Glycine for Sleep, Blood Sugar and Collagen: What the Trials Found

Glycine is the smallest amino acid and the most common one in collagen, where it makes up about one residue in three. The body can make it, which is why textbooks call it non-essential, but a 2009 metabolic analysis argued that the amount made plus the amount in a typical modern diet falls short of what collagen turnover needs. Most of the human research since then has looked at three questions: whether glycine before bed changes sleep, whether it changes the blood sugar response to a meal, and whether pairing it with cysteine restores glutathione in older adults. This article reports what those trials measured, which foods supply the most glycine, the doses researchers used, and what the safety literature records.

Key takeaways

• Glycine makes up roughly a third of the amino acids in collagen and gelatin. Bone broth, skin-on poultry, pork rinds and gelatin are the densest food sources.
• Three small Japanese trials found 3 g of glycine an hour before bed improved self-rated sleep quality and shortened time to fall asleep. All three were run by researchers employed by Ajinomoto, a company that sells glycine.
• In nine healthy adults, glycine taken with glucose cut the blood glucose rise by more than half. A three-month trial in metabolic syndrome patients using 15 g per day reported lower oxidative stress markers and systolic blood pressure.
• A 16-week Baylor trial of glycine plus N-acetylcysteine in older adults reported higher glutathione and better strength and gait. A separate Nestlé-funded trial found no change in glutathione.
• A 2022 mouse study found glycine worsened glucose tolerance in obese animals, so the blood sugar picture is not settled.
• The safety literature records mild stomach upset at high doses and little else, but there is no data on pregnancy or on people taking clozapine.

This article is for education only. It does not diagnose, treat, or prevent any condition. Ivy Ham is a certified clinical herbalist, not a physician. Do not start or stop any medication or supplement based on what you read here. Amino acid supplements can interact with prescription drugs, and the research below excludes people who are pregnant, have liver or kidney disease, or are recovering from surgery. Individual response varies. If you are in crisis, contact your local emergency number or a crisis line now.

What glycine is

Evidence: established biochemistry.

Glycine is one of the twenty amino acids the body uses to build protein. It is the only one with no side chain, which lets collagen fold into its tight triple helix, so every third position in a collagen strand is glycine. The body also uses free glycine for other jobs: it is one of the three building blocks of glutathione, it is needed to make heme, creatine and bile salts, and it works as an inhibitory neurotransmitter in the spinal cord and brainstem.⁹ Gelatin, which is cooked collagen, is about 30 percent glycine by amino acid count.⁵

The word glycinate on a supplement label means something different. Magnesium glycinate or zinc glycinate is a mineral bound to glycine, and the glycine amount in a typical dose is small. Plain glycine powder, sometimes sold as L-glycine, is the form used in the trials below.

Why researchers think the body runs short

Evidence: metabolic modeling, one research group. No direct human deficiency trial.

Because the body can make glycine, most nutrition references list no daily requirement. In 2009, Meléndez-Hevia and colleagues at the Institute of Cellular Metabolism in Tenerife published a stoichiometric analysis of the glycine synthesis pathway and concluded that human cells can only make about 3 g per day. Adding a typical diet's 1.5 to 3 g, they estimated a shortfall near 10 g per day for a 70 kg adult once collagen turnover, glutathione, creatine and other uses were counted.⁴ The paper proposed that this gap could explain why collagen repair slows with age.

This is a model, not a feeding study, and it has not been tested directly in humans. It is cited often because it gives a mechanism for why gelatin-rich diets might matter, and because the numbers line up with the doses that later trials found useful.

Glycine for sleep: what the trials measured

Evidence: three small randomized trials, all industry-run. Consistent results, no independent replication.

The glycine and sleep research comes almost entirely from one source. Ajinomoto, the Japanese company that manufactures amino acids, ran a series of trials through its own research labs between 2006 and 2012. The best known is Yamadera and colleagues' 2007 crossover trial in eleven adults who reported ongoing poor sleep. Participants took 3 g of glycine or a flavored placebo one hour before bed on two nights, with polysomnography recording brain activity. On glycine nights, self-rated sleep quality improved, time to fall asleep and time to reach slow-wave sleep both shortened, and sleep architecture was otherwise unchanged. Next-day sleepiness was lower and scores on a memory recognition task were higher.¹

An earlier trial from the same group, Inagawa 2006, gave 3 g to nineteen women who were unhappy with their sleep and found the same pattern on questionnaires.¹ Bannai and colleagues in 2012 restricted ten healthy men to 75 percent of their normal sleep for three nights and found that 3 g of glycine at bedtime reduced next-day fatigue on a visual analog scale, with a trend toward less sleepiness.²

How it might work

The mechanism proposed by the same lab is thermal. In rats, oral glycine raised skin blood flow and lowered core body temperature, and the effect disappeared when NMDA receptors in the suprachiasmatic nucleus, the brain's clock, were blocked.³ A falling core temperature is one of the normal signals that starts sleep, so the hypothesis is that glycine speeds a process that already happens. This would fit the trial finding that glycine shortened time to sleep without changing how much deep or REM sleep people got, which is different from how sedative drugs behave.

