Theanine, also gamma-glutamylethylamide or 5-N-ethyl-glutamine, is an amino acid and a glutamic acid analog commonly found in tea (infusions of Camellia sinensis), primarily in black tea, and also in the basidiomycete mushroom Boletus badius and in guayusa. More specifically, this compound is called L-theanine, being the L- amino acid (not to be confused with a levorotatory enantiomer). In 1950, the tea laboratory of Kyoto successfully separated theanine from gyokuro leaf, which has high theanine content. Theanine is an analog to glutamine and glutamate, and can cross the blood–brain barrier. It is sold in the US as a dietary supplement, and is classified by the FDA as a generally recognized as safe (GRAS) ingredient. However, the German Federal Institute for Risk Assessment (Bundesinstitut für Risikobewertung, BfR) has objected to the addition of isolated theanine to beverages.

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