Athletes often turn to nutritional supplements in order to maintain health and maximize athletic performance [1
]. Among them, proteins and amino acids represent the most consumed ergogenic aids, with a frequency of 35–40% [2
]. However, the use of nutritional supplements with vasodilatory function are increasing considerably in the sport field, given that there is strong evidence that its intake has a positive effect on athletic performance [3
]. In this sense, although nitrate and beetroot juice are the most studied vasodilatory supplements in this field [6
], arginine (Arg) is an amino acid that has shown a vasodilator effect because it participates in the synthesis and bioavailability of nitric oxide (NO) [4
]. For this reason, Arg supplementation has been used by athletes in order to obtain improvements in athletic performance [9
Although Arg is a non-essential amino acid for adults because is absorbed through dietary proteins [13
] and synthesized in the small intestine from proline, glutamate, and glutamine [8
], some research has shown that supplementation could be beneficial to increase athletic performance [15
]. The most relevant benefit of Arg is related to NO synthesis and its role as a cell signaling molecule with physiological relevant effects [16
]. NO has shown increased blood flow and improved muscle contraction, gas exchange, oxygen kinetics, and mitochondrial biogenesis [6
]. Otherwise, Arg has also been shown to stimulate the release of growth hormone (GH) [18
], which helps to promote cell growth and regulate the mobilization of fuels in the body that contributes to increase muscle mass and hypertrophy [20
]. Moreover, Arg supplementation has presented a reduction of ammonia, lactate, fatty acids, and fat oxidation levels after exercise [23
]. Likewise, Arg has shown an increase in glycerol post-exercise, with improved carbohydrate oxidation and oxygen efficiency [25
], considering these potential benefits in endurance sport performance. Thus, Arg has displayed effects on different physiological and metabolic pathways that could improve athletic performance in both, endurance or “aerobic” and high intensity or “anaerobic” athletic performance [15
Athletic performance in endurance sports, in which efforts commonly last 5 min or more and requires equal and/or less intensity than VO2
max, is related to the capacity of circulatory and respiratory systems to supply fuel and resynthesize adenosine triphosphate (ATP) by oxidative metabolism [27
]. Therefore, endurance performance is determined by maximal oxygen uptake (VO2
max), ventilatory thresholds, and energy efficiency or economy [28
]. In this sense, different Arg supplementation protocols (<7 days or acute) and dosages (6–10 g/day) have been shown to improve several physiological parameters and performance outcomes, such as time to exhaustion, mean power output, and exercise capacity in moderate-submaximal intensities [10
]. These results could be explained only due to improvements in blood flow and oxygen supply to the muscles, because it seemed that longer supplementation periods are needed to enhance mitochondrial respiration and oxidative phosphorylation through NO pathway [6
]. However, other studies did not show any improvement on total 5 km running time and on cycloergometer incremental test performance in experienced runners and recreationally active men, respectively [31
On the other hand, performance in high intensities or anaerobic sports, requires greater intensity than VO2
max and depends on different metabolic pathways related to exercise duration [33
]. In this sense, while high energetic phosphagen system (<6 s duration) is determinant for explosive disciplines, glycolysis represents the main energetic pathway for exercises between 1–5 min (with an increased contribution of oxidative phosphorylation proportionally with time), determined as high-intensity (~1 min) and intensive efforts (<5 min) [33
]. Regarding anaerobic performance, chronic Arg supplementation (45–56 days) with low (2 g/day) and high (12 g/day) dosages could led to improve performance in one maximum repetition (1RM) bench press, Wingate test, and VO2
max intensity test [9
]. These positive effects related with strength could be explained because of Arg enhance GH-releasing hormone, suppresses the endogenous GH-inhibiting hormone and increases insulin-like growth factor 1 (IGF-1) [37
]. Moreover, Arg plays an essential role in the synthesis of Creatine, main substrate for phosphagen system and anaerobic performance [8
]. However, other authors did not find any benefits on muscle strength, maximum number of repetitions, and sprint power after ingesting 6 g/day of Arg in both acute and chronic protocols [39
Although Arg supplementation could be effective on aerobic and high-intensity sport disciplines performance mediated by several effects, current evidence is controversial and confusing. In this sense, there are a few systematic reviews that analyze the effects of Arg supplementation on different physiological and metabolic mechanisms in older adults and disease patients [42
]. Moreover, a short systematic review examined the connection between Arg and citrulline and sport performance [15
]. However, to the best of the authors’ knowledge, there is not clearly and quantitatively analyzed information and distinction regarding the different involved effects in both aerobic and anaerobic performance capacities in the literature, and this may be necessary to better understand Arg supplementation reasons and protocols. Therefore, we proposed carrying out a systematic review and meta-analysis on the effects of Arg supplementation on exercise performance according to the main energy metabolism system used during exercise, with the main aim of analyzing current evidence and evaluating its impact on performance in both, aerobic and high-intensity or anaerobic, disciplines. In addition, this manuscript aims to show the effective doses and ideal moment of its intake.