Three things limit these results. The trials were small, with ten to nineteen participants each. All were designed, funded and authored by employees of a company that sells the product. And no independent group has published a replication in the years since. The findings are consistent and the design was sound, but they should be read as promising rather than proven.

Glycine and blood sugar

Evidence: one small crossover trial in healthy adults, two three-month trials in metabolic disease, one negative animal study. Mixed.

Gannon and colleagues at the Minneapolis VA Medical Center tested nine healthy adults on four separate mornings: water, 25 g of glucose, glycine alone, or glycine with the glucose. When glycine was taken with glucose, the rise in blood glucose over two hours was reduced by more than half compared with glucose alone, while the total insulin response was not significantly different. The authors suggested glycine triggers a gut hormone that helps insulin clear glucose from the blood.⁵ The glycine dose was set by lean body mass and worked out to several grams for most participants.

Two longer trials came from a group at Mexico's Instituto Mexicano del Seguro Social. Díaz-Flores 2013 gave 60 adults with metabolic syndrome either 15 g of glycine per day, split into three doses in water, or a starch placebo for three months. The glycine group showed lower markers of lipid peroxidation, higher antioxidant enzyme activity, and a drop in systolic blood pressure.⁶ A 2008 trial from the same group in type 2 diabetes, using the same dose, reported lower HbA1c and lower inflammatory cytokines; it is summarized in the 2023 GeroScience systematic review of every human glycine trial.¹⁰

The counterpoint is a 2022 mouse study. Alves and colleagues found that glycine supplementation in obese mice made glucose tolerance worse by increasing glucose production in the liver.¹¹ Mice are not people and the doses were not comparable, but it is a reminder that an amino acid the liver can turn into glucose might behave differently in a body that is already overproducing it. If blood sugar is your concern, the broader food and lifestyle picture in our blood sugar control article matters more than any single supplement.

Glycine, cysteine and glutathione

Evidence: one positive randomized trial from Baylor, one null trial funded by Nestlé. Unresolved.

Glutathione is the main antioxidant made inside cells, and it is assembled from three amino acids: glutamate, cysteine and glycine. Blood levels fall with age. A research program at Baylor College of Medicine led by Rajagopal Sekhar has spent more than a decade testing whether giving both limiting precursors, glycine and N-acetylcysteine, restores it. The combination is called GlyNAC.

Their 2023 randomized trial gave 24 older adults either GlyNAC or a placebo for 16 weeks. The GlyNAC group showed higher red blood cell glutathione, lower oxidative stress and inflammation markers, lower insulin resistance, and better gait speed and grip strength. The effects faded in the weeks after stopping.⁷ The doses were scaled to body weight and were high, in the range of 100 mg per kilogram per day of each amino acid.

A Nestlé Research team ran a larger trial the year before with 114 healthy older adults across three GlyNAC doses for 12 weeks. Glutathione did not rise compared with placebo on the primary endpoint. In a post-hoc analysis, participants who started with low glutathione and high oxidative stress did show an increase at the two higher doses.⁸ The difference between the trials may come down to who was enrolled: the Baylor group selected for measurable glutathione deficiency, while the Nestlé group took healthy volunteers. The honest reading is that GlyNAC seems to help people who are short on glutathione and does little for people who are not. For the cysteine half of the pair, see our article on what NAC is made from.

Foods high in glycine

Evidence: food composition data. Well established.

Glycine concentrates in connective tissue, so the richest foods are the parts of the animal that modern cooking tends to discard. Muscle meat contains some, but the ratio matters: a steak supplies far more methionine, cysteine and tryptophan than glycine, while skin, tendon, cartilage and bone supply the reverse.

