Glycine

Pharmacology & Research

Glycine is the smallest amino acid — a non-essential nutrient the body makes from serine, yet one with an outsized number of jobs: it is the most abundant amino acid in collagen (roughly every third residue), an inhibitory neurotransmitter at spinal and brainstem glycine receptors, an obligatory co-agonist at the NMDA glutamate receptor, and a rate-contributing substrate for glutathione (the body’s main intracellular antioxidant). Because of that breadth, the supplement is marketed for everything from sleep to skin to longevity, and the evidence is uneven across those claims. The best-characterised use is a modest sleep-quality effect at 3 g before bed; the strongest mechanistic story (glutathione, metabolic health, aging) still rests mostly on small trials and animal work. Glycine is not a classic “deficiency” nutrient — a typical diet plus endogenous synthesis usually covers needs — so most supplemental effects are pharmacological (a large dose doing something extra), not repletion of a shortfall. It is plain crystalline glycine in nearly all products, so form and bioavailability are not the confounders they are for minerals.

1. Sleep quality

The signal here comes from a small group of trials, mostly Japanese, in people with unsatisfactory or lightly-restricted sleep. A randomised, placebo-controlled study gave partially sleep-restricted healthy volunteers 3 g of glycine before bed for three nights and found improved subjective daytime fatigue and “clear-headedness,” with a significant improvement in psychomotor vigilance reaction time — though other objective performance tests did not reach significance ^{1Reference 1Bannai M et al. · 2012RCTThe effects of glycine on subjective daytime performance in partially sleep-restricted healthy volunteers — randomised controlled trialView study →}. Earlier work from the same programme, summarised in a pharmacology review, reported that 3 g bedtime glycine shortened the time to fall asleep and improved subjective sleep quality, and in a small polysomnography study reduced slow-wave-sleep latency ^{2Reference 2Bannai M · 2012ReviewNew therapeutic strategy for amino acid medicine: glycine improves the quality of sleep — reviewView study →}. Rat studies point to a mechanism: oral glycine raises plasma and CSF glycine, acts on NMDA receptors in the suprachiasmatic nucleus, and lowers core body temperature — a known sleep-promoting cue ^{3Reference 3Kawai N et al. · 2015The sleep-promoting and hypothermic effects of glycine are mediated by NMDA receptors in the suprachiasmatic nucleusView study →}. This is a pharmacological effect (not correcting a deficiency), the doses are small and safe, and the effect size is modest.

Gap: trials are small, several from one research group, and objective sleep architecture data (full polysomnography, independent replication) remain thin.

2. Schizophrenia (adjunctive)

Because glycine is a co-agonist at the NMDA receptor, and NMDA hypofunction is one theory of schizophrenia, high-dose glycine has been tested as an add-on to antipsychotics for the hard-to-treat negative and cognitive symptoms. Randomised crossover and parallel trials using ~0.8 g/kg/day (roughly 40–60 g) reported meaningful reductions in negative symptoms when added to conventional antipsychotics ^{4,5Reference 4Heresco-Levy U et al. · 1999RCTEfficacy of high-dose glycine in the treatment of enduring negative symptoms of schizophrenia — randomised controlled trialView study →Reference 5Heresco-Levy U et al. · 1996RCTDouble-blind, placebo-controlled, crossover trial of glycine adjuvant therapy for treatment-resistant schizophrenia — randomised controlled trialView study →}. A meta-analysis of glutamate positive modulators (glycine, D-serine, D-cycloserine, sarcosine) for the cognitive deficits of schizophrenia found no significant overall benefit (standardised mean difference 0.08, 95% CI −0.06 to 0.23) — so any adjunctive effect appears confined to negative symptoms rather than cognition ^{6Reference 6Iwata Y et al. · 2015Meta-analysisEffects of glutamate positive modulators on cognitive deficits in schizophrenia: a systematic review and meta-analysis of double-blind randomized controlled trialsView study →}. Two important limits: the benefit largely disappears when patients are on clozapine, and a later trial adding glycine to olanzapine/risperidone was underwhelming. This is a very high, medically supervised dose — not a wellness dose.

Gap: effect is modest and inconsistent, absent with clozapine, and requires gram-per-kilogram dosing that is impractical outside a trial; newer glycine-transporter drugs largely replaced this approach.

3. Glutathione & aging (GlyNAC)

Glutathione (GSH) is a tripeptide of glutamate, cysteine, and glycine; its synthesis can be limited by cysteine and glycine availability, and GSH falls with age. Supplementing glycine + N-acetylcysteine (GlyNAC) in older adults restored red-cell glutathione and improved markers of oxidative stress, mitochondrial fuel oxidation, inflammation, insulin resistance, muscle strength, and some cognitive measures — first in an open pilot ^{7Reference 7Kumar P et al. · 2021Clinical trialGlycine and N-acetylcysteine (GlyNAC) supplementation in older adults improves glutathione deficiency, oxidative stress, mitochondrial dysfunction, insulin resistance, muscle strength and cognition — pilot clinical trialView study →}, then in a small randomised trial ^{8Reference 8Kumar P et al. · 2023RCTSupplementing glycine and N-acetylcysteine (GlyNAC) in older adults improves glutathione deficiency, oxidative stress, mitochondrial dysfunction, inflammation, physical function, and aging hallmarks — randomised clinical trialView study →}. Mouse work from the same group shows lifespan and healthspan gains, and glycine alone modestly extended lifespan of genetically heterogeneous mice in the rigorous Interventions Testing Program (a 4–6% increase) ^{13Reference 13Miller RA et al. · 2019AnimalGlycine supplementation extends lifespan of male and female miceView study →}. The human evidence is genuinely interesting but early.

