A derivative of glutamic acid, theanine is classified as a non-protein amino acid. Theanine is harvested for use from certain species of the Camellia Sinensis plant. It may also be found in the mushroom Xerocomus badius. The leaves of the Camellia plant are often used as a primary ingredient in many Asiatic teas. It is estimated that nearly one-half of the entire amino acid content of these teas is represented by theanine. Japanese green teas contain this amino in the greatest abundance. Its water-soluble characteristics make it a perfect ingredient in these popular drinks.
In 1964, Japan became the first country to adopt theanine as a nutritional supplement. With the exception of baby foods, the Japanese approved the inclusion of theanine in over fifty adult foods, even being added to soft drinks and chewing gums.
The Japanese are legendary for there work ethic. Doctors in Japan have even approved the common usage for a term which describes one working him/herself to death; “Karoshi.” Subsequently, theanine’s main use is to aid in the increase of “calming” neurotransmitters in specific areas of the brain.
Theanine is one of the few nutrients able to cross the blood-brain-barrier with relative ease. Upon entering the brain, theanine stimulates the striatum, hypothalamus, and hippocampus. These specialized areas of the brain are responsible for the release of the aforementioned mood-enhancing agents, such as serotonin and dopamine. [1, 2] An increased output of these neurotransmitters is associated with relaxation and stress relief.
Theanine also acts as an antagonist to caffeine. Caffeine is the most popular stimulant in America. Caffeine decreases the output of the brain chemical and amino acid gamma-aminobutyric acid (GABA), and minimizes its sedative properties. Self-induced complications cause by the over consumption of caffeine include; tremors, headaches, and even loss of sleep. Theanine has been shown in various animal studies to counteract the stimulation affect of ingested caffeine, while increasing the output of GABA in the brain. 
Theanine is available in the majority of Asiatic tea preparations, deemed non-herbal. This amino acid constitutes nearly 2% of the entire dry weight of green tea leaves. Theanine is found in highest concentrations in umami (”ocha”), or Japanese green teas. This is the primary source that is recommended for dietary theanine supplementation. A food graph for this distinctive amino acid has been omitted.
Biochemists are unsure as to how theanine actually works in human physiology. There is speculation that theanine actually blocks the glutamate (NMDA) binding site in the brain. Glutamate is a toxic compound and is the most common excitatory neurotransmitter in the brain and spinal cord. When excessive amounts accumulate in the body it inhibits blood supply to the brain. This overload is marked by free radical damage in the brain, causing a condition known as “cerebral vascular dementia.” Theanine has been shown to reverse glutamate induced toxicities, and to protect against this type of dementia. [4, 5] More research is, however, necessary to assess this particular usage in human physiology.
Theanine may also increase the frequency and duration of alpha waves in the brain.  Brain waves, or oscillating electrical outputs, directly correspond to differing psychological states. These brain waves are classified into four groups; the Alpha, Beta, Delta, and Theta waves. The increase in alpha waves may promote the relief of anxiety, increased mental focus, improved concentration, and creativity. Unlike other medicines and herbs which can lead to lethargy, theanine may be effective in the reduction of tension, stress, and anxiety without inducing drowsiness.
Theanine may also increase immune system response, by enhancing the latent memory of the immune system. Various illnesses, caused by certain bacteria, are often recognized by the immune system as being previous detriments to the body’s overall health. These are our bodies natural defenses to diseases that have already been established, or “built in.” Theanine may ultimately assists in the recognization of previous specific pathogenic organisms, enabling the immune system to respond more effectively. Recent study has indicated that theanine may assist in both adaptive and innate acquired immunity. 
Environmental stressors are also associated with spontaneous hypertension in adults. Animal study has shown theanine as being effective at decreasing, or eliminating the onset of spontaneous increases in blood pressure.  This may be the direct result of theanine’s ability to promote calmness, concentration, and focus in stressful situations.
Cancer sufferers may also find benefit in supplemental theanine. Studies have concluded that Japanese adults consuming various amounts of green tea had a lower probability and incidence for the development of certain types of cancers. Past clinical application suggests that theanine may increase the effectiveness of cancer medications against this invasive disease. Doxorubicin (Adriamycin) is an intravenous medication used for the treatment of tumors. Research on mice given intravenous Adriamycin and supplemental theanine, resulted in the reduction of a nearly 62% average on overall tumor weight.  Theanine may also suppress tumor growth and proliferation, especially when used in conjunction with Doxorubicin. [10-12]
Because of its speculated antioxidant properties, researches have used green tea containing theanine as a chemoprotectant. Initial findings suggest that persons under going chemotherapy treatment may exhibit fewer, and less severe side effects when theanine is added to their diet. It is also suggested that drinking green tea may improve the quality of life and provide encouragement for clinical cancer patients undergoing medicinal therapy. 
Theanine supplementation may also assists in the symptoms of Premenstrual Syndrome (PMS) in women. Japanese researchers studied twenty women who supplemented their diets with theanine. The result of this study was promising. Theanine, supplemented at dosages of 100 milligrams twice per day, was shown to reduce the negative physical, mental, and social symptoms associated with this condition. 
There has been no established Recommended Dietary Allowance (RDA) for theanine. Clinical studies show that theanine is effective in dosages ranging from 50 - 200 milligrams per day, taken as necessary. Primary usage of theanine is often for the promotion of relaxation and the modulation of mood.
There have been no reported deficiencies or adverse side effects associated with minimal, or even excessive dosages, of supplemental theanine. Theanine is, nevertheless, contraindicated for individuals who are hypersensitive to this amino acid. In addition, pregnant women and nursing mothers should avoid L-theanine containing supplements.
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