Glycine

What is glycine and what does it do in the body?

Glycine is generally counted among the nonessential amino acids. ^[6] Apart from protein synthesis, it is also used in the synthesis of serine, glutathione, creatine, purines and the heme in hemoglobin. ^[3]

Collagen is a protein particularly rich in glycine: the tight packing of the triple helix requires every third residue to be glycine. ^[1] Collagen supplements are a separate subject; their evidence is covered in the collagen guide. Glycine being present in collagen does not mean that taking a glycine supplement would increase collagen production.

Does the body make enough glycine?

This question is the basis of the view that counts glycine as “semi-essential.” The 2009 study by Meléndez-Hevia and colleagues is a computational analysis comparing reported glycine production and consumption flows in an adult human (not a new experiment). According to the authors, more than 85% of glycine is synthesized from serine; the amount synthesized is about 3 g/day and that from food 1.5–3.0 g/day, and the total may fall short of the amount required by all uses, including collagen synthesis, in a 70 kg adult by about 10 g a day. From this the authors conclude that glycine is semi-essential and should be taken as a supplement. ^[2]

This conclusion is a hypothesis: it is not a measured deficiency or a health outcome corrected by supplementation, but the output of a flow calculation. The study did not measure a clinical outcome. For mean glycine intake from all sources in people, 3.21 g a day is cited based on US Food and Nutrition Board (FNB/IOM 2005) data. ^[3]

What are daily glycine intake and requirement?

Because glycine is an amino acid the body can synthesize from serine, it is generally not counted as essential. The review by Blachier and colleagues gives no requirement value (RDA or adequate intake) for glycine, only intake data: according to US Food and Nutrition Board (FNB/IOM 2005) data, mean glycine intake from all sources is 3.21 g a day. ^[3] Meléndez-Hevia and colleagues estimate intake from food in the range of 1.5–3.0 g/day. ^[2] The two estimates rest on different methods and may not represent the same population; a person’s actual intake varies with diet.

For whom does glycine supplementation work? Evidence summary

Does glycine improve sleep quality?

Among the authors of the three studies covered here are people linked to the same Japanese amino acid and food company. The 2006 study by Inagawa and colleagues is a randomized, double-blind, crossover trial comparing 3 g of glycine before bed with placebo in 19 volunteers who complained of sleep quality; the subjective state the next morning was assessed with two questionnaires, and glycine produced a significant improvement on the items “fatigue,” “vigor and energy” and “clear-headedness.” ^{[4] [7]} In the 2007 study by Yamadera and colleagues, in volunteers who persistently slept inadequately, 3 g of glycine before bed improved subjective sleep quality and sleep efficiency (sleep time/time in bed) and, on polysomnography, shortened the latency to sleep onset and to slow-wave sleep; sleep architecture did not change. ^[5] Bannai and colleagues reported, in healthy volunteers whose sleep was restricted by 25% for three nights, in a single-blind, randomized, crossover design, that 3 g of glycine significantly reduced the next-day fatigue score and showed a trend toward reduced sleepiness; the analysis rests on data from only 7 people. ^[6]

The limitations of these studies are as follows: among the authors are employees of the same company (according to the article citations, four of the five authors of Inagawa 2006 and three of the six authors of Yamadera 2007 are affiliated with the company’s research units; the authors of Bannai 2012 are from the company’s research laboratory, the study protocol was approved by the company’s ethics committee, and the article reports no conflict of interest); results rest on subjective questionnaires; the samples are small; only short-term use has been examined. As a second look, the systematic review published in GeroScience, covering 18 studies in healthy and 34 in patient populations, states that sleep improved in studies in which glycine was used for longer in healthy people, but that these studies had small samples and a high risk of bias. ^[7] The evidence is therefore read as “promising but limited.”

Does glycine work as an add-on treatment in schizophrenia?

