What to Combine Glycine With

Glycine is a 'helper' amino acid: it rarely acts on its own, but it participates in the synthesis of collagen, glutathione, and creatine. The editorial team examined which substances glycine is combined with most often, which combinations have confirmation in research, and which pairs are better avoided.

Why combine glycine at all

Glycine rarely works as a 'solo star.' Its role in the body is mostly auxiliary: it becomes a building block for collagen, glutathione, creatine, and porphyrins, and in the nervous system it acts as a mediator and co-agonist of NMDA receptors. So it is logical that the most interesting practical applications of glycine are related to combining it with other substances that close the 'second shoulder' of the same process.

For example, three amino acids are needed for glutathione synthesis - glutamate, cysteine, and glycine. If cysteine is lacking, extra glycine will change little. Similarly, for collagen it is important to have not only glycine, but also proline, lysine, and vitamin C as a cofactor of the enzymes that stabilize collagen fibers.

The editorial team examines combinations by three goals for which at least some evidence base exists: sleep and recovery of the nervous system, antioxidant protection, and support of connective tissue. Separately, we will explain what there is no sense in combining glycine with and when a doctor's consultation is needed.

Let us recall that glycine is an ordinary dietary supplement, so it is appropriate to discuss practical doses here. A guideline for most combinations is the amounts used in studies: about 3 g for sleep and several grams per day for metabolic goals.

Glycine and magnesium: a combination for sleep

The most popular pair is glycine and magnesium. Both substances are independently linked with sleep: glycine at a dose of about 3 g lowers core body temperature and eases falling asleep (Yamadera et al., 2007; Kawai et al., 2015), while magnesium participates in regulating neuronal excitability. Athletes with intense training and heavy sweating may have increased magnesium losses.

In practice, this combination is implemented in two ways: by taking glycine powder and magnesium (citrate, glycinate, bisglycinate) separately, or by choosing magnesium glycinate as a ready-made combination. It is worth understanding that in a standard dose of magnesium glycinate, the amount of glycine itself is only a fraction of the dose used in sleep studies.

So if the goal is specifically the effect of glycine on sleep, it is more sensible to add glycine powder separately, dissolving it in water 30-60 minutes before bed. Glycine has a slightly sweet taste, and many find it easy to take without sweeteners.

There are few direct randomized studies of the 'glycine + magnesium' combination specifically, so this pair is based on the logic of the separate effects. If sleep does not improve after two or three weeks of regular intake together with maintaining a routine, it is worth looking for the cause more deeply.

Glycine and N-acetylcysteine (GlyNAC)

The most scientifically 'formalized' combination is GlyNAC, that is, glycine together with N-acetylcysteine. The research group of Rajagopal Sekhar (Sekhar et al., 2011) showed that in elderly people reduced glutathione levels are linked precisely with a shortage of its precursors, and the combination of cysteine and glycine restored glutathione synthesis to the level of young participants.

A later pilot study by Kumar and colleagues (2021) reported improvement in a whole range of indicators - from oxidative stress and inflammation to insulin resistance and muscle strength - after several months of taking GlyNAC. However, the sample was small, and the results require confirmation in large independent trials.

For young healthy athletes with a full diet, the advisability of GlyNAC is less obvious. Moreover, high doses of antioxidants during the period of adaptation to training could theoretically blunt part of the signals that trigger adaptation. So the editorial team regards this combination primarily for older age and as a topic for discussion with a doctor.

If you are nevertheless considering GlyNAC, choose products with a transparent composition and discuss the advisability with a doctor, especially if you have chronic illnesses or take medications. The effect should be assessed not by sensations but by specific indicators that a specialist can order.

• NAC is a medicinal substance in many countries; it can interact with nitroglycerin and affect coagulation indicators.
• People with asthma should treat NAC with caution because of possible bronchospasm with inhaled forms.
• Optimal doses of GlyNAC for healthy people have not been established.

Glycine, collagen, and vitamin C for ligaments

For supporting tendons, ligaments, and cartilage, glycine is rarely taken in isolation. It is more sensible to consume a source that contains the whole 'set' of collagen amino acids: gelatin or hydrolyzed collagen, which are roughly one-third glycine. Additional vitamin C is needed as a cofactor of the enzymes that hydroxylate proline and lysine.

