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Theanine (specifically the L-theanine form) is an amino acid analog of the amino acids L-glutamic acid and L-glutamine. It is primarily found in black, green, and white teas prepared from Camellia sinensis. It’s uncommon in the typical Western diet.

Is it safe? Are there side effects?

Yes, L-theanine is classified as GRAS (generally regarded as safe) by the FDA.

Effects on Cognition


The general picture that emerges from L-theanine research is an anti-stress effect, with the strongest hypothesis being that anti-glutamate receptor activity is the primary driver of this effect. In particular, Kimura et al. 2002 demonstrated that stress (measured subjectively by the state trait anxiety index and objectively by heart rate variability) is significantly reduced as a result of L-theanine administration during a stressful task (Figures 1 and 2).

L-theanine, HRVFigure 1. L-theanine reduces heart rate variability during an acutely stressful task.

L-theanine, STAIFigure 1. L-theanine reduces the state trait anxiety index during an acutely stressful task.

There is also evidence to suggest that L-theanine administration prior to stress in academic settings can act to reduce stress levels as measured by questionnaires and the salivary marker alpha-amylase.

L-theanine supplementation has also been shown to increase brain α-waves (8-10 Hz range) which are associated with reduced stress and anxiety. Beyond relaxation, increased α-waves are associated with selective attention mechanisms and mental alertness. These altered wave functions are said to be evidence that theanine has ‘relaxing and attention promoting’ properties. There’s also literature that report increased theta wave function which is associated with learning and memory.

High levels of stress are well known to result in cognitive and health deficits in both humans and animal models. In one study of mice, L-theanine reversed the lifespan reduction, cognitive deficits, and behavioral depression associated with chronic stress. In another study in mice, L-theanine was shown to reverse 3 stress-induced, depression-like phenotypes in mice. These data point to robust effects of L-theanine on a variety of general and cognitive health correlates that are impacted by stress.


By itself, the effects of L-theanine on attention is not pronounced; however, there is a strong synergism between caffeine and L-theanine for improving attention.

Mechanisms of Action

Orally ingested theanine crosses the blood-brain barrier, so it is active in the brain. The increased brain concentrations of the compound is mediated via neutral-charge amino acid transport. Following oral intake, theanine reaches the brain within an hour remains elevated for up to 5 hours. After this time, it is eliminated and at 24 hours post ingestion, there is none left.

L-theanine inhibits binding of glutamate to its cortical neuronal receptors: AMPA, kainate, and NMDA. These excitatory receptors increase neuronal activity. Since glutamate is known to increase during stress, the inhibition of glutamate receptor activity is thought to contribute to the anti-stress effects of L-theanine. L-theanine has also been shown to reduce blood pressure in an animal model of hypertension. Further, it dampens the excitatory effects of caffeine, as measured by EEG and blood pressure.

Caffeine & L-theanine

Caffeine and L-theanine can act synergistically to improve alertness and attention. L-theanine can also potentially reverse some of the less favorable physiological effects of caffeine – including increased blood pressure, and reduced flow of oxygenated blood to the head.


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