Gap: the human trials are small, largely from a single laboratory, and test glycine combined with NAC — so they cannot isolate glycine’s own contribution; large independent RCTs are lacking.

4. Metabolic & liver health

Low circulating glycine is one of the most reproducible metabolic fingerprints of insulin resistance, type 2 diabetes, and non-alcoholic fatty liver disease (NAFLD). In mice, restoring glycine availability improves fatty-acid oxidation, glutathione synthesis, and the gut microbiome and ameliorates NAFLD ^{9Reference 9Rom O et al. · 2020Glycine-based treatment ameliorates NAFLD by modulating fatty acid oxidation, glutathione synthesis, and the gut microbiomeView study →}. But a large Mendelian-randomisation meta-analysis (80,003 people, 27 genetic loci) concluded that although higher glycine associates with lower coronary and diabetes risk, much of the low glycine seen in patients is a consequence of insulin resistance rather than a cause — genetic instruments gave weaker, partly bidirectional signals ^{11Reference 11Wittemans LBL et al. · 2019Meta-analysisAssessing the causal association of glycine with risk of cardio-metabolic diseases — Mendelian randomisation / meta-analysisView study →}. So the biomarker is real; the case that swallowing glycine fixes the metabolism is not yet made in humans.

Gap: essentially no randomised supplement trial in humans with a hard metabolic or liver endpoint; the human data are observational and genetic, and causality runs partly the “wrong” way.

5. Collagen & connective tissue

Glycine makes up about a third of collagen, so it is a genuine substrate for connective-tissue synthesis, and this underpins the skin/joint marketing. The best human data, however, are on gelatin or collagen peptides (which deliver glycine plus proline/hydroxyproline): vitamin-C-enriched gelatin taken before intermittent exercise raised a blood marker of collagen synthesis ^{10Reference 10Shaw G et al. · 2017RCTVitamin C-enriched gelatin supplementation before intermittent activity augments collagen synthesis — randomised crossover trialView study →}. That is a mechanistic readout, not a clinical outcome, and it is not the same as showing that isolated glycine improves skin elasticity, joint pain, or tendon strength. The related “anti-glycation” claim (glycine mopping up reactive carbonyls) is biochemically plausible but not demonstrated in supplement trials.

Gap: no controlled trial shows that glycine itself improves a clinical skin, joint, or connective-tissue endpoint; the supportive data use whole gelatin/collagen and surrogate markers.

Mechanisms

Target / pathway	Effect	Relevant to
Glycine receptor (GlyR, Cl⁻ channel)	Inhibitory neurotransmission (spinal/brainstem)	Sleep, muscle tone
NMDA glutamate receptor (glycine co-agonist site)	↑ NMDA signalling	Schizophrenia (adjunctive), sleep/SCN
Suprachiasmatic nucleus NMDA → vasodilation	↓ core body temperature	Sleep onset
Glutathione (γ-Glu-Cys-Gly) synthesis	Substrate; ↑ GSH with GlyNAC	Aging, oxidative stress, NAFLD
Collagen synthesis	~33% of collagen residues	Connective tissue
One-carbon / serine–glycine metabolism	Methyl-group and nucleotide supply	Metabolic health

Pharmacokinetics

Oral glycine is absorbed through intestinal amino-acid transporters and raises plasma glycine within ~30–60 minutes; rat data show it crosses into cerebrospinal fluid, consistent with a central effect at bedtime doses ^{2,3Reference 2Bannai M · 2012ReviewNew therapeutic strategy for amino acid medicine: glycine improves the quality of sleep — reviewView study →Reference 3Kawai N et al. · 2015The sleep-promoting and hypothermic effects of glycine are mediated by NMDA receptors in the suprachiasmatic nucleusView study →}. It is not stored — surplus glycine feeds one-carbon metabolism, glutathione, and is ultimately cleared via the glycine cleavage system and renal excretion, so there is no meaningful accumulation. Typical supplement doses (3–5 g) are modest relative to the body’s daily glycine turnover (endogenous synthesis is on the order of tens of grams), which is one reason effects are subtle. There are no clinically important food-timing requirements beyond taking the sleep dose near bedtime; it is a sweet-tasting powder that dissolves in water.

Clinical trials

Glycine is an off-patent, inexpensive nutrient, so industry-sponsored trial activity is limited; most human studies are small, investigator-initiated, and concentrated in sleep and adjunctive psychiatry, with a growing GlyNAC (aging/metabolic) programme. The large randomised trials that carry glycine in their name (GAIN International/Americas) tested a glycine-site antagonist drug, not the supplement, and were negative ^{12Reference 12Sacco RL et al. · 2001RCTGlycine antagonist (gavestinel) in neuroprotection for patients with acute stroke: GAIN Americas — randomised controlled trialView study →}.

