L-theanine

Several clinical trials have examined the effects of L-theanine, alone or in combination with caffeine or green tea, on brain health. No human studies have tested it for the prevention of dementia or cognitive decline.

Our search identified:
• 1 meta-analysis based on 10 randomized controlled trials testing acute effects of L-theanine alone or in combination with caffeine in healthy adults
• 6 randomized controlled trials (3 testing acute effects, 2 examining brain activity, and 1 with green tea extract in people with mild cognitive impairment)
• 1 open-label study in people with major depressive disorder
• 1 review
• Numerous preclinical studies

The long-term effects of L-theanine on cognitive health are unknown, but a few short-term human studies have shown small benefits. In a randomized controlled trial in patients with mild cognitive impairment, L-theanine taken with green tea extract for 16 weeks had no significant effect on memory and attention, though a trend for an improvement in memory was seen midway through the trial [1]. The patients who had more severe impairment at the start of the trial appeared more likely to benefit from treatment, but these results have not yet been replicated.

Combinations of L-theanine and caffeine have been reported to acutely improve attention and alertness in small clinical trials, but the positive effects have been primarily attributed to caffeine [2]. L-theanine may interact with caffeine, improving attention and ability to ignore distractions, and together, enhance performance on cognitively demanding tasks [3]. In a small randomized controlled trial of healthy adults, L-theanine alone did not affect attentional focus [4]. Other studies have reported mixed effects, with L-theanine decreasing the beneficial effects of caffeine on cognition and mood [5].

Preclinical studies have reported that L-theanine may reduce brain cell death [6] and oxidative damage [7], and increase levels of protective chemicals [8], but these effects have not been confirmed in humans.

For Dementia Patients

It is currently unknown whether L-theanine can improve cognition or slow cognitive decline in people with dementia. While some benefits have been observed in preclinical studies of Alzheimer's disease [6], L-theanine has not been clinically studied in dementia patients.

Large meta-analyses of long-term tea consumption (containing L-theanine) have found that side effects are mild [9][10] but these results on tea may not apply to supplements. In a short randomized controlled trial, an L-theanine supplement (400 mg/day) was well-tolerated with no significant adverse events [11]. Larger, long-term studies are needed. For more information on green tea, please review our separate report.

The interactions between L-theanine and other drugs have not been well-studied. Clinical data suggest that L-theanine lessens the blood pressure increase caused by caffeine [12][13]. Because of its possible effects on blood pressure [12][14], it may be dangerous to use in combination with blood pressure medications.

NOTE: This is not a comprehensive safety evaluation or complete list of potentially harmful drug interactions. It is important to discuss safety issues with your physician before taking any new supplement or medication.

L-theanine supplements are available in both capsule and powder forms. A single cup (200 ml) of tea can contain 5 to 85 mg of L-theanine depending on the type, quality, and preparation of tea. Clinical trials examining the effects of L-theanine on cognitive function have used doses ranging from 12–250 mg/day, with the majority of studies using 200 mg/day [2][15][16].

An analysis of commercially available products labeled as L-theanine.

United States Pharmacopeial Convention (USP) and FDA Information on Dietary Supplements offer information on the quality of specific supplements and can assist in finding a trusted brand.

1. Park SK, Jung IC, Lee WK et al. (2011) A combination of green tea extract and l-theanine improves memory and attention in subjects with mild cognitive impairment: a double-blind placebo-controlled study. J Med Food 14, 334-343.
2. Camfield DA, Stough C, Farrimond J et al. (2014) Acute effects of tea constituents L-theanine, caffeine, and epigallocatechin gallate on cognitive function and mood: a systematic review and meta-analysis. Nutr Rev 72, 507-522.
3. Owen GN, Parnell H, De Bruin EA et al. (2008) The combined effects of L-theanine and caffeine on cognitive performance and mood. Nutr Neurosci 11, 193-198.
4. Kelly SP, Gomez-Ramirez M, Montesi JL et al. (2008) L-theanine and caffeine in combination affect human cognition as evidenced by oscillatory alpha-band activity and attention task performance. J Nutr 138, 1572S-1577S.
5. Dodd FL, Kennedy DO, Riby LM et al. (2015) A double-blind, placebo-controlled study evaluating the effects of caffeine and L-theanine both alone and in combination on cerebral blood flow, cognition and mood. Psychopharmacology (Berl) 232, 2563-2576.
6. Kim TI, Lee YK, Park SG et al. (2009) l-Theanine, an amino acid in green tea, attenuates beta-amyloid-induced cognitive dysfunction and neurotoxicity: reduction in oxidative damage and inactivation of ERK/p38 kinase and NF-kappaB pathways. Free Radic Biol Med 47, 1601-1610.
7. Nishida K, Yasuda E, Nagasawa K et al. (2008) Altered levels of oxidation and phospholipase C isozyme expression in the brains of theanine-administered rats. Biol Pharm Bull 31, 857-860.
8. Cho HS, Kim S, Lee SY et al. (2008) Protective effect of the green tea component, L-theanine on environmental toxins-induced neuronal cell death. Neurotoxicology 29, 656-662.
9. Hartley L, Flowers N, Holmes J et al. (2013) Green and black tea for the primary prevention of cardiovascular disease. Cochrane Database Syst Rev, CD009934.
10. Jurgens TM, Whelan AM, Killian L et al. (2012) Green tea for weight loss and weight maintenance in overweight or obese adults. Cochrane Database Syst Rev 12, CD008650.
11. Lyon MR, Kapoor MP, Juneja LR (2011) The effects of L-theanine (Suntheanine(R)) on objective sleep quality in boys with attention deficit hyperactivity disorder (ADHD): a randomized, double-blind, placebo-controlled clinical trial. Altern Med Rev 16, 348-354.
12. Rogers PJ, Smith JE, Heatherley SV et al. (2008) Time for tea: mood, blood pressure and cognitive performance effects of caffeine and theanine administered alone and together. Psychopharmacology (Berl) 195, 569-577.
13. Yoto A, Motoki M, Murao S et al. (2012) Effects of L-theanine or caffeine intake on changes in blood pressure under physical and psychological stresses. J Physiol Anthropol 31, 28.
14. Siamwala JH, Dias PM, Majumder S et al. (2013) L-theanine promotes nitric oxide production in endothelial cells through eNOS phosphorylation. J Nutr Biochem 24, 595-605.
15. Kahathuduwa CN, Dassanayake TL, Amarakoon AM et al. (2016) Acute effects of theanine, caffeine and theanine-caffeine combination on attention. Nutr Neurosci.
16. White DJ, de Klerk S, Woods W et al. (2016) Anti-Stress, Behavioural and Magnetoencephalography Effects of an L-Theanine-Based Nutrient Drink: A Randomised, Double-Blind, Placebo-Controlled, Crossover Trial. Nutrients 8.