Glycine: the forgotten sports performance nutrient?

The role of glycine

Like all amino acids, the amino acid known as ‘glycine’ consists of a carbon backbone, with an amino (NH2) group at one end of the molecule and a carboxylic (COOH) group at the other end (see figure 2). These amino and carboxylic groups are the linkage points that allow the body to assemble long chains of amino acids – ie to make proteins. Importantly, these protein chains of amino acids can be built into complex shapes, which allows them to perform key functional roles in the body.

Figure 2: Structure of glycine

Although it is classed as a non-essential amino acid, glycine nevertheless plays a fundamental role in the synthesis of various essential biological molecules in the body, such as glutathione, creatine, and purine nucleotides (found in DNA). Moreover, glycine constitutes an essential element in the structure of numerous proteins – notably in collagen (see this article) and other types of connective tissue. In the bloodstream, glycine accounts for 11.5% of total amino acids and contains 20% of the total nitrogen content in body proteins, thereby contributing significantly to total protein.

Sources of glycine

As figure 1 shows, glycine is relatively abundant in both animal and plant proteins. Animal proteins rich in glycine include pork, beef, chicken, fish, dairy products, and eggs. Plant proteins rich in glycine include almonds, peanuts, walnuts and pulses such as beans and lentils. Given this abundance, and that the recommended dietary intake of glycine ranges between 1.5 and 3.0 grams per day⁽²⁾, it’s natural to assume that getting enough glycine in the diet for optimum health and performance is not an issue. However, some research suggests that in certain cases such as the elderly⁽³⁾ and athletes who train hard and compete⁽⁴⁾, glycine intakes might not meet the required demand for optimum well being.

For this reason, some researchers have proposed that there could be a need for athletes in heavy training to increase their glycine intake via the use of supplements⁽⁵⁾. However, there are two issues here: Firstly, there is a dearth of data on glycine supplementation and athletic performance, with some of the reported benefits being theoretical in nature rather than being observed in actual studies. Secondly, the safety profile of supplemental glycine is not well established; whilst a lot of research has found glycine to be a safe and non-toxic nutrient when ingested at modest levels⁽⁶⁾, at very high doses of glycine supplementation, there is some evidence that the risk of strokes may be increased⁽⁷⁾.

New research

To try and get a better understanding of whether and how extra glycine intake could help athletic performance (and what the safe limits are) a team of Mexican researchers has just carried out a review of the accumulated research into glycine⁽⁸⁾. Published in the journal ‘Sports Basel’, the researchers’ aim was to investigate and clarify the data on glycine and performance, focusing on a number of areas relevant to athletes such as endurance performance, muscle growth and strength, and sleep quality.

Why is glycine a critical nutrient for important for athletic performance?

In the first part of their review, the researchers explained why glycine is such an important nutrient for athletes. Without delving too much into the nutrient’s biochemistry, it turns out that glycine plays key roles in the body, including being a cytoprotective nutrient (protecting cell integrity and the transmission of nerve signals) an antioxidant, immune supportive and an anti-inflammatory^(9-11). Due to the complex biochemistry involved (beyond the scope of this article), these actions help maintain cell membrane integrity and stability, and prevent inflammation and cellular death, especially during ATP depletion, hypoxia (lack of oxygen), and tissue injury during high stress – exactly the conditions experienced in muscle tissue during intense exercise^(12,13).

Muscle growth

Research on glycine supplementation conducted in animals suggests that it can significantly affecting skeletal muscle metabolism, neuromuscular action, and exercise performance, resulting in preserved muscle mass, reduced inflammation, and increased growth hormone levels (see table 1).

Table 1: Studies on glycine

The muscle protein synthesis effects might be explained by the fact when combined with high-intensity exercise, glycine is implicated in the up-regulation of a vital signalling molecule known as ‘mTORC1’ (mammalian target of rapamycin). As we have explained in a previous SPB article, mTORC1 is a crucial pathway for muscle protein synthesis, and anything that can stimulate the release of mTORC1 is likely to help maintain muscle mass or boost muscle growth^(16-18). In addition, glycine (in combination with arginine) activates an enzyme known as ‘arginine glycine amidinotransferase’. This enzyme is important because it is required for creatine synthesis in skeletal muscle, which is a vital molecule for energy production, especially during high-intensity exercise.

Connective tissue

In addition to muscle growth, glycine is a critically important nutrient for collagen synthesis⁽¹⁹⁾. Collagen is an essential component of the extracellular connective tissue matrix in muscle tissues. This matrix provides structural support to muscles, enhancing their strength and resistance. In one study carried out on cultured cells, increasing the glycine concentration in the solution around the cells to a level that 5-fold higher than that found in human plasma resulted in a 2.5-fold increase in collagen synthesis⁽¹⁹⁾.

Could extra collagen synthesis help athletes to build more resilient muscles? In theory yes, but to date, there’s not much data on this topic. One randomized clinical study on marathon runners drew a blank; it found that injuries to muscle tissue that occurred after completion of the marathon and approximately 48 hours later were similar between athletes with or without glycine supplementation (10 grams, three times a day for 14 days before the run)⁽²⁰⁾. However, given that it takes weeks/months remodel (break down and resynthesize) connective tissue, a 14-day intervention is likely to have been too short to produce a meaningful outcome. What’s needed is an intervention study where glycine supplementation is carried out over a time period of months!

