Evidence supporting the use of: Theanine
For the body system: Parasympathetic Nervous System

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Synopsis

Source of validity: Scientific
Rating (out of 5): 3

L-Theanine is an amino acid primarily found in green tea (Camellia sinensis) and is commonly used as a supplement to promote relaxation and reduce stress. Its use to support the parasympathetic nervous system (PNS)—the branch of the autonomic nervous system responsible for "rest and digest" functions—is supported by a moderate body of scientific evidence, though more research is needed for definitive conclusions.

Multiple human and animal studies have demonstrated that L-theanine can increase alpha brain wave activity, which is associated with a relaxed, yet alert, mental state. Some research suggests that L-theanine supplementation may lower heart rate and reduce stress responses, potentially by enhancing parasympathetic activity and/or reducing sympathetic nervous system dominance. For example, a randomized controlled trial (Nobre et al., 2008) found that L-theanine increases alpha wave activity in the brain, which correlates with relaxation without drowsiness. Other studies, such as Kimura et al. (2007), reported that L-theanine intake led to reduced heart rate and decreased salivary immunoglobulin A responses under acute stress, both indicators of parasympathetic activation.

While these findings are promising, the evidence base is still developing, and studies often have small sample sizes. There is scientific plausibility for L-theanine’s support of the parasympathetic nervous system, but high-quality, large-scale clinical trials are required to fully establish efficacy.

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