The Cochrane review (search up to 2003) covered 18 short-term trials (358 participants; 6–51 people per trial, at most 12 weeks) examining glutamatergic substances as an add-on to antipsychotic medicines; the substances used were glycine, D-serine, D-cycloserine and the ampakine CX516. Glycine and D-serine, co-agonists of the NMDA receptor, produced a moderate-size reduction in negative symptoms (n = 132; SMD −0.66; 95% CI −1.0 to −0.3); however, there was inconsistency in the response-rate analysis for glycine, and no significant effect was seen on positive symptoms and cognitive function. The authors stated that the data were too few for definitive conclusions. ^[8] These data are old and small; treatment of schizophrenia is carried out by a physician with antipsychotic medicines, and glycine has been examined as a substance added on top of them, not in place of them.

Does glycine work in metabolic syndrome and aging?

In the study by Díaz-Flores and colleagues in Mexico, 60 volunteers with metabolic syndrome (30 glycine 15 g/day, 30 placebo) were followed for 3 months. In the glycine group, plasma TBARS (a marker of lipid peroxidation) fell 25% compared with placebo; superoxide dismutase activity fell 20%; systolic blood pressure fell significantly only in men given glycine (p = 0.043). ^[9] These are markers of oxidative stress, not clinical outcomes such as cardiovascular events; it is a single, small study.

In the GlyNAC study of Kumar and colleagues, 24 older adults took glycine + N-acetylcysteine (GlyNAC) or an isonitrogenous alanine placebo for 16 weeks (12 versus 12); 12 young adults also took GlyNAC for 2 weeks. In older adults, baseline indicators of glutathione deficiency, oxidative stress and mitochondrial dysfunction improved with GlyNAC and did not with placebo; the product was safe and well tolerated. The study was funded by the US National Institutes of Health (NIH/NIA) and a charitable donation, and no conflict of interest was reported. ^[10] The result belongs to the glycine + NAC combination and cannot be attributed to glycine alone. In Türkiye, N-acetyl-L-cysteine was not found suitable for use as an active substance in food supplements. ^[11]

Is glycine safe?

The review by Blachier and colleagues cites the no-observed-adverse-effect level (NOAEL) for glycine in humans as 129.0 mg/kg body weight/day, citing the short-term Inagawa 2006 sleep study; this is not an official upper limit, is derived only from the dose tried in that study, and was used in the review to scale to usual intake. One of the authors of the review is an employee of the same company. ^[3] In the GlyNAC study, 16 weeks of use was reported as safe and well tolerated. ^[10] There is a large difference between the 15 g/day used in the metabolic syndrome study and the 3 g in the sleep studies; safety data also differ with the purpose and the person for whom the dose is used.

Are magnesium glycinate and free glycine the same?

The substance used in the studies in this guide is the free amino acid glycine. Mineral–amino acid compounds such as magnesium glycinate are separate products and were not used in the sleep, schizophrenia or metabolic syndrome studies examined here; the results of these studies therefore cannot be transferred to those products. Magnesium forms are covered in a separate guide: magnesium forms.

Which questions should be asked when evaluating a supplement?

For what purpose will it be used: a sleep complaint, a psychiatric illness, a vague expectation of renewal? Is there a controlled trial for this purpose, in whom was it done and who wrote it? Is the dose used in studies (3 g, 15 g) above the supplement limit in Türkiye (1,000 mg/day)? ^[11] Does the product contain glycine alone or another active substance together with glycine (for example N-acetylcysteine)? ^[10] The answers to these questions vary from person to person; for this reason the results should not be read as personal recommendations.

What is glycine’s status in Türkiye?

Glycine appears in the Ministry of Agriculture and Forestry’s Restricted Substances List for food supplements. In the list dated 21/09/2026, chemical synthesis and bovine protein hydrolysis are given as sources and a maximum daily intake of 1,000 mg for ages 11 and over; no limit is entered for the 4–10 age group. ^[11] The 3 g in sleep studies and the 15 g in the metabolic syndrome study are above this limit. The Food Supplements Communiqué also makes mandatory on the labels of these products the statement “not a medicine” and the warning that a doctor should be consulted in pregnancy, breastfeeding, illness or medicine use. ^[12] Safety data specific to glycine supplements in pregnancy and breastfeeding are limited.

This guide does not recommend a personal dose; if there is a persistent sleep problem, psychiatric illness or regular medicine use, the decision is made with a physician.