The study by Shaw and colleagues (2017) showed that 15 g of gelatin with vitamin C, taken about an hour before a short jumping load, raised markers of collagen synthesis in the blood. The authors explain this by the fact that mechanical loading enhances the delivery of amino acids to connective tissue precisely in this time window.

Whether it is worth additionally adding pure glycine to collagen is an open question. The hypothesis of Melendez-Hevia and colleagues (2009) about the body's limited ability to synthesize glycine for collagen theoretically supports such an idea, but the editorial team is not aware of clinical studies that would compare collagen with and without additional glycine.

The practical conclusion for an athlete: if the goal is ligaments and tendons, the priority is a collagen product with vitamin C at the right time before exercise. Separate glycine can be left for evening intake, where it has its own niche - supporting sleep.

What is not worth combining

The popular idea that 'glycine will enhance creatine because it is its precursor' is not confirmed. Ready-made creatine monohydrate already bypasses the synthesis stage, so extra glycine does not increase its stores in the muscles. Such a combination can be used, but you should not count on synergy.

Combining glycine at night with evening caffeine, pre-workout complexes, or stimulants deprives it of meaning. If you train in the evening, it is worth considering whether you need caffeine at such an hour, rather than trying to 'neutralize' it with an amino acid.

Glycine should also not be combined with alcohol to 'improve sleep': alcohol itself worsens sleep architecture, suppressing the REM phase, and no supplement compensates for this.

Special attention is required for people taking psychotropic drugs, in particular antipsychotics. Glycine acts on NMDA receptors, and in clinical studies it was used as an add-on to schizophrenia therapy in high doses (Heresco-Levy et al., 1999). Such combinations are possible only under the supervision of a psychiatrist, not as a self-directed experiment.

Editorial conclusions

Glycine works best in 'teams,' where the other members cover the needs of the same process. The most well-founded pairs are glycine with N-acetylcysteine for glutathione synthesis (mainly in older age), collagen sources of glycine with vitamin C for connective tissue, and glycine with magnesium as part of an evening routine.

At the same time, the level of evidence for most combinations is moderate or low, and none of them replaces a full diet, sufficient protein, and a sleep routine. Combining 'just in case' without a clear goal only complicates assessing what actually works.

For a deeper dive, we recommend our articles 'Myths about glycine,' a review of magnesium for athletes, and material on collagen and vitamin C for ligament recovery.

References

1. Yamadera W, Inagawa K, Chiba S, et al. Glycine ingestion improves subjective sleep quality in human volunteers, correlating with polysomnographic changes. Sleep Biol Rhythms. 2007;5(2):126–131.
2. 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.
3. Sekhar RV, Patel SG, Guthikonda AP, et al. Deficient synthesis of glutathione underlies oxidative stress in aging and can be corrected by dietary cysteine and glycine supplementation. Am J Clin Nutr. 2011;94(3):847–853.
4. Kumar P, Liu C, Hsu JW, et al. Glycine and N-acetylcysteine (GlyNAC) supplementation in older adults improves glutathione deficiency, oxidative stress, mitochondrial dysfunction, inflammation, insulin resistance, endothelial dysfunction, genotoxicity, muscle strength, and cognition: results of a pilot clinical trial. Clin Transl Med. 2021;11(3):e372.
5. Shaw G, Lee-Barthel A, Ross ML, Wang B, Baar K. Vitamin C-enriched gelatin supplementation before intermittent activity augments collagen synthesis. Am J Clin Nutr. 2017;105(1):136–143.
6. Meléndez-Hevia E, de Paz-Lugo P, Cornish-Bowden A, Cascante M. A weak link in metabolism: the metabolic capacity for glycine biosynthesis does not satisfy the need for collagen synthesis. J Biosci. 2009;34(6):853–872.
7. Heresco-Levy U, Javitt DC, Ermilov M, et al. Efficacy of high-dose glycine in the treatment of enduring negative symptoms of schizophrenia. Arch Gen Psychiatry. 1999;56(1):29–36.
8. Razak MA, Begum PS, Viswanath B, Rajagopal S. Multifarious beneficial effect of nonessential amino acid, glycine: a review. Oxid Med Cell Longev. 2017;2017:1716701.