Among the amino acids, the essential constituents of proteins, that of greatest interest is Theanine (gamma-glutamylethylamide), with a higher percentage in green teas.

Theanine can pass through the blood-brain barrier, favouring the release of neurotransmitters in the brain that have a "calming" action, thus inhibiting the effects of caffeine on the central nervous system.

It also appears to prevent ischemic neuronal damage.