Glycine benefits

How is the role of glycine separated from the benefit of a supplement?

Glycine is used in the body in processes such as protein and glutathione synthesis; these roles do not show that a supplement would improve the same processes. The computational analysis by Meléndez-Hevia and colleagues suggests that glycine synthesis in adults plus dietary intake may not meet all uses including collagen; this is a hypothesis and measures no clinical outcome. ^[1] In the headings below, for each claim, who was studied, who wrote it and what the result does not cover are written out.

Summary of evidence and funding

Does glycine improve sleep quality?

All three studies are short-term and based on subjective measures. Inagawa and colleagues compared 3 g of glycine with placebo before bedtime in 19 volunteers complaining of sleep quality in a randomized, double-blind, crossover design; the “fatigue,” “vigor and energy” and “clear-headedness” items, assessed with two questionnaires the next morning, improved significantly with glycine. ^[2] In the study by Yamadera and colleagues, 3 g of glycine in volunteers with chronic insufficient sleep increased subjective sleep quality and sleep efficiency, shortened both sleep onset latency and the latency to slow-wave sleep on polysomnography without changing sleep architecture; daytime sleepiness decreased and performance in a memory recognition task increased. ^[3] In the single-blind, randomized, crossover study by Bannai and colleagues, sleep duration in healthy volunteers was restricted to 25% below normal for three nights (the analysis rests on data from 7 people); 3 g of glycine before bedtime significantly reduced the fatigue score, showed a trend toward reduced sleepiness and improved the psychomotor vigilance test. ^[4]

There are three constraints in reading these results: the samples are small; the measures are subjective or short-term performance tests; and the authors include people affiliated with a single company. The systematic review published in GeroScience, which examined glycine in healthy (18) and patient (34) populations, also states that longer-term use in healthy people improved sleep, but that these studies have small samples and a high risk of bias; no conflict of interest was reported in the review. ^[5] The evidence is therefore read as “promising but limited”; glycine has not been shown as a treatment for chronic insomnia.

Does glycine work in schizophrenia?

The Cochrane review by Tuominen and colleagues (search up to 2003) pooled 18 short-term trials (358 participants; 6–51 people per trial, the longest 12 weeks) examining glutamatergic substances as an add-on to antipsychotic drugs. Glycine and D-serine provided a moderate reduction in negative symptoms (n = 132; SMD −0.66; 95% CI −1.0 to −0.3). However, for glycine the difference in response rates was inconsistent: for improvement above 20%, n = 62, RR 0.70 (0.3–1.71); for improvement above 50%, there was only a borderline significant difference (RR 0.87; 0.8–1.00). There was no significant effect on positive symptoms and cognitive function. The authors stated that glycine and D-serine may improve negative symptoms to some extent, but that the results are not fully consistent and the data are insufficient for a firm conclusion. ^[6] This analysis is old and based on small trials; glycine was examined here as an add-on to antipsychotics, not as a replacement for treatment. The systematic review (Soh 2023) reports improvement in psychiatric symptoms with long-term glycine administration in psychiatric populations. ^[5]

Does glycine work in metabolic syndrome?

Díaz-Flores and colleagues followed 60 volunteers with metabolic syndrome (30 on 15 g/day of glycine, 30 on placebo) for 3 months. In the glycine group, plasma TBARS fell 25% relative to placebo; superoxide dismutase specific activity fell 20%; systolic blood pressure fell significantly only in men taking glycine (p = 0.043). ^[7] These are oxidative stress markers; they are not clinical outcomes such as cardiovascular events, blood sugar or weight. The subgroup (men) result should not be counted as the main outcome. The 15 g/day used is 15 times the 1,000 mg/day supplement limit in Türkiye. ^[9]

Does GlyNAC (glycine + N-acetylcysteine) work in older adults?

In the study by Kumar and colleagues, 24 older adults received GlyNAC or an isonitrogenous alanine placebo for 16 weeks (12 vs 12); 12 young adults received GlyNAC for 2 weeks. The older adults had baseline markers of glutathione deficiency, oxidative stress and mitochondrial dysfunction; GlyNAC improved these, placebo did not; the product was safe and well tolerated. ^[8] The study is small, the result belongs to the combination of two substances and cannot be attributed to glycine alone. The study was funded by NIH/NIA and a charitable donation, and the authors reported no conflict of interest. ^[8] In Türkiye, N-acetyl-L-cysteine has not been found suitable for use as an active ingredient in food supplements. ^[9]

Does glycine increase collagen synthesis?

Glycine is abundant in collagen, and it has been suggested through a computational analysis that the body’s glycine production may not meet collagen requirements; the analysis estimates a deficit of about 10 g per day. ^[1] This is a hypothesis. Glycine supplementation has not been shown in clinical trials to improve collagen synthesis or skin, joint or connective tissue outcomes. The evidence for collagen supplements is covered separately in the collagen guide.

This guide does not recommend personal doses; the doses in the trials belong to the trial conditions and are higher than the supplement limit in Türkiye.