Theanine (C7H14N2O3) is a naturally-occurring amino acid derivative found in plants and fungi, and especially in tea leaves. Under standard conditions, it is a white solid that is soluble in water.
Due to its structural similarity to glutamic acid, theanine can act as a weak agonist to glutamate receptors. It also reduces the reuptake of glutamate. Additionally, it increases serotonin, dopamine, and gamma-aminobutyric acid (GABA) levels in the brain.
l-Theanine is biosynthesised from glutamic acid and ethylamine via theanine synthase in tea trees, and is the most common non-proteinogenic amino acid in these plants.
Theanine enhances the savoury tastes of certain foods, such as tea, and is therefore used as a food additive in certain countries like Japan. It is also consumed as a dietary supplement in countries such as the United States.
In a combination with caffeine, theanine has been shown to promote alertness and to enhance reaction times. It may also boost the body’s immune system by enhancing its ability to combat foreign infections.