Amino Acids DOI 10.1007/s00726-015-2106-y

MINIREVIEW ARTICLE

Guanidinoacetic acid as a performance‑enhancing agent Sergej M. Ostojic1,2 

Received: 2 September 2015 / Accepted: 22 September 2015 © Springer-Verlag Wien 2015

Abstract  Guanidinoacetic acid (GAA; also known as glycocyamine or guanidinoacetate) is the natural precursor of creatine, and under investigation as a novel dietary agent. It was first identified as a natural compound in humans ~80 years ago. In the 1950s, GAA’s use as a therapeutic agent was explored, showing that supplemental GAA improved patient-reported outcomes and work capacity in clinical populations. Recently, a few studies have examined the safety and efficacy of GAA and suggest potential ergogenic benefits for physically active men and women. The purpose of this review is to examine possible applications of GAA supplementation for exercise performance enhancement, safety, and legislation issues. Keywords  Creatine · Exercise performance · Guanidinoacetic acid · Dietary supplement · Ergogenic · Side effects Abbreviations ADP Adenosine diphosphate l-arginineglycine amidinotransferase AGAT  ATP Adenosine triphosphate CRF Chronic renal failure GAA Guanidinoacetic acid GABA Gamma-amino butyric acid

Handling Editors: T. Wallimann and R. Harris. * Sergej M. Ostojic [email protected] 1

Biomedical Sciences Department, Faculty of Sport and Physical Education, University of Novi Sad, Lovcenska 16, Novi Sad 21000, Serbia

2

University of Belgrade School of Medicine, Belgrade, Serbia





GAMT Guanidinoacetate N-methyltransferase GH Growth hormone SAH  S-adenosylhomocysteine

Background The global dietary supplement market is worth approximately $60 billion (Future Market Insights 2015), with worldwide demand for vitamins and supplements continuing to climb. It seems that the majority of the adult population in Western countries routinely use dietary supplements (Bailey et al. 2012), eagerly looking for novel and more effective products to improve the quality of nutrition and life. Almost 2000 new products appear in the dietary supplement market every year (Committee on the Framework for Evaluating the Safety of Dietary Supplements et al. 2004), frequently without the support of any evidencebased research, in search of the ‘magic pill.’ Among others, dietary agents that may boost energy and improve exercise performance particularly attract the attention of both industry and consumers. Besides other candidate agents, guanidinoacetic acid (GAA) could be of interest since it occurs naturally in the human body and acts as an immediate precursor of creatine and its phosphorylated derivative, phosphocreatine, the latter a high-energy molecule able to fuel cellular activities as well as moderate the accumulation of adenosine diphosphate (ADP) from adenosine triphosphate (ATP) during high rate of cellular metabolism (Wallimann et al. 2011). GAA was first identified as a natural compound in humans approximately 80 years ago (Weber 1934). Its use as a therapeutic agent began in the 1950s (Borsook and Borsook 1951). GAA has recently attracted new interest as a dietary additive due to its creatine-recovery effect and its high stability in aqueous solutions (Table 1). In this paper,

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S. M. Ostojic Table 1  Physicochemical properties of guanidinoacetic acid Characteristics

Molar mass Appearance (color) Appearance (form) Odor Thermal decomposition Melting point Acidity (pKa) Solubility in water (15 °C) Stability

C3H7N3O2 Glycocyamine, guanidinoacetate, guanyl glycine, betacyamine, N-amidinoglycine 117.11 g/mol White to off-white Powder or crystals Odorless >190 °C 284 °C 3.41 3600 mg/L Not a limit at 20 °Ca

Shelf life

2 years

Molecular formula Synonyms

a  The stability of crystalline GAA has been studied under accelerated conditions (3 days at 85 °C, 3 weeks at 60 °C, and 3 months at 45 °C). Under those experimental conditions, a negligible loss of crystalline GAA was observed (99.6, 99.7, and 99.7 % GAA after 3 days, 3 weeks and 3 months, respectively) (European Food Safety Authority 2009)

I review GAA’s applicability as a dietary supplement for exercise performance enhancement and provide information regarding its safety as well as legislation issues.

