Glycine
Inhibitory neurotransmitter in brainstem and spinal cord; glutathione precursor; modulates the methionine cycle and supports collagen synthesis. Co-supplementation with NAC has been studied as a "GlyNAC" intervention for age-related glutathione decline.
Glycine is the smallest of the twenty proteinogenic amino acids and the only one that is not chiral. It accounts for roughly one-third of the residues in collagen, the most abundant protein in the human body, and serves additional roles as an inhibitory neurotransmitter (in brainstem and spinal cord), a precursor of glutathione, creatine, and porphyrins, and a participant in the methionine cycle via the glycine N-methyltransferase reaction.
Aging-research interest centers on the GlyNAC paradigm — supplementation with glycine and N-acetylcysteine to restore glutathione (GSH) biosynthesis, which declines markedly with age. A series of trials from Baylor College of Medicine has reported that GlyNAC supplementation in older adults improves indices of mitochondrial function, insulin sensitivity, endothelial function, cognition, and muscle strength alongside normalization of intracellular GSH levels. Independent replication is ongoing.
Beyond GSH, glycine itself influences metabolic flux: it has been studied as a glycine receptor partial agonist (with implications for sleep architecture) and as a modulator of the methylglycine cycle in liver. Endogenous glycine synthesis is generally sufficient for healthy adults; supplementation effects in deficient or aged populations remain an active research area.