Exercise performance

Can additional glycine help with exercise performance? In a 2021 randomized clinical trial, researchers found that reported that supplementing with 0.1 grams of glycine per kilo of bodyweight per day (7 grams for a 70kg adult) for 24 weeks resulted in decreased inflammation, improved blood flow, increased strength and exercise capacity, and lowered body fat and waist circumference in older and healthy adults⁽²¹⁾. Moreover, these benefits declined after stopping supplementation for 12 weeks.

However, these study adults were older and not engaged in vigorous exercise, which makes it hard to draw firm conclusions for athletes. Nevertheless, a 2024 study also found that glycine supplementation was able to improve improving metabolic health and mitochondrial (the cells’ ‘energy factories’) function, and also reduce the progression of obesity-related disorders⁽²²⁾. Given how important mitochondrial function and density (number per unit volume) are for endurance performance⁽²³⁾, this is most definitely an area that needs further investigation!

Glycine and sleep quality

As regular readers will know, getting good quality sleep is a vital part of athletic recovery (see this article). Therefore any nutrient that can improve sleep quality could help athletes to recover better. Beyond its effects on muscle function, it also turns out that glycine appears to have a role in promoting high sleep quality, which could therefore help with performance. Human studies on otherwise healthy people but who suffer difficulty sleeping found that taking three grams of glycine before bedtime not only improved sleep quality, but also reduced daytime sleepiness, and fatigue induced by acute sleep restriction^(24,25).

These findings are supported by animal studies where it was found that glycine ingestion was able to reduce the activity of neurons that fire in the ‘awake’ state⁽²⁶⁾. In addition, it seems that pre-bedtime glycine can also induce sleepiness by increasing the flow of blood to the extremities of the body (vasodilation), which lowers the core temperature⁽²⁷⁾. That’s relevant because a lowering of core temperature is known to occur before the onset of sleep, and to help the process of inducing sleep⁽²⁸⁾.

Recommendations for athletes

Given our current knowledge about glycine, should you consider taking extra in order to help your performance? Unfortunately, this is a difficult question to answer. While there are a number of theoretical performance benefits to glycine supplementation, the data from human studies involving athletes in training is lacking. Without this hard data, it’s not really possible to make a firm recommendation. There’s also the issue of toxicity; because glycine supplementation has not been studied in great depth, it’s necessary to proceed with caution.

Having said the above, athletes who wish to try taking some extra glycine at low doses would appear to have little to lose. Anecdotally, some athletes who take extra glycine report that they feel more energized and recover faster. In the study on older healthy adults, benefits were seen at around 7-8 grams of glycine per day for 24 weeks with absolutely zero ill effects. And given that the marathon runners investigated for injury prevention managed 30 grams per day with no ill effects, it is very unlikely that a dose of 5-8 grams per day would have any ill effects, yet is still an amount that might produce benefits. If you are going to try glycine for performance, take it away from other protein foods and divide your doses into 2-gram portions – eg 2 grams, three times a day.

Going further, athletes who struggle with sleeping lso have little to lose from taking three grams of glycine before bedtime. This assumes of course that you are already following good sleep hygiene measures – see this article. Another scenario where glycine supplementation is almost certainly warranted is an addition to a why protein recovery drink. In a recent article, we discussed new research on how whey drinks can temporarily induce a relative shortfall of glycine, which might impair the synthesis of connective tissue compared to the synthesis of muscle tissue. In that article, we discussed research that recommended adding 5-10 grams of collagen protein (rich in glycine) to a whey drink in order to balance out the amino acid profile. Collagen is rich in glycine so of course another option is to take some glycine (around 3 grams or so) with your whey drink, which will achieve the same effect!

References

1.       Nutrition Reviews, Volume 77, Issue 4, April 2019, Pages 197–215,

2.       Amino Acids. 2021;53:1313–1328

3.       Nutr. Neurosci. 2003;6:269–275

4.       Nutrients. 2023;15:4003. doi: 10.3390/nu15184003

5.       Eur. Sport. Manag. Quar. 2018;18:175–192

6.       Rev. Med. Chem. 2016;17:15–32

7.       J. Nutr. 2015;145:720–728

8.       Sports (Basel). 2024 Sep 25;12(10):265. doi: 10.3390/sports12100265

9.       Transl. Med. 2021;11:e372

10.     Anim. Model. Exp. Med. 2023;6:168–177

11.     J. Immunol. 2019;202:1704–1714

12.     Neurochem. Res. 2003;28:893–901

13.     Eur. J. Cell Biol. 2011;90:333–341

14.     Surgery. 2001;129:231–235

15.     Clin. Transl. Med. 2021;11:e372

16.     Endocr. Rev. 2021;42:56–76

17.     Front. Nutr. 2019;6:172

18.     J. Appl. Physiol. 2019;127:581–590

19.     Amino Acids. 2018;50:1357–1365

20.     Int. J. Sports Med. 1999;20:315–321

21.     Clin. Transl. Med. 2021;11:e372

22.     Front. Endocrinol. 2024;15:1343738

23.     J Med Food. 2018 Jan;21(1):30-38

24.     Front. Neurol. 2012;3:61

25.     Sleep Biol. Rhythm. 2006;4:75–77

26.     PLoS ONE. 2011;6:e20360

27.     Neuropsychopharmacology. 2015;40:1405–1416

28.     Chronobiol Int. 1994;11:126–131