Last checked: July 2026.

Dietary Sources

Glycine is both eaten and made internally, so a normal diet plus endogenous synthesis (from serine, choline, and threonine) usually covers day-to-day needs — it is a non-essential amino acid. The richest dietary sources are the collagen-heavy parts of animals: skin, connective tissue, bone, and the gelatin made from them. Muscle meat, fish, dairy, legumes, and some seeds contribute smaller amounts. Diets built around lean muscle meat and refined foods (little skin, bone broth, or gelatin) deliver relatively little glycine, which is one argument behind “nose-to-tail” and bone-broth eating patterns.

Food	Approx. glycine
Gelatin / collagen powder	very high (~19–27 g per 100 g protein)
Skin, tendon, bone broth	high
Pork, poultry, red meat (muscle)	moderate (~1–1.5 g per 100 g)
Fish, dairy, eggs	moderate–low
Legumes, sesame/pumpkin seeds	low–moderate (plant sources)

Endogenous synthesis contributes on the order of tens of grams’ worth of turnover per day, so supplemental grams are modest relative to the body’s total glycine economy. Authoritative food-composition and requirement figures are best drawn from national nutrient databases; there is no NIH Office of Dietary Supplements fact sheet for glycine specifically, as it is not a regulated essential nutrient.

Dosage

These are doses studied in research, not a personal recommendation.

• Sleep: 3 g of plain glycine dissolved in water, taken shortly before bed, is the dose used in the sleep-quality trials ^{1,2Reference 1Bannai M et al. · 2012RCTThe effects of glycine on subjective daytime performance in partially sleep-restricted healthy volunteers — randomised controlled trialView study →Reference 2Bannai M · 2012ReviewNew therapeutic strategy for amino acid medicine: glycine improves the quality of sleep — reviewView study →}.
• General / connective-tissue / glutathione support: 3–5 g/day is the common supplemental range; GlyNAC studies pair glycine (dosed by body weight, ~1.3 mmol/kg/day) with N-acetylcysteine ^{7,8Reference 7Kumar P et al. · 2021Clinical trialGlycine and N-acetylcysteine (GlyNAC) supplementation in older adults improves glutathione deficiency, oxidative stress, mitochondrial dysfunction, insulin resistance, muscle strength and cognition — pilot clinical trialView study →Reference 8Kumar P et al. · 2023RCTSupplementing glycine and N-acetylcysteine (GlyNAC) in older adults improves glutathione deficiency, oxidative stress, mitochondrial dysfunction, inflammation, physical function, and aging hallmarks — randomised clinical trialView study →}.
• Schizophrenia (adjunctive, supervised only): trials used ~0.8 g/kg/day — roughly 40–60 g/day in divided doses — which is a clinical, not a consumer, dose and requires medical supervision ^{4Reference 4Heresco-Levy U et al. · 1999RCTEfficacy of high-dose glycine in the treatment of enduring negative symptoms of schizophrenia — randomised controlled trialView study →}.

Glycine is a free amino acid (not a mineral salt), so there is no elemental-vs-compound-weight distinction and no need to correct the label figure. It is sweet-tasting and water-soluble. There is no established Tolerable Upper Intake Level; very high grams-per-day mainly cause reversible gastrointestinal effects.

Safety

Glycine is generally recognised as safe and is well tolerated at the doses used for sleep and general support. The main reported adverse effect is mild gastrointestinal upset (nausea, soft stool, stomach discomfort), which becomes the dose-limiting issue as intake climbs into the tens of grams used in schizophrenia trials.

The most important interaction is with clozapine: because glycine enhances NMDA-receptor signalling and clozapine may act partly through the same site, adjunctive glycine appears to blunt clozapine’s antipsychotic benefit — so glycine supplementation should be avoided or medically supervised in anyone taking clozapine, and more broadly in anyone on antipsychotic or other psychiatric medication. Beyond this, systematic interaction screening has not been done for the consumer dose range.

People with significant kidney or liver impairment should be cautious with high-gram dosing, since amino-acid load is handled by those organs; this is precaution rather than a documented harm at 3–5 g.

Pregnancy & lactation

Verdict: not established — avoid supplemental doses without clinician guidance. Glycine is a normal dietary and endogenous amino acid, but supplemental glycine has not been studied for safety in pregnancy or lactation. Absence of reports is not evidence of safety; pregnant or breastfeeding people should not take supplemental grams without professional advice.

Scope disclosure (REQUIRED honesty — not a claim, no citation):

• Interactions assessed? Partially — the clinically important clozapine/antipsychotic interaction is documented; broad drug-class screening for consumer doses was not performed.
• Pregnancy/lactation assessed? No — no supplement safety data; treated as not established.
• Upper Limit (UL) established? No UL set. This does not imply unlimited intake is safe; high grams cause GI effects and long-term high-dose safety is uncharacterised.

Never imply safety from absence of reports. Where glycine was not studied, this monograph says “not assessed.”

References