1) Stress

Theanine may help reduce the negative effects of stress, according to a small study published in Biological Psychology in 2007. In an experiment involving 12 people, researchers found theanine-treated participants were less likely to experience an increase in heart rate when performing a stressful task.

2) Immunity

Taking green-tea-derived theanine and antioxidants in supplement form may help prevent the flu, suggests a study published in BMC Complementary and Alternative Medicine in 2011. Every day for five months, the study's 197 participants took either a placebo or a capsule containing theanine and catechins (a class of antioxidants found in green tea).

 3) Brain Health

A 2010 study from Nutritional Neuroscience suggests that a combination of theanine and caffeine may help improve cognitive performance. For the study, 44 young adults took either a placebo or a combination of theanine and caffeine prior to performing a demanding cognitive task. Researchers found that the combination of theanine and caffeine significantly improved accuracy and alertness and helped to increase focus while performing the task.

Additionally, findings from an animal-based study published in Free Radical Biology & Medicine in 2009 suggest that theanine may inhibit buildup of amyloid beta (a substance that forms the brain plaques associated with Alzheimer's disease).

 4) Depression

Theanine may help relieve depression, according to an animal-based study published inPhytotherapy Research in 2011. In tests on mice, scientists discovered that theanine produced an "antidepressant-like effect."

 5) Weight Loss

Theanine shows promise as a weight loss aid, according to animal-based research published in the journal In Vivo in 2004. In an experiment involving mice fed green tea powder, researchers found that theanine and caffeine found in the powder appeared to prevent weight gain and fight the buildup of fat.