The main objective of this systematic review and meta-analysis was to analyze and summarize the current evidence around the effects of Arg supplementation on aerobic and high-intensity anaerobic athletic performance. Results obtained mainly indicate that Arg specific supplementation protocols led to significant improvements both on anaerobic (>VO2max) and aerobic (≤VO2max) performance. Regarding dosage, differences are observed between acute and chronic protocols, suggesting that both strategies could have different mechanisms and responses. In that line, 0.15 g/kg body mass (≈10–11 g) of Arg supplementation ingested between 60–90 min before exercise (acute protocol) have shown to improve on anaerobic and aerobic performance outcomes. In addition, 1.5–2 g/day Arg supplementation for 4–7 weeks (chronic protocol) and longer doses (10–12 g/day for 8 weeks) may also be beneficial to enhance aerobic and anaerobic performance, respectively.
Regarding the well-known role of Arg related with NO production and increased blood flow and vasodilation, it has been suggested to favorably impact on training adaptations [64
]. However, there are few studies analyzing these effects. In fact, due to the limited and controversial results about Arg supplementation on performance variables, Arg supplementation is not included as an ergogenic aid with strong evidence [4
]. In this line, although 6 g/day seems to be the most used protocol, there are no clear timing and dose protocols related with sport performance. While, results obtained in this systematic review and meta-analysis indicated that an Arg supplementation with 6 g/day did not have significative effects on aerobic and anaerobic performance outcomes, higher doses (≈10 g) seem to be more effective on both anaerobic and aerobic performance outcomes. In this sense, these results depend on the timing of Arg ingestion. Given that nitrate and nitrite blood levels are increased 2.5–3 h after nitrate-rich supplement intake [65
], and Arg pharmacokinetic tests have shown a maximum plasma concentration between 30–90 min after ingestion [66
], acute ingestion protocols of Arg and other supplements related with NO metabolism have been analyzed in several studies. However, chronic supplementation protocols seem to be necessary to obtain major adaptations in other NO supplements as beetroot juice or nitrate [6
]. Present work suggests that when acute protocols are used, higher doses (≈10 g) are needed to show improvements on aerobic performance, while when chronic ingestion protocols are followed, lower doses (1.5–3 g/day) may be enough to show benefits. Therefore, it suggests that studying differences between acute and chronic supplementation protocols could be essential to understand what dosage is more adequate in order to obtain performance optimization.
The physiological and metabolic differences between aerobic and high-intensity sport disciplines are relevant [33
]. While anaerobic performance depends on the capacity to rapidly synthesize ATP mainly via phosphagen system or cytosolic glycolysis (overreaching the capacity to oxidize lactate), performance in aerobic sports like road cycling or endurance running is determined by the capacity to oxidize lactate and supply ATP demands via oxidative phosphorylation [28
]. In this line, nutrition and supplementation represents an important tool to optimize sport performance, especially in aerobic disciplines [5
]. Given that NO pathway is involved in some beneficial physiological aerobic mechanisms, such as improved economy, gross efficiency, ventilatory kinetics, and metabolic responses, trying to optimize them with supplementation could be an adequate strategy [68
]. Arg plays an essential role on endogenous NO synthesis and, thus, on some major physiological responses. Therefore, Arg supplementation could lead to improve training and competing performance due to the higher capacity to maintain work while prolonged exercising [9
]. Because of that, Arg supplementation could be recommended for the improvement of anaerobic and aerobic sport disciplines performances.