Highest sources

• Gelatin. About 30 percent glycine by amino acid count.⁵ One tablespoon of powdered gelatin, around 7 g of protein, gives roughly 2 g of glycine. Beef gelatin and collagen peptides have the same amino acid profile; the peptides are just pre-broken into shorter chains so they dissolve cold.
• Bone broth and stock made from joints, feet, knuckles and skin. Glycine content depends entirely on how much collagen dissolved, which is why a broth that sets firm in the fridge is the goal. A thin broth that stays liquid has little in it.
• Skin-on poultry, chicken feet and wings. The skin and cartilage are the source, not the breast meat.
• Pork rinds and pork skin. Fried pork skin is one of the highest glycine foods by weight because it is nearly pure collagen.
• Oxtail, shank, brisket, short ribs and other slow-cooked cuts. Long, moist cooking converts the connective tissue to gelatin.
• Fish skin and small whole fish such as sardines eaten with the bones.

Plant foods contain glycine in smaller amounts. Pumpkin seeds, spirulina and sesame are among the higher ones, but none approach gelatin, and no plant food carries the collagen ratio that makes animal connective tissue useful for this purpose.

Ray Peat's view

The physiologist Ray Peat wrote about gelatin for decades and treated it as a food rather than a supplement. His argument was that a whole animal is roughly half collagen by protein, so a diet built only on muscle meat is skewed toward the amino acids he considered pro-inflammatory, tryptophan and cysteine, and short on glycine, which he described as protective and calming.¹² He recommended eating gelatin-rich foods with meat rather than taking isolated glycine, and reported using it for sleep and for bleeding problems. His full framework is collected in our Ray Peat diet guide.

Start here The Ray Peat Diet Guide Every food he mentioned, his hormone framework in order, and links to his interview audio transcripts, and articles. Open the guide →

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How much glycine the trials used

Evidence: doses reported in the published trials. Not a recommendation.

The doses below are what researchers gave study participants. They are listed so you can read the trials in context, not as instructions. Anyone on prescription medication should run their full list past a pharmacist or a clinical herbalist before adding an amino acid supplement.

• Sleep trials: 3 g of glycine dissolved in water, taken 30 to 60 minutes before bed.^1,2
• Metabolic syndrome and type 2 diabetes trials: 15 g per day, split as 5 g three times daily, for three months.^6,10
• GlyNAC trials: roughly 100 mg per kilogram of body weight per day of glycine, paired with the same amount of N-acetylcysteine, for 12 to 16 weeks.^7,8
• Schizophrenia adjunct trials: up to 0.8 g per kilogram per day, which is around 60 g for an average adult, under psychiatric supervision.¹⁰

Two tablespoons of gelatin or a large mug of well-set bone broth supplies roughly the amount used in the sleep trials. Glycine powder tastes sweet, dissolves easily in warm water, and is inexpensive, which is part of why it has attracted so little commercial research funding outside Ajinomoto.

Cautions on record for glycine

Evidence: trial adverse-event reports, one animal study, and gaps in the literature.

The 2023 GeroScience systematic review pooled every published human glycine trial, at doses from under 1 g to more than 40 g per day, and found the adverse-event record thin.¹⁰ What the literature records:

• Stomach upset. Loose stools and mild nausea appear in some reports at higher doses. Ajinomoto's own high-dose tolerability study, cited in the Yamadera paper, reported no serious events.¹
• Blood sugar in obesity. The 2022 mouse study found worsened glucose tolerance in obese animals through increased liver glucose output.¹¹ No human trial has shown this, but no human trial has specifically looked in obese participants without metabolic syndrome either.
• Clozapine. Pharmacology texts note that glycine acts at the NMDA receptor and that the antipsychotic clozapine may already be working through related pathways. Trials adding glycine to clozapine did not find benefit, and some reviewers flag the combination as one to avoid.
• Pregnancy, breastfeeding, kidney and liver disease. No trial data exists in these groups. Glycine is cleared through the kidneys and metabolized in the liver, so reduced function in either organ changes how it behaves.
• Sedation. Because glycine is an inhibitory neurotransmitter, the same property behind the sleep findings could in theory add to the effect of sedating medications. This is a pharmacological inference; no interaction study has been published.

Frequently asked questions

Evidence: summarized from the sections above.

Does glycine help you sleep?

Three small randomized trials found that 3 g of glycine an hour before bed improved self-rated sleep quality, shortened the time to fall asleep, and reduced next-day fatigue. All three were run by Ajinomoto researchers and no independent group has replicated them.

How much glycine did the sleep studies use?

3 g, dissolved in water and taken 30 to 60 minutes before bed. That is about the amount in two tablespoons of gelatin.

What foods are high in glycine?

Gelatin, bone broth that sets firm, chicken skin and feet, pork rinds, oxtail and other slow-cooked connective-tissue cuts, and fish skin. Muscle meat contains some but is weighted toward other amino acids.