Fig. 1  Guanidinoacetic acid (GAA) metabolism. GAA is synthesized from glycine and l-arginine, with reaction catalyzed by the enzyme l-arginine:glycine amidinotransferase (AGAT). AGAT is located mainly in the kidneys, pancreas and liver. After transported to the liver (also pancreas, spleen, reproductive organs, muscle), GAA is methylated to yield creatine. This reaction is catalyzed by the enzyme guanidinoacetate N-methyltransferase (GAMT) and requires transfer of a methyl group from S-adenosylmethionine (SAM) to GAA, to form creatine and S-adenosylhomocysteine (SAH). Dashed gray lines

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GAA metabolism: synthesis and utilization Guanidinoacetic acid (chemical name: N-[aminoiminomethyl]-glycine) is a naturally occurring amino acid derivative that acts as a direct precursor of creatine. Its role as a creatine precursor was hypothesized for the first time ~90 years ago (Hunter 1928), with GAA isolated from the urine of dogs and humans by Weber in 1934. A little later, Davenport and co-workers (1938) described the synthesis of endogenous GAA and its further metabolism to creatine (Fig. 1). GAA is formed from arginine and glycine by the reaction catalyzed by l-arginine:glycine amidinotransferase (AGAT). This step of GAA biosynthesis appears to be rate limiting, as AGAT is subject to feedback inhibition by creatine at a pretranslational stage (Van Pilsum 1971; Stead et al. 2001; Derave et al. 2004). GAA synthesis takes place mainly in kidney and pancreas, yet several studies have suggested that the degree of extrarenal endogenous synthesis of GAA might be considerable. In humans, renal GAA synthesis may account for ~20 % of total GAA production, suggesting that GAA must be synthesized in other tissues as well (e.g., pancreas, liver, and muscle), although kidney clearly plays a role (Edison et al. 2007). The second enzyme in GAA biotransformation is guanidinoacetate N-methyltransferase (GAMT), which catalyzes the transfer of a methyl group from S-adenosylmethionine to GAA

indicate other possible performance-related roles of GAA, including stimulation of insulin secretion, sparing of arginine and facilitate its use for other physiological functions, activating gamma-amino butyric acid (GABA)-A receptor and modulating GABA and growth hormone (GH) production, and acting as a direct substrate for cytosolic creatine kinase (CK). Black circles indicate enzymes AGAT (1), GAMT (2) and CK (3). ATP adenosine triphosphate, ADP adenosine diphosphate

Guanidinoacetic acid as a performance-enhancing agent

to form creatine and S-adenosylhomocysteine (SAH), with this process taking place mainly in the liver. It has been estimated that about 40 % of all labile methyl groups are used to synthesize creatine by the reaction catalyzed by GAMT (Edison et al. 2007; Mudd et al. 2007). For GAMT, no feedback control by creatine has been observed (Walker 1979). GAMT activity was also detected in extrahepatic tissues (e.g., muscle, reproductive organs, spleen, myocardium, and brain), with GAMT activity in skeletal muscle being calculated to have the potential to synthesize all of the creatine needed in this tissue (Daly 1985). In addition, the brain is dependent on its own creatine synthesis as well (Braissant et al. 2001). However, this hypothesis of extrahepatic synthesis of creatine from GAA in humans has not been tested in vivo. Finally, creatine is released into the circulation from where it can be taken up by various tissues (Edison et al. 2007) via a specific creatine transporter (SLC6A8, a Na+/Cl− creatine co-transporter situated in the cell membrane) (Christie 2007). S-adenosylhomocysteine (SAH), another end-product of GAA utilization, can reversibly be hydrolyzed to homocysteine and adenosine (Walker 1979), with homocysteine either catabolized to cysteine, remethylated to methionine, or exported to the circulation (Edison et al. 2007). GAA can be excreted via the kidney (Wyss and Kaddurah-Daouk 2000). Besides serving as a precursor of creatine by methylation via GAMT, no other physiological roles of GAA have been suggested so far. The natural daily turnover of GAA is balanced between endogenous production/utilization and kidney excretion, due to the fact that only a minimal amount of GAA is consumed from food sources (~10 mg/kg of meat) (European Food Safety Authority 2009). Reference-matched laboratory values for control plasma and urine GAA concentrations are 2.3 ± 0.8 µmol/L and 31.2 ± 21.7 mmol/mol of creatinine, respectively (Joncquel-Chevalier Curt et al. 2013). However, under pathological conditions, the GAA equilibrium might be disturbed by metabolic disorders or renal failure. For example, GAMT deficiency, an inborn error of creatine metabolism, is characterized by creatine depletion and accumulation of GAA in the brain and body fluids (serum levels >16 µmol/L), accompanied by muscle weakness, epilepsy, and mental retardation in affected children (Leuzzi et al. 2000). A GAA-lowering diet in combination with supplementary creatine significantly improves the symptoms of this creatine-deficiency syndrome (Schulze et al. 2006). On the other hand, GAA depletion (serum levels <1.9 µmol/L) has been found in chronic kidney disease and diabetes mellitus patients (Torremans et al. 2006; Tsubakihara et al. 2012), with decreased muscle mass and muscle power in uremic patients that might be restored by long-term GAA provision (Tsubakihara et al. 1999). The above studies highlight two important aspects of GAA metabolism that should be addressed in the context

of exogenous GAA utilization: (1) a possible toxic effect of supraphysiological GAA levels and (2) a need to address a possible GAA deficit under physiologically demanding situations.