4.1. Effect on Anaerobic Performance (>VO2max)
Explosive (6 s), high-intensity (<1 min) and intensive (<5 min) efforts are determined by the anaerobic power and capacity, but there are some differences on metabolic and physiological demands between them [33
]. From metabolic point of view, the main fuel substrates to rapidly synthesize ATP in anaerobic exercise is phosphocreatine and lactate. Therefore, intramuscular creatine content, glycolytic power, and buffer capacity are determinant factors [35
]. On the other hand, physiologically, neuromuscular mechanisms, muscle contraction function, and structural factors such as muscle fiber composition (type II) are essential to ensure high intensity performance [36
]. In this line, NO-pathway related supplements have shown, among other effects, to have specific impact on type II fibers improving anaerobic performance [69
]. Moreover, Arg supplementation may have direct effects improving creatine resynthesis and reducing lactate accumulation [23
Regarding anaerobic performance, the most commonly used measurement tests are Wingate test, short time trials (1–2 km), isokinetic exercises, 1RM and, in team sports, the RSAT [70
]. In this sense, high doses (12 g/d) of chronic Arg supplementation (8 weeks) led to significant improvements in Wingate test parameters (peak power, time to peak power, and rate of fatigue), as well as enhanced 1RM performance in well resistance-trained subjects [20
]. However, although they did not observe significant effects on RSAT total sprint time [60
] and time to complete and average power output during a 1 km TT [11
], high doses of Arg (0.15 g/kg ≈ 10–11 g) ingested 60–90 min before exercise led to a clear improvement tendency in these anaerobic parameters. In the same line, 2–6 g/d of acute Arg supplementation (60–90 min before exercise) did not show significant performance improvements in Wingate Test, 20” all out sprint and 1 km TT, although tendency in this last trial was also positive [11
]. Likewise, 6 g/d for 7–14 days did not show any significant improvements in 1 min all out cycling sprint in recreationally active men [32
] and in anaerobic sprint capacity (6 consecutive sprints) in soccer players [41
]. Moreover, 3.7–6 g/d of Arg supplementation did not display significant beneficial effects regarding acute (30–80 min) or chronic (8 weeks) protocols on strength performance and sprinting capacity. Although it could be reasonable to expect greater anaerobic improvements with Arg supplementation due to enhanced blood flow and reduced lactate accumulation, among other mechanisms, positive results obtained in trials showed small significant effects but these had a tendency that was clear in some of analyzed trials. However, when global analysis was made, it showed that these improvements were represented with a higher and significant statistical power.
Therefore, these results suggest that regardless of fitness level and timing of supplementation, lower dosages are insufficient to improve high intensity anaerobic performance and that higher doses could be needed in both acute and chronic protocols. Regarding this, a supplementation of 0.15 g/kg body weight taken 60–90 min before exercise (acute protocol) or 10–12 g Arg supplementation for 8 weeks (chronic protocol) could led to improvements on anaerobic performance.
