Theanine

L-theanine is a naturally occurring non-proteinogenic amino acid that is found in tea and other natural sources. Unlike the 20 standard proteinogenic amino acids that are encoded in the genetic code and incorporated into proteins in the body, non-proteinogenic amino acids such as L-theanine are not used as building blocks for protein synthesis. Depending on the type of tea, L-theanine content is generally present in amounts under 50 mg per serving. When higher doses are desired, L-theanine is also commonly taken as a dietary supplement.

The main benefits of L-theanine are associated with promoting a relaxed state without causing drowsiness. L-theanine may also have mild cognitive-enhancing effects, especially when taken with caffeine.^[1] L-theanine has also been shown to suppress the blood pressure increasing effects of caffeine^[2] and may also counteract blood pressure increases under stressful conditions.^[3] Though it does not provide any sedative-like effects, L-theanine enhances sleep quality by promoting a more relaxed state in the brain.^[4]

L-theanine has remarkably low toxicity, with one rodent study failing to find any toxic effects at 4,000 mg of L-theanine per kg of body weight daily for 13 weeks.^[5] Additional animal studies have reported similar results with no observed toxicity at remarkably high doses, and L-theanine seems to be equally well-tolerated in humans, with no known reports of adverse effects or toxicity. Due to the ability of L-theanine to reduce blood pressure in certain contexts, people taking prescription blood pressure medication should consult with their physician before taking L-theanine supplements.

After consumption, L-theanine can cross the blood-brain barrier^[6] and affect brain activity by promoting increased alpha-waves,^[7] a pattern of brain activity associated with a more relaxed state. The tendency of L-theanine to increase alpha waves may explain its stress and anxiety reducing effects.^[8] In animal studies, L-theanine has been shown to affect neurotransmitter signaling in the brain, but these effects have been less explored in humans. The mechanisms behind the synergistic effect of L-theanine and caffeine on cognition are not completely understood, but its relaxation-promoting properties are likely responsible for the reduced anxiousness and jitters associated with caffeine use.