Glycine is the smallest amino acid — a building block of proteins and collagen — and at the same time an inhibitory neurotransmitter in the central nervous system. It acts as a ligand for glycine receptors (influencing neural tone and relaxation) and as a co-agonist of the NMDA receptor (synaptic plasticity, learning processes). At the tissue level it supplies substrate for the synthesis of collagen types I and III, and in metabolism it participates in bile-acid conjugation, detoxification and one-carbon cycling. This dual role — structural and signalling — explains why it works well for many supplementation goals.