Theanine , 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; in 1950, it was isolated from gyokuro leaves. Theanine provides a unique brothy or savory (umami) flavor to green tea infusions. The name theanine without a prefix generally implies the enantiomer, L-theanine, which is the form found in tea leaves and as a dietary supplement ingredient. Most studies have used L-theanine. The opposite enantiomer, D-theanine, has been studied less. The regulatory status of theanine varies by country. In Japan, L-theanine has been approved for use in all foods, with some restrictions in the case of infant foods. In the United States, the Food and Drug Administration (FDA) considers it to be generally recognized as safe (GRAS) and allows its sale as a dietary supplement. The German Federal Institute for Risk Assessment, an agency of their Federal Ministry of Food and Agriculture, objects to the addition of L-theanine to beverages. In 2011 the European Food Safety Authority, when asked to provide a scientific opinion, concluded that a cause and effect relationship had not been established between consumption of L-theanine and improved cognitive function, alleviation of psychological stress, maintenance of normal sleep, or reduction of menstrual discomfort. Therefore, health claims for L-theanine are not recognized in the European Union. Structure and properties The chemical name N5-ethyl-L-glutamine and other synonyms (see box) for theanine reflect its chemical structure. The name theanine, without prefix, is generally understood to imply the L- (S-) enantiomer, derived from the related proteinogenic L-amino acid glutamic acid. Theanine is an analog of this amino acid, and its primary amide, L-glutamine (also a proteinogenic amino acid). Theanine is a derivative of glutamine that is ethylated on the amide nitrogen (as the name N5-ethyl-L-glutamine describes), or alternatively, to the amide formed from ethylamine and L-glutamic acid at its γ- (5-) side chain carboxylic acid group (as the name γ-L-glutamylethylamide describes). Relative to theanine, the opposite (D-, R-) enantiomer is largely absent from the literature, except implicitly. While natural extracts that are not harshly treated are presumed to contain only the biosynthetic L- enantiomeric form, mishandled isolates and racemic chemical preparations of theanines necessarily contain both theanine and its D-enantiomer (and from racemic syntheses, in equal proportion), and studies have