La teanina, gamma glutamiletilamida o 5-N-etil-glutamina es un aminoácido y un ácido glutámico análogo, encontrado comúnmente en el té (infusiones de Camellia sinensis), capaz de producir una sensación de relajación. La teanina produce estos efectos aumentando el nivel de la producción del ácido gamma-aminobutírico (GABA), afecta los niveles del serotonina y de dopamina en el cerebro e inhibe la toxicidad excitable del ácido glutámico. Este aminoácido está disponible principalmente en el té verde, el té negro y otras variedades orientales; también se encuentra en el hongo basidiomiceto Boletus badius e Ilex guayusa. Se comercializa además como suplemento alimenticio. Más específicamente, este compuesto se llama L-teanina que es el enantiómero levógiro. En 1950, el laboratorio de té de Kyoto, aisló con éxito la teanina de la hoja del gyokuro, una variedad de té verde de Japón, que tiene un alto contenido en teanina. La teanina es un análogo de la glutamina y el glutamato, y puede cruzar la barrera hematoencefálica. Se vende en los EE.UU. como suplemento dietético, y ha sido autorizado por la FDA reconocido generalmente como seguro. El Ministerio de Salud y Bienestar Social de Japón aprobó el uso de la teanina para su consumo universal en 1964.

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Theanine /ˈθiːəniːn/, also known as L-γ-glutamylethylamide and N5-ethyl-L-glutamine, is an amino acid analogue of the proteinogenic amino acids L-glutamate and L-glutamine and is found primarily in particular plant and fungal species. It was discovered as a constituent of green tea in 1949 and in 1950 was isolated from gyokuro leaves, which have high theanine content. Appearance of the name "theanine" without a prefix is understood to imply the L-enantiomer, which is the form found in fresh teas and in some, but not all dietary supplements. The opposite D-enantiomer has far less studied pharmacologic properties, but is present in racemic chemical preparations, and substantially in some studied theanine supplements. As an analogue of glutamate and glutamine, the theanine in common preparations is absorbed in the small intestine after oral ingestion; its hydrolysis is to L-glutamate and ethylamine and occurs both in the intestine and liver. It can also cross the blood–brain barrier intact, and register pharmacological effects directly. In Japan, L-theanine has been approved for use in all foods, including herb teas, soft drinks, desserts, etc. with some restrictions applying to infant foods. It provides a unique brothy or savory (umami) flavor to green tea infusions. It is generally recognized as safe (GRAS) as an ingredient by the Food and Drug Administration (FDA), and is sold as a dietary supplement in the US. However, the German Federal Institute for Risk Assessment, an agency of their Federal Ministry of Food and Agriculture, has objected to the addition of isolated theanine to beverages. The European Food Safety Authority EFSA advised negatively on health claims related to L-theanine and cognitive function, alleviation of psychological stress, maintenance of normal sleep, and reduction of menstrual discomfort. Therefore, health claims for L-theanine are prohibited in the European Union.

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