Glycine

Glycine is a non-essential (and conditionally essential) amino acid that serves several distinct physiological roles: it is an inhibitory neurotransmitter in the spinal cord and brainstem, acting on strychnine-sensitive glycine receptors; it is one of three precursors for glutathione (the body's principal endogenous antioxidant); it is the most abundant amino acid in collagen; and it is required for creatine synthesis. The breadth of these roles has generated interest across sleep, longevity, metabolic, and musculoskeletal research. The most developed area of human evidence concerns sleep quality. Bannai and Kawai (2012) summarized human pilot data showing that 3 g of glycine taken before sleep improved subjective sleep quality and reduced daytime fatigue. A distinctive proposed mechanism — reduction in core body temperature via peripheral vasodilation — distinguishes glycine from sedative sleep aids and is biologically plausible given what is known about thermoregulatory sleep initiation. However, the clinical trials in this literature are small (n ≈ 11), and the field lacks large, registered, pre-specified RCTs with polysomnographic endpoints in well-characterized populations. A growing body of work on the glycine + N-acetylcysteine (GlyNAC) combination is notable. Kumar et al. (2017, expanded in subsequent work by Sekhar's group) found that GlyNAC supplementation over 24 weeks in older adults corrected glutathione deficiency, reduced markers of oxidative stress and mitochondrial dysfunction, and improved physical function across several hallmarks-of-aging endpoints. These findings are mechanistically compelling and clinically interesting, but the samples remain small and the evidence is early-stage. Importantly, the GlyNAC studies do not isolate glycine's individual contribution from NAC's. A modeling analysis (Meléndez-Hevia et al., 2013) argued that glycine demand from collagen turnover and metabolic pathways substantially exceeds what endogenous synthesis and typical protein intake can supply — a "hidden" dietary gap attributable to the decline of collagen-rich foods (bone broth, skin, connective tissue) in modern diets. This hypothesis is thought-provoking and increasingly cited in the popular health space, but it rests on metabolic modeling rather than clinical intervention data. Overall, glycine has a favorable safety record — no established Tolerable Upper Intake Level (UL), and doses of 15–60 g/day have been studied (for schizophrenia) without serious adverse events. The sleep evidence is suggestive and mechanistically grounded but not yet robust. The glutathione/longevity evidence is early but directionally consistent. Individuals interested in glycine for sleep or antioxidant support may consider it a low-risk, evidence-informed option while acknowledging that the clinical evidence base is still maturing.