An amino acid found in tea, L-Theanine has psychoactive properties that have been shown in studies to help reduce stress.
One study ((Kimura K, Ozeki M, Juneja L, Ohira H (2007). “L-Theanine reduces psychological and physiological stress responses”. Biol Psychol 74 (1): 39-45.)) found that the injestion of L-Theanine “resulted in a reduction in the heart rate and salivary immunoglobulin responses to an acute stress task relative to the placebo control condition.” Translation: L-Theanine was effective in lowering stress.
Does lower stress correlate directly with weight loss? No. Is it helpful? Yes. Diet pills will often include L-Theanine in order to counteract the stimulants that are often included. Some stimulants often cause subjects to have adverse side effects such as nervousness and the jitters. L-Theanine is meant to soften those side effects by reducing the amount of “stress” that you encounter.
So like many diet pill ingredients, L-Theanine by itself won’t cause you lose weight at all, but it is included for good reason in many diet pills. It has been proven to be effective in stress reduction. Will it effectively get rid of any jitters from the stimulants in some diet pills? That has yet to be determined, but that is the hope of diet pill makers.