GAA as a nutritional additive In the early 1930s, several researchers began to evaluate in animal studies the influence of feeding different amino acids and related substances on creatine metabolism. Dietary GAA emerged as the ‘mother substance’ of endogenous creatine as feeding 1 g of GAA raised the level of muscle creatine in young rats by an average of 48.5 % (Beard and Barnes 1931). The process was also fairly rapid, reaching its maximum within 17–24 h after ingestion of the supplement. Compared with all other substances tested, GAA gave the largest increases of creatine over the control level, even when compared to the equivalent amount of creatine. These early trials helped to advance our knowledge about the function of GAA in the body and were groundbreaking for its use as feed additive in animal nutrition. Today, supplemental GAA is used as a feed additive for chickens and pigs to improve growth, breast meat yield, and the feed conversion ratio (Ringel et al. 2008; Michiels et al. 2012; Wang et al. 2012; Mousavi et al. 2013; Heger et al. 2014), as well as for efficacious replacement of dietary arginine in young chicks (Baker 2009; Dilger et al. 2013). GAA has also been used to improve reproductive parameters and postnatal progeny performance in quails (Murakami et al. 2014). The European Food Safety Authority (2009) concluded that GAA, if administered at a dosage not higher than 600–800 mg GAA per kg feed could be considered as an effective and safe feed additive for chickens for improving performance characteristics. Based on data from five efficacy trials, the European Food Safety Authority (2009) summarized that creatine and ATP content in breast muscle of chickens were both significantly increased by GAA addition to feed. The panel also recognized that only little information was available concerning GAA absorption, utilization, and excretion after feeding diets supplemented with the additive. However, several pilot studies suggested a high digestibility and availability of exogenous GAA in animal nutrition with a true fecal digestibility of ~99 % for GAA in colon-fistulated broilers (Lemme et al. 2007). The first reported use of GAA as an experimental nutritional intervention in humans dates back to the early 1950s when researchers from Caltech University and the US Naval School of Aviation Medicine administered GAA (along with betaine) to patients with cardiovascular and neuromuscular disorders (Borsook and Borsook 1951; Graybiel and Patterson 1951; Van Zandt

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and Borsook 1951). Treatment with GAA was provided in dosages of up to 5 g per day, and the duration of treatment lasted between one and 12 months. The majority of patients demonstrated an improved sense of well-being, less fatigue, and advanced clinician-reported outcomes. In addition, GAA intervention induced considerable excretion of urinary creatine in all patients, while no data were available for plasma or muscle GAA and creatine levels. In the following years, the effectiveness of GAAbetaine therapy was investigated in patients with arthritis (Higgins et al. 1952), acute and chronic poliomyelitis (Borsook et al. 1962; Fallis and Lam 1952; Watkins 1953; Basom et al. 1955), anxiety and depression (Dixon et al. 1954), motor-neuron disease (Liversedge 1956), and neuromuscular disease (Aldes 1957). Since then, supplemental GAA has been suggested as a dietary supplement to compensate for GAA deficiency in patients with chronic renal failure (Tsubakihara et al. 1999), and as a dietary supplement for increasing creatine availability in healthy volunteers (Ostojic et al. 2013a) and patients with chronic fatigue syndrome (Ostojic et al. 2015a). Tsubakihara et al. (1999) demonstrated the effects of administering a single dose of GAA (1 g) to normal and chronic renal failure (CRF) patients, and the effects of medium-term administration of GAA (0.5 g/day of GAA, three times a day for 4 weeks). These authors reported that GAA was absorbed well in all subjects and that skeletal muscle creatine appeared to be significantly increased by regular GAA administration in CRF patients when assessed indirectly through urinary creatine excretion. Our group provided evidence that supplementation with up to 4.8 g of GAA per day during 6 weeks affected the levels of guanidino compounds (GAA, creatine, creatinine) in the serum and urine of healthy volunteers and additionally elevated serum homocysteine (Ostojic et al. 2013a). In addition, no influence of dietary GAA was found on vitamin B status, important co-factors that are heavily involved in the metabolism of end-products of GAA utilization (Ostojic et al. 2014). We have also shown that the daily administration of 2.4 g of GAA for 12 weeks increased skeletal muscle creatine by ~36 % in humans (Ostojic et al. unpublished results). GAA was suggested to be readily absorbed from the gastrointestinal tract and rapidly metabolized to creatine, with a single-dose oral GAA exhibiting dose-dependent pharmacokinetics in healthy men and women (Ostojic and Vojvodic-Ostojic 2015). However, no preclinical or clinical biotransformation studies are available at the moment to describe the disposition of dietary GAA within the human body. Future studies with isotopic GAA tracer (e.g., guanidino-[13C2] acetic acid) are warranted to better understand metabolic behavior of exogenous GAA.

