Glycine is a conditionally essential amino acid involved in protein synthesis, collagen production, neurotransmission, glutathione synthesis, one-carbon metabolism, and endogenous creatine synthesis.
The research library should be transparent that several healthy-aging trials have studied glycine in combination with N-acetylcysteine, known as GlyNAC. Those studies cannot establish that glycine alone produced the reported effects.
Key References
1. Razak MA, Begum PS, Viswanath B, Rajagopal S. Multifarious beneficial effect of nonessential amino acid, glycine: a review. Oxidative Medicine and Cellular Longevity. 2017;2017:1716701. doi:10.1155/2017/1716701.
PubMed | Full text
Why it matters: This broad review summarizes glycine’s biological roles in glutathione synthesis, collagen, metabolic regulation, inflammation, neurological signaling, and cellular protection.
2. Wang W, Wu Z, Dai Z, Yang Y, Wang J, Wu G. Glycine metabolism in animals and humans: implications for nutrition and health. Amino Acids. 2013;45(3):463-477. doi:10.1007/s00726-013-1493-1.
PubMed
Why it matters: This highly cited review provides a detailed overview of glycine metabolism and explains why endogenous glycine production may not always meet metabolic demand.
3. Bannai M, Kawai N. New therapeutic strategy for amino-acid medicine: glycine improves the quality of sleep. Journal of Pharmacological Sciences. 2012;118(2):145-148. doi:10.1254/jphs.11R04FM.
PubMed
Why it matters: This review summarizes small human studies suggesting that glycine consumed before bedtime may improve subjective sleep quality and next-day function. Larger independent trials are still needed.
4. Sekhar RV, Patel SG, Guthikonda AP, et al. Deficient synthesis of glutathione underlies oxidative stress in aging and can be corrected by dietary cysteine and glycine supplementation. The American Journal of Clinical Nutrition. 2011;94(3):847-853. doi:10.3945/ajcn.110.003483.
PubMed | Full text
Why it matters: This metabolic study demonstrated that glycine plus cysteine supplementation restored glutathione synthesis in older adults. Because both precursors were administered, the findings should not be attributed to glycine alone.
5. Kumar P, Liu C, Hsu JW, et al. Supplementing glycine and N-acetylcysteine in older adults improves glutathione deficiency, oxidative stress, mitochondrial dysfunction, inflammation, physical function, and aging hallmarks: a randomized clinical trial. The Journals of Gerontology: Series A. 2023;78(1):75-89. doi:10.1093/gerona/glac135.
PubMed | Full text
Why it matters: This randomized trial reported improvements in several physiological outcomes with GlyNAC. It supports glycine’s role as a glutathione precursor but does not demonstrate that glycine alone produces the same effects.