences. 100 (10): 6009–14. Bibcode:2003PNAS..100.6009K. doi:10.1073/pnas.1035603100. JSTOR 3147533. PMC . PMID 12719524.
- Kim, N. H.; Jeong, H. J.; Kim, H. M. (2011). "Theanine is a candidate amino acid for pharmacological stabilization of mast cells". Amino Acids. 42 (5): 1609–18. doi:10.1007/s00726-011-0847-9. PMID 21344174.
- Nathan, PJ; Lu, K; Gray, M; Oliver, C (2015-04-20). "The neuropharmacology of L-theanine(N-ethyl-L-glutamine): a possible neuroprotective and cognitive enhancing agent". J Herb Pharmacother. 6: 21–30. doi:10.1300/J157v06n02_02. PMID 17182482.
- Y. Orihara; T. Furuya (1990). "Production of theanine and other γ-glutamyl derivatives by Camellia sinensis cultured cells". Plant Cell Reports. 9 (2): 65–68. doi:10.1007/BF00231550. PMID 24226431.
- E.K. Keenan; M.D.A. Finnie; P.S. Jones; P.J. Rogers; C.M. Priestley (2011). "How much theanine in a cup of tea? Effects of tea type and method of preparation". Food Chemistry. 125 (2): 588–594. doi:10.1016/j.foodchem.2010.08.071.