In conclusion, L-theanine is rapidly absorbed in healthy
participants after intake of capsules and green tea. The major
part of L-theanine seems to be hydrolyzed to ethylamine and
glutamic acid, which are excreted by urine. A minor part is
retained in erythrocytes. The kinetics and urinary excretion of
L-theanine, ethylamine, and glutamic acid are comparable after
intake of capsules and green tea. Thus, functional effects of
L-theanine intake may result from L-theanine, ethylamine, or
glutamic acid. Because glutamic acid is converted to glutamine,
future studies should investigate if L-theanine may be an
alternative source of glutamine, which is essential for critically
ill patients.