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Performance‑enhancing effects of GAA Pioneering studies in the 1950s provided the first evidence about the performance-enhancing effects of GAA in humans, revealing reduced fatigue, improved endurance, and muscular strength in clinical patients supplemented with a GAA-betaine formulation (Borsook and Borsook 1951, Fallis and Lam 1952). Specifically, daily administration of ~5 g of GAA for 8 weeks improved exercise tolerance, as measured by the Harvard step test, in patients with heart disease (Graybiel and Patterson 1951). In addition, a substantial increment of motor unit activity and muscle power occurred with GAA therapy in 26 out of 31 patients with acute poliomyelitis observed from three to 11 months (Borsook et al. 1962). This so-called ‘sthenic’ effect appeared early (within 1 week after the beginning of the treatment), and persisted throughout the intervention period. The authors suggested that the positive effects of GAA were mediated by an enhanced synthesis of creatine, providing additional energy for cellular bioenergetics. They also suggested that the GAA-betaine therapy was physiologically superior to creatine administration (Borsook et al. 1962). Although the findings of these preliminary trials were encouraging, all studies were open-label without a control group, undertaken on small samples, with limited objective data provided on exercise performance. Our group recently evaluated the performance-enhancing effects of GAA in young healthy men and women. The original randomized, double-blind, placebo-controlled study was initiated in 2009 to examine the safety and efficacy of 6-week supplementation of oral GAA in healthy volunteers (Ostojic 2010). The study enrolled 48 men and women aged 20–25 years who received either oral doses of GAA (up to 4.8 g/day) or a placebo (inulin) for 6 weeks, with exercise performance outcomes observed at baseline and following 6 weeks of intervention (Ostojic et al. 2015b). We found that oral GAA significantly improved handgrip strength (by approximately 6 kg) as compared with the placebo, suggesting an ergogenic effect of GAA on maximal isometric strength of the hand and forearm muscles (Fig. 2). In addition, GAA significantly improved muscular endurance in the upper body as assessed through bench press performance, with the number of repetitions increased for up to 8.9 repetitions on average with the highGAA dose, as compared with the placebo. We also reported a trend (p = 0.07) for improved lower body muscle endurance (leg press repetitions) after 6 weeks of GAA supplementation. However, the remaining measures, body composition, aerobic, and anaerobic performances values were not significantly improved with GAA supplementation. Dose–response relationships between GAA administration and exercise performance outcomes were not found

Guanidinoacetic acid as a performance-enhancing agent Fig. 2  Percentage change in exercise performance end points after 6-week supplementation with three different guanidinoacetic acid (GAA) supplementation protocols (1.2, 2.4, and 4.8 g/days) in healthy men and women. Asterisk indicates significant interaction effect (trial vs. group) at p < 0.05. Data are calculated from Ostojic et al. (2015b)

