Theanine, a major amino acid in green tea, can reduce stress in mice and humans, according to researchers from the School of Pharmaceutical Sciences at University of Shizuoka--Japan.

As explained in thestudy, the amount of theanine, which is the most abundant amino acid in green tea leaves, depends on nitrogen supply absorbed from the roots.

In a study with animals and a preceding clinical trial, research showed that high levels of theanine and arginine in matcha led to significant stress-reducing effect.

Matcha tea leaves, as researchers explain, are protected from direct sunlight, and so their amino acid content, especially theanine, remains high because the hydration of theanine used in the biosynthesis of catechin is therefore lowered.

Out of 39 participants in the clinical trial, half consumed test-matcha, which was expected to have a stress-reducing effect, and half consumed a placebo-matcha, where no effect was expected.

Researchers concluded that anxiety, a reaction to stress, was significantly lower in the test-matcha group than in the placebo group.

"An effective stress-reducing outcome was only possible when the molar ratio of caffeine and epigallocatechin gallate (EGCG) to theanine and arginine was less than two," according to the researchers, who also noted, "To predict mental function of each matcha, both the quantity of theanine and the ratios of caffeine, EGCG, and arginine against theanine need to be verified."

The study also noted that, out of 76 teas sold in Japan, 32 had both the correct amount of theanine and the proper ratio of theanine to caffeine, EGCG, and arginine necessary to reduce stress, whereas overseas, out of 67 teas, only one met the necessary conditions.