L-Theanine: an Amino Acid in Tea to Help Mild Cognitive Impairment

by Ari Magill, MD

Tea has been long recognized in the Orient to have a calming and soothing effect, which has become increasingly appreciated in Western countries.1 L-theanine, an amino acid  found in black, green, and white teas from Camellia Sinensis,2 was initially extracted by scientists from green tea.3

The Takeaway:

An amino acid derived from the Camellia Sinensis plant and found in black, green, and white teas could help you relax and bolster your memory.

Animal Studies

Tian et al used a mouse model to investigate whether L-theanine could protect against stress-induced impairments in cognition and memory.4 A physical restraint, administered by suddenly placing animals in well-ventilated tubing for eight hours a day, was used as the stressor. In the experimental group, mice were administered L-theanine 30 minutes prior to restraint.

A water mazea and a step-throughb test evaluated their cognitive skills. Restraint-induced stress resulted in profound impairment on the cognitive tests and increased levels of stress-induced biomarkers in the serum and brain, including serum cortisol and certain neurotransmitters. L-theanine administration prevented cognitive impairment and biomarker changes.

In a laboratory rat model, the impact of L-theanine on memory formation was assessed on a molecular level by studying the phenomenon of long-term potentiation (LTP), a molecular process whereby nerve cells become sensitized and linked to each through communication interfaces known as synapses.These studies show that L-theanine could circumvent the deleterious impact of stress on LTP formation.5,6 It was also shown to lower the animals’ elevated blood pressure.7

Human Studies

Japanese food scientists found an inexpensive way to mass produce L-theanine using enzymes in a lab without requiring extraction from tea leaves.1 It increased the frequency of alpha waves (associated with restful wakefulness but not sleep1) on an EEG (used to monitor patient’s brain waves) in areas at the back of the brain at doses of 50 to 200 mg.7 Administration of 50 mg of L-theanine in another study resulted in an increased incidence of these waves both during rest with eyes closed and while participating in a passive activity.8 Kelly et al speculated that the substance might augment concentration ability given the prominent role alpha waves play during these tasks.

L-theanine has a tempering effect on caffeine-induced stimulation.1 Kelly et al examined the combined effects of L-theanine and caffeine in subjects who had to focus on a particular area of the screen (after given a cue) and locate one out of many targets.8 Caffeine aided in ultimate identification, but the ability to discriminate between correct and incorrect marks were enhanced when they gave participants a combination of the two agents.

Another study showed that L-theanine significantly improved performance in a task that required switching attentional focus, although it did not increase feelings of subjective alertness.9

One trial found that it significantly improved reaction time and performance of tasks requiring visual attention while increasing alpha wave activity and decreasing heart rate– but only in subjects who rated high on a self-reported anxiety questionnaire.10

Park et al suggests benefit in mild cognitive impairment (MCI).11 Patients scoring at the lowest end of MCI had significant improvement in a mental status exam after 16 weeks of taking a pill combining 60 mg of L-theanine with other green tea extracts.

Kuriyama et al investigated whether there was a relationship between green tea consumption and cognitive performance, and found that people who drank greater quantities of green tea had significantly decreased risk of cognitive impairment.12

Possible Mechanisms of Action

L-theanine enhances nitric oxide production within the cells of the inner lining of blood vessels.13 It serves as a chemical messenger to tell the vessels to dilate, which could enhance cognitive performance by improving cerebral blood flow that feeds brain cells.

L-theanine also seems to work by counteracting excitotoxicity, a phenomenon where excessive excitation leads to nerve cell injury and death due to oxidative damage and other mechanisms. The substance binds to the same receptors as glutamate, the most common neurotransmitter in the brain that excites other nerve cells, but with greatly reduced affinity.14 However, it does not activate that receptor. By occupying the binding spot, glutamate cannot land there, resulting in nerve-cell relaxation without actively inhibiting them. It was also shown to inhibit glutamine build-up within both neurons and nerve cell helper cells.15


a Water-maze: a test in which the mouse has to remember the location of a solid “island” within a “sea” of surrounding water.

b Step-through exercise: a bright box is connected to a dark box by a “guillotine” door. The mouse is placed in the bright box with the connecting door closed and allowed to explore (although they instinctively prefer dark, hidden places). The connecting door is opened; the mouse will instinctively move through the door into the black box, but then receives a shock, so it has to learn to refrain from moving into the black box.



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  13. Siamwala JH, et al. L-theanine promotes nitric oxide production in endothelial cells through eNOS phosphorylation. J Nutr Biochem. 2013;2(3):595-605. doi: 10.1016/j.jnutbio.2012.02.016.
  14. Kakuda T, et al. Inhibition by theanine of binding of [3H] AMPA,[3H] kainate, and [3H] MDL 105,519 to glutamate receptors. Biosci Biotech Biochem. 2002;66(12): 2683-2686. doi: 10.1271/bbb.66.2683.
  15. Kakuda T, et al. Theanine, an ingredient of green tea, inhibits [3H] glutamine transport in neurons and astroglia in rat brain. J Neurosci Res. 2008;86(8):1846-1856. doi: 10.1002/jnr.21637.

About the Author

Dr. Ari Magill is a holistic neurologist and medical consultant based in Mesa, AZ. He received medical school training at University of Texas Southwestern in Dallas and residency training at the University of Arizona in Tucson. He is passionate about finding innovative treatments for cognitive impairment, emphasizing lifestyle change and natural supplements.