The function and application potential of L-theanine: From biotechnological production, biosynthesis, bioactivity to...

L-theanine (γ-glutamylethylamide), a unique amino acid predominantly biosynthesized in tea plants (Camellia sinensis), exhibits multifaceted roles in human health and food innovation. This review elucidates its biosynthesis regulated by nitrogen availability and epigenetic mechanisms, alongside sustainable biotechnological production strategies, including extraction technologies to biotransformation. Mechanistically, L-theanine modulates glutamate receptors to enhance neuroprotection (BDNF upregulation, cortisol reduction), activates AMPK for anti-obesity effects (lipid oxidation, adiponectin secretion), and activates of γδ T cells to bolster immunomodulation. Emerging food applications include cognitive-enhancing beverages, plant-based preservatives (metal chelation), and low-sodium umami enhancers. By integrating molecular insights with scalable production and functional food design, this work positions L-theanine as a pivotal nutraceutical bridging metabolic health, sustainable biotechnological innovations, and consumer-driven dietary trends.