for the dose-range investigated (Ostojic et al. 2015b). We concluded that dietary GAA could provide a performanceenhancing benefit with the greatest effect in muscle groups with lower initial levels of strength (e.g., chest, shoulders, and arms). This is in line with the results from a previous study that investigated the efficacy of GAA in the treatment of late residual muscular weakness resulting from poliomyelitis (Watkins 1953). The latter author reported an improvement in muscular strength after GAA administration (3.0–5.5 g/day of GAA, three times a day for 8 weeks) only in underdeveloped muscle groups that had never received progressive-resistance exercises. In addition, we recently reported an improvement in muscular performance (total isometric strength of quadriceps increased by 4.2 %) in 12 adult patients suffering from chronic fatigue syndrome, who had been supplemented with 2.4 g/day of GAA for 4 weeks (Ostojic et al. 2015a). The above preliminary findings suggest that oral GAA may have a favorable effect on muscle strength in healthy individuals with lower levels of muscular fitness, novice athletes, or in clinical populations with muscular weakness. However, more studies are needed to closely evaluate performance-enhancing effects of dietary GAA in those with poor exercise capacity (e.g., cardiorespiratory disorders, anemia, and mitochondrial diseases) before recommending GAA as a potentially useful nutritional intervention. Several possible mechanisms have been suggested to explain the ergogenic effects seen with GAA administration. However, definitive evidence for each mechanism is so far insufficient or lacking. The most obvious explanation for the observed effects of GAA is that it acts as a direct

precursor of creatine, stimulating biosynthesis in the liver and other organs, with increased accumulation of creatine in the muscle (Ostojic et al. 2015b). However, only preliminary data are available concerning enhanced creatine availability in the skeletal muscle after GAA intervention, with no human study describing fundamental aspects of GAA/creatine biotransformation and regulation, such as the effect of endogenous GAA synthesis on the rate of extrarenal synthesis of creatine. A second proposal was that exogenous GAA might provide an anabolic stimulus through stimulation of insulin secretion (Aynsley-Green and Alberti 1974), with GAA-driven insulin release appearing to be more potent than that produced by arginine or creatine (Alsever et al. 1970). This might favorably affect exercise performance, since insulin is well recognized as an ergogenic agent (Sonksen 2001). Thirdly, it was suggested for broilers that dietary GAA, as an arginine-sparing additive, might positively affect the metabolic utilization of arginine (Baker 2009), which may further increase muscular growth and performance. Preliminary findings suggest a minimal effect of dietary GAA on serum arginine levels in healthy human volunteers supplemented with 2.4 grams of GAA for 8 weeks (Ostojic et al. unpublished data). No data are available regarding arginine utilization after GAA intervention and possible ergogenic effects relating to arginine (e.g., promoted vasodilation through increased nitric oxide production, improved recovery via increased substrate utilization and metabolite removal after exercise) (Álvares et al. 2011). Fourthly, GAA might act as a gamma-amino butyric acid (GABA)-A receptors agonist (Neu et al. 2002) and as a possible modulator of GABA

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metabolism in the brain and peripheral tissues. Since the modulation of GABA utilization could affect the secretion of growth hormone (GH) (Powers 2012), with GH being a well-known performance-enhancing agent (Stacy et al. 2004), dietary GAA might boost GH response and affect exercise capacity. We recently confirmed that GAA loading (3 g daily for 3 weeks) affected plasma GABA levels and potentially modulates GABA synthesis in peripheral tissues (Ostojic and Stojanovic 2015), but currently no exercise performance data are available concerning the GABAergic action of oral GAA or the GH response to GAA intervention. Finally, phosphorylated GAA might act as a compensatory phosphagen and a direct substrate for cytosolic but not for mitochondrial creatine kinase (Boehm et al. 1996), thus providing enhanced energy transfer and serving as a creatine mimetic, at least when the availability of creatine is reduced. This hypothesis has not been tested in humans so far.

GAA pharmacovigilance The safety of dietary GAA has been evaluated from the beginning of its use in human studies. Clinical trials in patients with heart disease (Borsook and Borsook 1951, Van Zandt and Borsook 1951; Graybiel and Patterson 1951) demonstrated no signs of GAA toxicity as assessed by routine blood and urine analyses and patientreported outcomes, except for minor disturbances in the gastrointestinal tract (e.g., mild nausea, loss of appetite, abdominal bloating, and diarrhea) reported early in the intervention. Subsequent clinical trials in the 1950s confirmed GAA to be a nontoxic and well-tolerated preparation (Dixon et al. 1954), with isolated cases of slight gain in weight and occasional muscle cramping after GAA intervention (Watkins 1953). No adverse events of dietary GAA were reported by patients with kidney failure, and orally administered GAA had no significant effect on blood chemistry, hematological indices, and urinary solutes in CRF patients, except for slightly elevated urinary calcium excretion (Tsubakihara et al. 1999). Our recent study (Ostojic et al. 2013a) suggested that dietary GAA had an acceptable side-effect profile when orally administered for 6 weeks in healthy men and women, with liver and muscle enzyme profiles being unaffected by GAA intervention, and with a rather low incidence of adverse events after ingestion (e.g., weight gain, nausea, bloating, muscle cramping, and abdominal pain). A higher incidence of elevated serum creatinine was observed in participants supplemented with GAA (88.3 vs. 9.1 % with placebo) yet no other markers of kidney damage were found. Under these circumstances, where muscle and total body creatine levels are elevated by GAA, elevated serum

