Glycine

Glycine is the smallest and simplest amino acid, non-essential under normal conditions but conditionally essential in aging, pregnancy, and disease states where demand may exceed endogenous synthesis from serine and threonine. It is the most abundant amino acid in collagen and a structural backbone of glutathione (as the third residue of the γ-Glu-Cys-Gly tripeptide), explaining its role as a rate-limiting substrate for glutathione synthesis in older adults whose glycine levels are typically low. Glycine also acts as an inhibitory neurotransmitter in the spinal cord and brainstem, modulates NMDA receptor activity, and participates in one-carbon metabolism, bile acid conjugation, and creatine synthesis. Dietary sources include gelatin, skin, bones, and connective tissue; modern lean-meat-focused diets provide relatively little. Animal studies show lifespan extension by glycine supplementation in mice (ITP, Miller 2019); evidence from C. elegans is indirect, arising mainly from methionine restriction and one-carbon metabolism studies rather than direct glycine trials. In humans, glycine deficiency in older adults is increasingly recognized, and supplementation trials (particularly in the context of GlyNAC) have shown restoration of glutathione levels and improvements in multiple aging-related biomarkers.