Is glycine the same as magnesium glycinate?

No. Glycinate means a mineral has been bound to glycine to improve absorption. The glycine dose in a mineral glycinate capsule is small compared with the 3 g used in the sleep trials.

Does glycine raise blood sugar?

In healthy adults, glycine taken with glucose lowered the blood glucose rise by more than half. In obese mice, it made glucose tolerance worse. Human data in obesity without metabolic syndrome does not exist yet.

Does glycine cause diarrhea?

Loose stools and mild nausea are reported in some participants at higher doses. The sleep-trial dose of 3 g was not linked to digestive complaints in the published reports.

What is the difference between glycine and collagen powder?

Collagen peptides and gelatin are whole proteins that are about 30 percent glycine, with proline and hydroxyproline making up most of the rest. Glycine powder is the single amino acid on its own. Gram for gram, collagen gives you less glycine but supplies the other amino acids the body uses alongside it to rebuild connective tissue.

Sources

1. Yamadera W, Inagawa K, Chiba S, Bannai M, Takahashi M, Nakayama K. Glycine ingestion improves subjective sleep quality in human volunteers, correlating with polysomnographic changes. Sleep and Biological Rhythms. 2007;5(2):126-131. doi:10.1111/j.1479-8425.2007.00262.x. Funding and authorship: Ajinomoto Co., Inc., a glycine manufacturer. The Inagawa 2006 sleep trial and the Inagawa 2006 high-dose tolerability study are cited in this paper's reference list.
2. Bannai M, Kawai N, Ono K, Nakahara K, Murakami N. The effects of glycine on subjective daytime performance in partially sleep-restricted healthy volunteers. Frontiers in Neurology. 2012;3:61. doi:10.3389/fneur.2012.00061. Lead authors employed by Ajinomoto Co., Inc.
3. Kawai N, Sakai N, Okuro M, et al. The sleep-promoting and hypothermic effects of glycine are mediated by NMDA receptors in the suprachiasmatic nucleus. Neuropsychopharmacology. 2015;40(6):1405-1416. PMC4397399. Rat study; authors affiliated with Ajinomoto Co., Inc.
4. Meléndez-Hevia E, De Paz-Lugo P, Cornish-Bowden A, Cárdenas ML. A weak link in metabolism: the metabolic capacity for glycine biosynthesis does not satisfy the need for collagen synthesis. Journal of Biosciences. 2009;34(6):853-872. doi:10.1007/s12038-009-0100-9.
5. Gannon MC, Nuttall JA, Nuttall FQ. The metabolic response to ingested glycine. American Journal of Clinical Nutrition. 2002;76(6):1302-1307. PMID 12450897.
6. Díaz-Flores M, Cruz M, Duran-Reyes G, et al. Oral supplementation with glycine reduces oxidative stress in patients with metabolic syndrome, improving their systolic blood pressure. Canadian Journal of Physiology and Pharmacology. 2013;91(10):855-860. PMID 24144057.
7. Kumar P, Liu C, Suliburk J, et al. Supplementing glycine and N-acetylcysteine (GlyNAC) in older adults improves glutathione deficiency, oxidative stress, mitochondrial dysfunction, inflammation, physical function, and aging hallmarks: a randomized clinical trial. Journals of Gerontology Series A. 2023;78(1):75-89. doi:10.1093/gerona/glac135. Baylor College of Medicine has patent filings covering GlyNAC on which the senior author is named; see the paper's disclosure statement.
8. Lizzo G, Migliavacca E, Lamers D, et al. A randomized controlled clinical trial in healthy older adults to determine efficacy of glycine and N-acetylcysteine supplementation on glutathione redox status and oxidative damage. Frontiers in Aging. 2022;3:852569. doi:10.3389/fragi.2022.852569. Funded and authored by Nestlé Research.
9. 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. PMID 23615880.
10. Aguayo-Cerón KA, et al. The effect of glycine administration on the characteristics of physiological systems in human adults: a systematic review. GeroScience. 2024;46(1):219-239. doi:10.1007/s11357-023-00970-8. Table 1 lists every included trial with dose and duration, including Cruz 2008, the schizophrenia adjunct trials, and the Inagawa 2006 sleep trial.
11. Alves A, Lamarche F, Lefebvre R, et al. Glycine supplementation in obesity worsens glucose intolerance through enhanced liver gluconeogenesis. Nutrients. 2022;15(1):96. doi:10.3390/nu15010096. Mouse study.
12. Peat R. Gelatin, stress, longevity. raypeat.com. Independent author; no industry funding.