Benefits
Subjective Sleep Quality
In three small Japanese crossover trials (about 19, 11 and 7 people), 3 grams before bed improved self-rated sleep quality; one found faster sleep onset on brain-wave recordings and one found less next-day fatigue, with no significant difference in sleepiness. All three were linked to Ajinomoto, which sells glycine for sleep, and we found no independent repeat.
Blood Sugar Markers
In 9 healthy people, glycine taken with sugar cut the blood glucose response by more than half in a single test, and small Mexican trials in type 2 diabetes and metabolic syndrome reported lower lab markers. A genetic study of 80,003 people suggests insulin resistance may lower glycine rather than the reverse.
Collagen Building Block
Glycine makes up about a third of collagen. Whether extra glycine changes joint or skin collagen in people has not been tested in a published human trial.
Glutathione (with NAC)
Glycine combined with N-acetylcysteine (GlyNAC) raised glutathione and improved aging markers in small trials from one Baylor group, which holds a patent licensed to Nestle; a larger Nestle trial found no significant difference in its main glutathione measure. These results cannot be credited to glycine alone.
High-Dose Psychiatric Research
At about 60 grams a day under medical supervision, glycine added to some antipsychotics improved symptoms in small crossover trials, but the largest trial (157 patients) found no significant difference from placebo. These are drug-level research doses, not supplement use.
Mechanism of action
Neurotransmitter Role
Glycine is an inhibitory neurotransmitter in the central nervous system, acting on glycine receptors to hyperpolarize neurons, reducing neuronal excitability and promoting calmness.
Thermoregulation
In rats, oral glycine lowered core body temperature by widening skin blood vessels, acting through NMDA receptors in the suprachiasmatic nucleus, the brain's clock region; this has not been confirmed in peer-reviewed human studies.
NMDA Receptor Modulation
Glycine is a co-agonist at NMDA receptors, enhancing glutamatergic signaling critical for synaptic plasticity, learning, and memory.
Inhibitory Signaling
Its action on glycine receptors reduces neuronal excitability in the spinal cord and brainstem; no controlled trial of glycine for depression or anxiety at supplement doses was found.
Collagen Synthesis
Glycine occupies every third position of the collagen triple helix, about a third of its amino acids.
Anti-inflammatory Effects
In cell and animal studies, glycine dampens pro-inflammatory cytokines such as TNF-alpha; a hemodialysis trial found no significant effect on cytokines.
Insulin Sensitivity
Glycine may support insulin signaling, possibly through glutathione synthesis; human evidence is limited to small lab-marker trials.
Anti-inflammatory
It inhibits NF-κB activation, reducing systemic inflammation linked to metabolic disorders.
Detoxification
Glycine is a precursor to glutathione, a key antioxidant that neutralizes reactive oxygen species and supports liver detoxification.
Cytoprotection
In animal studies, glycine reduced oxidative damage in liver cells after alcohol or toxins; no human liver trial of glycine alone was found.
Protein Synthesis
Glycine supports mTOR signaling and provides building blocks for muscle protein synthesis, counteracting muscle breakdown.
Clinical trials
Randomized, single-blind crossover trial: 3 g glycine or placebo 30 minutes before bed during 3 nights of sleep cut by 25%. Volunteers were full-time Ajinomoto employees and 3 of 5 authors worked for Ajinomoto. (Bannai et al. 2012, Front Neurol)
10 healthy men enrolled, 7 analyzed.
Next-day fatigue on a visual scale was lower on glycine (p = 0.022) and reaction time improved; sleepiness showed no significant difference (p = 0.098). Sleep itself was tracked only by sleep logs.
Double-blind, placebo-controlled trial in 24 older adults (mean age about 71) randomized to GlyNAC or an alanine placebo for 16 weeks, plus 12 young adults given GlyNAC for 2 weeks. Dose: 100 mg/kg/day each of glycine and NAC. Outcomes: glutathione, oxidative stress, mitochondrial markers, physical function, body composition. (J Gerontol A Biol Sci Med Sci, 2023)
24 older adults (12 GlyNAC, 12 placebo) plus 12 young adult comparators. 16-week intervention.
Reported improvements in glutathione, oxidative stress, mitochondrial markers, inflammation, insulin resistance, endothelial function, gait speed and body composition, mostly as within-group changes; cognition was not measured. The arms differed by sex (8 men in GlyNAC vs 4 in placebo). Small sample, single research group (Sekhar lab at Baylor); Baylor holds a GlyNAC patent licensed to Nestle Health Science. Every result reflects glycine plus NAC, not glycine alone.
Randomized controlled trial in 114 healthy older adults receiving GlyNAC at low (2.4 g/day), medium (4.8 g/day), or high (7.2 g/day) doses for 2 weeks. Outcomes: glutathione redox status, oxidative damage, tolerability. (Lizzo/Migliavacca 2022, Front Aging)
114 healthy older adults. 2-week dose-finding.
Primary endpoint negative: over the 2-week intervention, GlyNAC did not significantly raise total glutathione or the glutathione redox ratio in the overall cohort. Only a post-hoc subset with high baseline oxidative stress and low baseline glutathione, on the medium or high dose, showed increased glutathione generation. Well-tolerated across doses. Because this trial was run by Nestle, which licenses GlyNAC patents, rather than by the Baylor group, its negative primary result tempers rather than confirms the glutathione-raising claims from the single-group Sekhar GlyNAC studies.
Open-label, controlled trial in 56 patients with severe COVID-19 requiring mechanical ventilation receiving enteral glycine vs standard care. Outcomes: clinical recovery, ICU outcomes, mortality. (2024 Mexican trial)
56 severe COVID-19 ICU patients.
Primary endpoint negative: enteral glycine did not improve major clinical outcomes (mortality, ventilator-free days, ICU LOS) in severe COVID-19. Negative finding important; extrapolating GlyNAC anti-aging signals to acute critical illness was not supported.
Double-blind crossover trial in 13 people with cystic fibrosis aged 6 to 23 taking oral glycine 0.5 g/kg/day or placebo for 8 weeks. Outcomes: symptom score, spirometry (FEV1), inflammatory markers. (Vargas et al. 2017, BMC Pulm Med)
13 people with CF aged 6 to 23, crossover.
Symptom scores and lung function measures improved on glycine versus placebo; inflammatory marker changes were mostly not significant. A small pilot only, at a supervised dose far above supplement amounts; modern CF care relies on CFTR modulators, and glycine is not a substitute.
Double-blind, placebo-controlled crossover trial in 22 patients with treatment-resistant schizophrenia receiving high-dose glycine (0.8 g/kg/day, about 60 g for a 75 kg adult) added to antipsychotics for 6 weeks. (Heresco-Levy 1999, Arch Gen Psychiatry)
22 treatment-resistant schizophrenia patients. Crossover.
Negative symptoms and total symptom scores improved versus placebo. Context: in the largest trial, CONSIST (157 patients), neither glycine nor D-cycloserine beat placebo, and added to clozapine glycine did not help and may have worsened positive symptoms. Modern treatment has not adopted high-dose glycine; these are drug-level research doses.