Theanine May Help You Fight Off Disease

Theanine May Help You
Fight Off Disease

Green tea amino acid enhances immune function, protects brain cells, and may promote relaxation
By Aaron W. Jensen, Ph.D.

ave you heard the good news about tea? Of course you have—unless you’ve been living in a cave somewhere. The wonders of tea, especially green tea, have been featured in scores of stories in the news media. And with good reason: tea really is a wonder beverage, full of natural chemical compounds that promote good health and protect against chronic diseases, especially heart disease and cancer.

About 20 years ago, green tea began to enter the public health consciousness, eventually capturing the spotlight as the hot beverage of choice. Now we know that the three major kinds of tea—green (unfermented), oolong (partially fermented), and black (fully fermented), all of which are made from leaves of the tea tree, Camellia sinensis—pack a powerful health punch. A great deal of research supports the health benefits of green tea in particular, but this article is about certain health benefits associated with all true teas (i.e., excluding herbal teas, which are fine beverages but are not true teas).

Theanine Is Found in All True Teas

The unremarkable appearance of tea leaves belies the exotic biochemistry brewing inside them as they grow. Most important, from the health point of view, is a class of polyphenolic compounds called catechins, the most abundant and potent of which is epigallocatechin gallate, or EGCG. Many polyphenolic compounds are powerful antioxidants, which are highly beneficial. Other biologically active compounds in tea include a variety of beneficial flavonoids, as well as a fair amount of caffeine (which can be considered a friend or foe, depending on how close it is to bedtime).

The Harvard researchers stated,
“These data show that ingestion of
tea . . . results in a markedly
enhanced innate immune
response to bacteria that is

Yet another compound found in tea leaves is theanine (L-theanine for purists; the L has to do with a detail of the molecular structure). This rare amino acid—it is found nowhere else in nature except in two other Camellia species and one species of mushroom—is basking in the spotlight now (we’ll soon see why). Theanine constitutes 1–2% of the dry weight of tea leaves, where it exists only in the free form (i.e., not as a component of proteins) and accounts for about 50% of the total free amino acid content. In tea beverages, theanine is found in roughly the same concentrations in green, oolong, and black teas. This means that, unlike EGCG, theanine is unaffected by the oxidative fermentation process that transforms green to oolong, and oolong to black.

Theanine Produces Ethylamine—Which Is Good

To add to the already glowing reputation of tea as a health beverage, a research group at Harvard Medical School has discovered that the theanine in tea helps boost the immune system.1 Their interest was piqued by the fact that theanine is a precursor to a compound called ethylamine, an antigen molecule that primes the body to produce a type of immune-system blood cell called the gamma-delta T cell. That type of cell is known to be the body’s first line of defense against hostile microbial invaders, particularly bacteria, viruses, and parasites. Gamma-delta T cells act by secreting a protein called interferon-gamma, which is a key part of the body’s chemical defense system against infection.

Drinking theanine solutions
tended to generate alpha-waves,
which are typically associated
with a relaxed but wakeful
(not drowsy) mental state.

Ethylamine molecules are commonly found on the surfaces of bacteria, viruses, and parasites. That may sound bad, but it’s not, because it’s the ethylamine, acting as an antigen, that triggers the immune response (the formation of antibodies) to those invaders. How interesting—and fortunate—it is that ethylamine is also found in tea leaves, where it is formed from theanine; when tea is drunk, still more of the theanine content is converted to ethylamine in the liver. (Ethylamine, by the way, is also found, in lesser amounts, in some other foods, notably mushrooms, apples, and wine, and it and a number of closely related compounds are found in various bodily fluids of healthy humans: blood, urine, breast milk, vaginal secretions, and amniotic fluid.)

