L-theanine, an amino acid that occurs in green tea leaves, reduced heart rate and lowered signs of stress in a new study. In order to create a severely stressful situation, researchers gave a mental arithmetic task to 12 participants in four separate trials. In one of the four double-blind trials, participants took L-theanine at the beginning of the mental math task. In a second variation, participants took L-theanine halfway through the task. In a third trial, participants took a placebo at the beginning of the task and in a fourth, nothing at all.

Compared to the placebo group, those in the L-theanine group had lower heart rates and lower amounts of a stress marker in the saliva (immunoglobulin), which the immune system releases as a response to foreign objects such as bacteria or viruses.

The researchers theorized that L-theanine suppresses the sympathetic nervous system, which is the part of the involuntary (autonomic) nervous system that prepares the body to respond to acute stress or emergency situations. According to the scientists, L-theanine blocks the chemical (L-glutamic acid or glutamate) that is primarily responsible for carrying electrical signals (neurotransmission) from nerve cells to other cells in the body. The doctors stated that L-theanine may influence both body (physiology) and mind (psychology) under stress and concluded that taking an oral L-theanine supplement may reduce the effects of acute stress.

Reference: Biol Psychol. 2007;74(1):39-45.