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creatinine levels are to be expected, and are not reflective of kidney damage per se. However, increased plasma homocysteine concentrations were found in 58.3 % of participants receiving GAA as compared to 18.2 % subjects receiving placebo. The observation of elevated plasma homocysteine as a metabolic consequence of GAA ingestion has been confirmed in another study (Ostojic et al. 2014), with the daily dose of 4.8 g of GAA being particularly effective in this respect. Since hyperhomocysteinemia has been recognized as an independent risk factor for cardiovascular and arteriosclerotic diseases (Refsum et al. 1998; Morris 2003), a progressive increase in plasma homocysteine after GAA intake should be considered as a possible adverse effect of the intervention. Therefore, administration of GAA in a clinical environment needs careful monitoring of serum homocysteine and a possible dose titration trial during GAA administration. In addition, it seems reasonable to use methyl group donors (e.g., betaine, choline, and vitamin B) as nutritional additives to suppress the hyperhomocysteinemia induced by GAA loading (Ostojic et al. 2013b). The question arises, however, whether in light of hyperhomocysteinemia caused by GAA administration, this treatment can be recommended to healthy individuals, e.g., to athletes who would want to use GAA as an ergogenic aid. At the moment it seems that significantly more studies are needed, especially for long-term usage of GAA for this population of potential GAA consumers. Considering consumer safety information, European Food Safety Authority (2009) concluded that from the available evidence, GAA has no mutagenic or genotoxic properties neither does it pose a risk to the environment. Several studies reported neurotoxic or pro-oxidant effects of GAA administration in animal models and in vitro studies (Zugno et al. 2004, 2008). In addition, recent neurological research reported accumulation of GAA in body fluids of children with inborn errors of creatine metabolism (Stromberger et al. 2003), suggesting that extra GAA might contribute to neurological complications. Although dietary GAA causes an increase in plasma GAA after administration, the possibility that dietary GAA accumulates and induces neurotoxicity in the brain is highly unlikely due to the limited permeability of the blood–brain and blood–cerebrospinal fluid barriers for GAA (Braisant 2012). In addition, baseline GAA values (fasting state) after oral administration are well below pathological levels of plasma GAA, which are reported to be 20–30 times higher in patients with creatine-deficiency syndrome compared to normal values (Almeida et al. 2004). At this point, no final conclusion on the safety of GAA orally administered to healthy humans can be drawn, and definitely more studies are needed to evaluate long-term safety of GAA for human nutrition.

Guanidinoacetic acid as a performance-enhancing agent

Legislation aspects of GAA GAA has been authorized in the European Union as a novel feed additive for chicken under Commission Regulation EC No 904/2009 and is categorized as a member of the functional group of amino acids, salts, and analogs (European Food Safety Authority 2009). Concerning human nutrition, the safety of GAA for human consumption is not yet approved by government agencies such as the US Food and Drug Administration (FDA) or the European Food Safety Authority (EFSA). The substance is currently not cataloged or listed as a dietary/food supplement. So far, no documentary standards of identity, quality, and associated analytical methods for GAA are available in the US Pharmacopeia Dietary Supplements Compendium.

Conclusion As a highly bioavailable precursor of creatine, supplemental GAA seems to improve muscular performance (both isometric and dynamic strength) in a clinical and athletic environment, with no dose–response relationships between GAA administration and exercise performance outcomes. During medium-term application, GAA has an acceptable side-effects profile with a low incidence of biochemical abnormalities, except for elevated plasma homocysteine that should be closely monitored or potentially prevented with the concomitant use of methyl group donors. Preliminary findings suggest that GAA might be considered as an innovative way to improve exercise performance, yet more studies are needed to evaluate its kinetics and the long-term safety of GAA supplementation, along with possible performance-related mechanisms (besides that of muscle creatine elevation). Acknowledgments  This work was supported by the Serbian Ministry of Science (Grant No. 175037), Faculty of Sport and Physical Education, University of Novi Sad (2015 Annual Award), and AlzChem AG, Trostberg, Germany. The author is grateful to Theo Wallimann and Roger Harris for their assistance during the manuscript preparation. Compliance with ethical standards  Conflict of interest  The author declares no conflict of interest.

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