Theanine Boosts Immune Function

The Harvard researchers recruited 11 healthy, non-tea-drinking subjects and instructed them to drink 5–6 cups (about 600 ml) per day of black tea for either 2 weeks or 4 weeks. This volume of tea contained about 230 mg of theanine, or about 42 mg per cup if you take 5 1/2 cups as the average. As a control group, 10 healthy, non-tea- and non-coffee-drinking subjects were instructed to drink 5–6 cups per day of instant coffee (which contains caffeine but not theanine) for the same time periods. Blood samples were drawn from all the subjects at one-week intervals, and certain white blood cells were isolated to investigate the effect that either tea or coffee had on immune function.

Two weeks into the study, 7 of the 11 tea drinkers showed an enhanced gamma-delta-T-cell response, vs. only 1 of the 10 coffee drinkers. And after 1 to 4 weeks of tea drinking, there was a 2–3-fold increase in interferon-gamma production; with coffee drinking there was no increase. These data show that drinking tea can boost the capacity of the body’s gamma-delta T cells to produce interferon-gamma.

In a related experiment with white blood cells from the same groups of test subjects (but tea drinkers only in this case), the researchers used killed bacterial cells of E. coli and M. morganii to see whether an increase in interferon-gamma production would be elicited. After 1 week of tea drinking, there was a 2–4-fold increase. The authors stated, “These data show that ingestion of tea . . . results in a markedly enhanced innate immune response to bacteria that is gamma-delta-T-cell-dependent.”

Green Tea Protects Against Stroke

An epidemiological study conducted in Japan in 1989 showed that drinking more than five cups of green tea per day significantly lowered the risk for stroke.2 The protective effect is believed to be due to the tea catechins, especially EGCG. But when a stroke does occur, what about the damage done to brain neurons (nerve cells) from oxygen deprivation due to the obstruction of blood flow? Even if the blockage is only short-lived, neurons may die. An important factor in this process is believed to be the excessive release of glutamic acid, a brain neurotransmitter.

Recently, Japanese researchers postulated that theanine might confer a neuroprotective effect on stroke-affected neurons.3 Why theanine? Because it’s chemically similar to glutamic acid, and the idea was that if theanine would bind—harmlessly—to the glutamic acid receptors on the neurons, it would reduce the amount of glutamic acid that could do so, by literally getting in the way. Thus the theanine would stifle glutamic acid’s ability to induce cell death.

What’s So Good About Catechins?

Catechins are the primary active polyphenolic compounds found in green tea, and they’re believed to be responsible for most of the health benefits associated with this marvelous beverage. The most potent—and, through a great stroke of luck, the most abundant—of the green-tea catechins is epigallocatechin gallate (EGCG), but a number of other catechins in green tea are also known to be biologically active. Catechins have long been known to have multiple health benefits, including antibacterial, antiviral, antioxidant, and anticarcinogenic effects.

The best-known benefits, however, are associated with decreasing the risk of heart problems, as there are numerous studies that link a high intake of green tea with a reduced risk of heart disease. There is much evidence to show that green tea helps to lower cholesterol levels. At the same time, its antioxidant properties may help to inhibit the oxidation of LDL-cholesterol, an additional factor that makes it a key player in maintaining good heart health.

Theanine Protects Brain Neurons

To test their hypothesis, the Japanese team artificially induced strokes in gerbils, 30 minutes after having injected a theanine solution or a saline solution (as the control) directly into the region of the brain that would be affected by the stroke. Then, after 7 days, they killed the gerbils and examined their brains for evidence of neuronal damage. As expected, the damage was severe in the control brains, but in the theanine-treated brains, it was drastically reduced: the neuronal survival rate was 60 to 90%.

Curiously, the researchers found that the degree to which theanine displaced glutamic acid at the neuronal receptors was actually rather small, so they were forced to conclude that some other mechanism must account for the bulk of theanine’s neuroprotective effect. In any case, there’s a world of difference between injecting theanine into a gerbil’s brain and drinking tea that contains theanine, so this work has no direct relevance to humans—but it’s interesting and provocative.