4.2. Effect on Aerobic Performance (≤VO2max)
Aerobic performance requires a great metabolic flexibility and a great cardiovascular capacity that supplies oxygen needs during prolonged exercise [28
]. Although genetic predisposition represents an important requirement to perform in aerobic disciplines, looking for training and nutrition protocols to improve performance in such demanding disciplines is essential [74
]. Beyond nutritional and hydration aids like carbohydrate-rich foods and electrolyte drinks, ergogenic aids such as caffeine or vasodilatory aids are often used [1
]. In this regard, 1.5–3 g/d of chronic Arg supplementation (4–7 weeks) improved physical working capacity at the fatigue threshold in amateur and healthy-untrained subjects [9
], showing lower fatigue related metabolites concentration such as ammonia and lactate, as well as a higher muscular power, endurance, and blood flow. On the other hand, a range of 2.85–5.7 g/d Arg supplementation for 4–8 weeks did not show statistical effects on time to exhaustion in running and cycling in endurance well-trained athletes [31
]. Even higher dosage of 12 g/d Arg supplementation during more prolonged duration (8 weeks), did not present any improvements on time to exhaustion while running an incremental test protocol in resistance trained athletes [20
]. These controversial results could be explained attending to subjects physical fitness, considering that Arg supplementation could have lower positive effects in well-trained subjects comparing with untrained ones as documented in other vasodilatory supplements [75
Regarding acute supplementation, 0.15 g/kg (≈10–11 g) of Arg ingested between 60–90 min before exercise, led to improve time to exhaustion and power output in cycling endurance test in both trained and untrained subjects [10
]. These effects could be explained due to reduced VO2
slow components, anaerobic reserves use, reduced metabolites of fatigue, and ATP cost of muscle force production [10
]. However, Arg supplementation within 5.7–6 g between 30–90 min previous to exercise, seemed not to show beneficial effects on total running time and average power output at ventilatory threshold in both endurance-trained and recreationally active subjects [11
]. These differences suggest that when it comes to improving the physiological and metabolic responses related with enhanced aerobic performance, acute Ag supplementation protocols with higher doses (≈10–11 g absolute or 0.15 g/kg), independently of fitness status, should be followed.
Chronic and acute supplementation protocols could have different physiological mechanisms of action and these may be closely and directly related with different Arg dosages. In this regard, results obtained in this systematic review and meta-analysis demonstrated that aerobic performance could be optimized when Arg supplementation is followed in both acute (higher dosage) and chronic (lower dosage) protocols. However, these effects have to be taken with caution due to the publication bias found in the included aerobic performance included studies. Finally, considering that different physiological responses are documented on trained and untrained athletes when other NO-related supplement (beetroot juice) is ingested [6
], it suggests that Arg supplementation dosages may need to be adjusted according to the subjects fitness level.
4.3. Strength, Limitations, and Future Lines of Research
The few total number of studies (n = 18) carried out in this systematic review and meta-analysis related to Arg supplementation could represent a limitation. On the other hand, the exhaustive methodology followed by authors in both systematic review and meta-analysis, concerning studies selection and outcomes analysis, interpretation, and quantification represents also a strong point. In fact, when assessing the review and meta-analysis quality by AMSTAR guidelines, the score obtained was ‘high quality’ [76
]. In the same line, this meta-analysis was carried out with a strong statistical power and confidence, which constitute an important point in favor. Regarding supplementation, all included studies use only Arg with no other supplements, compared with a placebo, what limits the possibility to obtain synergistic or antagonistic effects along with other compounds. Moreover, there are some other limitations regarding supplementation protocols, doses and timing. Due to controversial results around Arg supplementation and its effects in the current literature [5
], adequate dosages and supplementation duration are not yet well established. Because of that, there were found out some different and heterogeneous supplementation and timing protocols that limited the extraction of strong conclusions. In addition, performance outcomes and protocols used in studies were dissimilar and, beyond their classification into anaerobic and aerobic performance, differences were found within each section regarding mainly used metabolic pathways. In the same line, the heterogeneity and the publications bias obtained in aerobic performance studies, mean that practitioners should take these results with caution when applying the analyzed outcomes to reality.
Future lines of research should focus on homogenizing Arg supplementations protocols regarding dosages and timing, due to the current miscellaneous methods. To our understanding, and taking into account the results obtained, new Arg dosages should be proposed in favor of higher and chronic supplementation protocols, as well as studying the differences between elite and amateur subjects. Finally, a novel research line should be considered studying the Arg absorption capacity and involved mechanisms in athletes, as this could represent a crucial understanding of supplementation knowledge.