Synonyms: Gamma-glutamylethylamide,
5-N-ethylglutamine
Introduction
Theanine, pronounced "tea-anene" (as in tea), is an amino acid found
only in tea and a single species of mushroom (Xerocomus badius) in its
free amino acid form. It was first identified and isolated as a major
component of green tea in the late 1940's, and it was later found in
other forms of tea as well [1-2]. Theanine constitutes around 50% of
the amino acids in tea and makes up 1-2% of the dry weight of tea on
average [2-3], although one source indicates that the number is closer
to 1.5-3% [1]. This places the content in a normal cup of tea in the
20-60 mg range.
The amino acids, especially theanine, are one of the primary reasons
for the taste of tea [2]. The amount of theanine in a cup of tea shows
the highest correlation of any constituent with tea quality (as
determined by tea tasters) [3]. The taste of theanine is described as
"umami" or "brothy," which is separate from the four other basic tastes
(sweet, sour, salty, bitter) [4]. About 20-25% of the population cannot
distinguish this taste [5].
Theanine has been found to be involved in many of the effects of tea,
which is known to have numerous benefits (see the
green tea article).
Effects on Mood & Cognition
Theanine may have has shown to support mood. In
human studies, oral administration of theanine has been reported to
dose-dependently increase the production of alpha waves in the
occipital and parietal regions of the brain [6-8]. According to one
article, this signifies a state of being awake, alert, and relaxed at
the same time. This effect occurs within 30-40 minutes of oral
administration of 50-200 mg. It should be noted that the study
population in this research consisted only of females 18-22 years of
age, and this effect has not yet been confirmed in other human
populations [8].
The behavioral effects of theanine on rodents have also been the
subject of study. Theanine does not change activity level or
exploration behavior in normal animals. However, it does improve
performance on a number of tests of memory and learning, including the
active avoidance test, the passive avoidance test, and the Morris water
maze [9]. Using electroencephalography, it has also been determined
that theanine antagonizes the effect of caffeine. If about eight times
as much theanine than caffeine is given (on a per milligram basis), the
effect of caffeine is completely blunted [2, 10]. It has been suggested
that theanine is responsible for the relaxing effect of tea despite the
caffeine content.
Animal studies have also explored the possible mechanism by which
theanine leads to these effects. Theanine reliably causes a significant
increase in CNS dopamine (DA) levels, especially in the striatum,
hypothalamus, and hippocampus [2, 9, 11-12]. The effect theanine has on
levels of other neurotransmitters is less well-established. It has been
reported to both increase and decrease central serotonin levels
[11-12]. One study indicated that it may decrease central
norepinephrine [3]. Finally, a study in the early 70's reported that
theanine may have an effect on the formation of gamma-aminobutyric acid
(GABA) [13]. The direct mechanism for these changes is not clear,
although it may be related to an effect at N-methyl-D-aspartate (NMDA)
receptors [14].
Neuroprotection
Theanine has neuroprotective properties in experimental models. In animals, theanine has also been
reported to increase the synthesis rate of nerve growth factor (NGF)
[6].
The neuroprotective properties are thought to be primarily due to the
fact that it is structurally similar to glutamate. Glutamate is an
excitatory neurotransmitter, and in some instances it can cause
excitotoxicity [14]. It has been traditionally thought that theanine
prevents toxicity by acting as a competitive antagonist to glutamate at
ionotropic glutamate receptors, of which there are three types, NMDA,
alpha-amino-3-hydroxy-5-methylisoxazol-4-propionic acid (AMPA), and
kainate (KA) [15]. In rat cortical neurons, theanine acted at all three
subtypes, but had less binding affinity than glutamate. Also, the
binding affinity to NMDA receptors was an order of magnitude lower than
to AMPA and KA receptors [14]. It is possible that actions at all of
the subtypes could play important roles in the activity of theanine.
Because the effective concentration in antagonizing ionotropic
glutamate receptors is so low compared to glutamate, a more recent
study hypothesized that the neuroprotective effect was related to an
effect at metabotropic glutamate receptors instead [11]. This study
found that inhibition of glutamate-induced neuronal death by theanine
could be inhibited by an antagonist of group I metabotropic glutamate
receptors. This indicates that this may be the mechanism for the
neuroprotective effect of theanine. Given the current state of
research, it is too early to draw any solid conclusions, as the effect
of theanine could ultimately prove to be due to an action at any
combination of these receptors. An effect on glutamate transporters may
also be involved [14].
A final mechanism of neuroprotection is the antioxidant effect of
theanine. One study suggested that one of the protective mechanisms was
inhibition of lipid peroxidation. Vitamin E has also been found to
inhibit ischemia induced by MCA occlusion [15].
Other Benefits
Tea drinking has been found to support the immune system
in humans, and theanine and related compounds may be the reason.
Theanine and its precursor ethylamine can act as antigens, which
"prime" T cells and make them more able to respond effectively to
challenges to the immune system by pathogens.
A final use for theanine is to hide the bitter taste of some foods or
supplements. Although umami is not a strong taste, it effectively masks
bitter tastes, such as the taste of caffeine, ginseng, some
antioxidants, and bitter foods [13].
Suggested Use
After oral administration, theanine is absorbed in the intestine and
then easily crosses into the brain from the bloodstream through an
amino acid transporter [11, 15]. The increase in alpha wave activity in
the brain is measurable within 30 minutes and maximized at the 40
minute point [8]. Although the pharmacokinetic profile in humans is not
well-known, in rats, it is metabolized in the kidney to L-glutamate and
ethylamine and is completely absent 24 hours after administration [1]
[26].
No toxic effects of theanine have been reported, and green tea has a
long history of safe use [1, 8]. Toxicology studies have failed to find
any side effects or toxic effects associated with theanine, even in
large amounts. In Japan, it underwent the 28 Day Subacute Toxicity
Study, the 78 Week Evaluation of Toxicity and Carcinogenicity, the
Acute Toxicity/LD50 Determination, and the Ames Salmonella Mutagenicity
tests, and was subsequently approved for unlimited use as a food
additive except in infant foods [13]. The LD50 is 5 g/kg [8].
The generally recommended dosage of theanine is 100-200 mg 2-3 times
daily or as needed, although some sources recommend even higher doses.
It can be used on a regular basis for general stress prevention and
good health or saved for periods of high stress. Theanine does not have
a strong taste and mixes easily in water.
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