And here’s something paradoxical: when we drink tea, some of the theanine is converted by the liver not just to ethylamine (as noted earlier), but also to . . . glutamic acid! Theanine, in fact, consists of a chemically bound combination of those two molecules, as is reflected by its proper chemical name: gamma-ethylamino-L-glutamic acid. Thus, ingested theanine is a source of glutamic acid, but injected theanine combats the effects of excessive amounts of glutamic acid. It is known, though, that some ingested theanine does enter the bloodstream, and it gains access to the brain by crossing the blood-brain barrier.

Theanine Counteracts Caffeine

Who doesn’t enjoy a nice, relaxing cup of tea between dinner and bedtime? Many people make evening tea a part of their ritual as they prepare for a good night’s sleep. But doesn’t tea contain caffeine, which might make you restless and keep you awake? Well, yes, it does (unless you have decaffeinated tea), but oddly, that may not be a problem, because tea also contains theanine, which blunts the stimulatory effect of caffeine.

The antagonistic interaction between theanine and caffeine has been investigated in laboratory animals. When rats are given caffeine, their EEG (electroencephalography) readings indicate stimulatory brain activity, but when they get caffeine and theanine together (as we humans do when we drink tea), their EEG readings show no stimulatory activity.4

Theanine for that Nice, Relaxed Feeling

Well, those were rats, not humans—and here’s another paradox: although tea is regarded as a stimulant beverage for humans, it’s also regarded as being relaxing. Perhaps it’s possible to be stimulating in a relaxing kind of way, just as physiological stress, e.g., can be simultaneously both bad and good for us.

In any case, a different group of Japanese researchers investigated how human volunteers (50 women, aged 18–22) reacted to theanine by measuring their brain waves, which are the result of electrical activity in the brain.5* They found that drinking aqueous theanine solutions (50 mg or 200 mg) tended to generate alpha-waves, which are typically associated with a relaxed but wakeful (not drowsy) mental state. The alpha-waves appeared within about 40 minutes after the tea was drunk, presumably because it takes that long for the theanine to get into the bloodstream and reach the brain.

*It should be noted that four of the five authors of this paper were listed as being employed by Taiyo Kagaku Company, the world’s largest producer of theanine.

Have Your Tea and Drink It Too

Next to water, tea is the most widely consumed beverage on earth. And that’s a good thing, because this phytochemically rich brew is brimming over with health benefits—provided you drink at least five cups a day, so as to obtain optimal amounts of such potent compounds as theanine and EGCG. Of course, these compounds are also available in the form of nutritional supplements, which many people prefer for their convenience and consistency: it’s an easy way to get the equivalent of many cups of tea every day.

Anyhow, the next time you’re prying your eyes open in the morning with your favorite tea blend, sipping iced tea by the pool, or relaxing in your favorite chair with a steaming cup of tea—or when you’re downing a capsule containing some of tea’s most potent molecules—smile at your wise decision to make tea a featured part of your health program.


  1. Kamath AB, Wang L, Das H, Li L, Reinhold VN, Bukowski JF. Antigens in tea-beverage prime human Vg2Vd2 T cells in vitro and in vivo for memory and nonmemory antibacterial cytokine responses. Proc Natl Acad Sci 2003 May 13:100(10):6009-14.
  2. Sato Y, Nakatsuka H, Watanabe T, et al. Possible contribution of green tea drinking habits to the prevention of stroke. Tohoku J Exp Med 1989;157: 337-43.
  3. Kakuda T. Neuroprotective effects of the green tea components theanine and catechins. Biol Pharm Bull 2002;25:1513-8.
  4. Kakuda T, Nozawa A, Unno T, Okamura H, Okai O. Inhibiting effects of theanine on caffeine stimulation evaluated by EEG in the rat. Biosci Biotechnol Biochem 2000;64:287-93.
  5. Juneja LR, Chu D-C, Okubo T, Nagato Y, Yokogoshi H. L-Theanine—a unique amino acid of green tea and its relaxation effect in humans. Trends Food Sci Tech 1999;10:199-204.

Dr. Jensen is a cell biologist who has conducted research in England, Germany, and the United States. He has taught college courses in biology and nutrition and has written extensively on medical and scientific topics.

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