L-theanine (𝛄-ethylamino-L-glutamic acid) is an amino acid found almost exclusively in the leaves of the tea plant (Camelia sinensis). L-theanine is thought to possess physiological activity in humans, and in particular may be able to affect the relaxation state of the brain. L-theanine makes up about 1 to 2 % of the dry weight of tea leaves, and in non-proteinaceous, in that it only appears in its free form. Of the amino acids in tea, it makes up over 50 % of the total, making it the most numerous amino acid in tea. Biochemical studies suggest that L-theanine acts antagonistically against caffeine. As well as its physiological effects in humans, studies show that L-theanine contributes significantly to the taste of tea. The detailed effects of L-theanine are beginning to be uncovered, and studies to date have shown that L-theanine can modulate brain wave activity by increasing α-wave generation. As α-waves are known to be associated with a relaxed state of mental capacity, this may explain the relaxing effects of tea.
L-theanine is well absorbed in humans, with studies showing that the amino acid is rapidly absorbed from the gastrointestinal tract, and in animal models can accumulate in the brain within 30 min of administration. To enter the brain of rats, L-theanine may use the L-type transporter system (for transporting large neutral amino acids), and in this respect may inhibit other amino acids that use this system including tryptophan, tyrosine and the branched chain amino acids. It has been suggested that L-theanine is absorbed from the small intestine via the methionine carrier transport system, and in this respect may show antagonism to amino acids that use the same system, notably methionine, valine, leucine and isoleucine. High doses of L-theanine therefore inhibit the absorption of these amino acids to the brain, as has been demonstrated in animal models. In contrast concentrations of alanine, serine, glycine, aspartic acid and glutamic acid may not be affected by L-theanine administration.
Once in the brain L-theanine may interact with a number of neurotransmitter systems. This may include both direct interaction with enzymes as well as indirect action because of the altered ratio of amino acids reaching the brain. Some evidence suggests that L-theanine increases levels of dopamine in certain parts of the brain, including the striatum. L-theanine may also affect the serotonin system, through various mechanisms. Firstly the L-theanine may increase the content of tryptophan in the brain. This may result from the inhibitory effect of L-theanine on the uptake of certain amino acids that normally compete with tryptophan for entry to the brain across the blood brain barrier on the L-amino acid transporter. However, while L-theanine may increase tryptophan levels, serotonin (a product of tryptophan) levels may actually fall. Serotonin synthesis may therefore be inhibited, or the degradation and excretion of serotonin enhanced. However, the understanding of these interactions is still in its infancy.
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