Nutritional Ergogenic Aids in Racquet Sports: A Systematic Review

nutrients Review Nutritional Ergogenic Aids in Racquet Sports: A Systematic Review Néstor Vicente-Salar 1,2,*,†, Guillermo Santos-Sánchez 3,†,‡and Enrique Roche 1,2,4 1Biochemistry and Cell Therapy Unit, Institute of Bioengineering, University Miguel Hernandez, 03201 Elche, Spain; [email protected] 2Department of Applied Biology-Nutrition, Alicante Institute for Health and Biomedical Research (ISABIAL-FISABIO Foundation), University Miguel Hernandez, 03201 Elche, Spain 3Departamento de Tecnología de la Alimentación y Nutrición, Universidad Católica de Murcia, 30107 Murcia, Spain; [email protected] 4CIBER Fisiopatología de la Obesidad y Nutrición (CIBEROBN), Instituto de Salud Carlos III (ISCIII), 28029 Madrid, Spain *Correspondence: [email protected] †To be considered as equal first author. ‡Present Address: Departamento de Bioquímica Médica y Biología Molecular e Inmunología, Universidad de Sevilla, 41009 Seville, Spain. Received: 27 August 2020; Accepted: 15 September 2020; Published: 17 September 2020   Abstract: A nutritional ergogenic aid (NEA) can help athletes optimize performance, but an evidence-based analysis is required in order to support training outcomes or competition performance in specific events. Racquet sports players are regularly exposed to a high-intensity workload throughout the tournament season. The activity during a match is characterized by variable durations (2–4 h) of repeated high-intensity bouts interspersed with standardized rest periods. Medline/PubMed, Scopus, and EBSCO were searched from their inception until February 2020 for randomized controlled trials (RCTs). Two independent reviewers extracted data, after which they assessed the risk of bias and the quality of trials. Out of 439 articles found, 21 met the predefined criteria: tennis (15 trials), badminton (three trials), paddle (one trial), and squash (two trials). Among all the studied NEAs, acute dosages of caffeine (3–6 mg/kg) 30–60 min before a match have been proven to improve specific skills and accuracy but may not contribute to improve perceived exertion. Currently, creatine, sodium bicarbonate, sodium citrate, beetroot juice, citrulline, and glycerol need more studies to strengthen the evidence regarding improved performance in racquet sports. Keywords: racquet sports; ergogenic aid; performance; sport supplement 1. Introduction Racquet sports are included in the family of ball sports and more specifically, among those using an implement. They are characterized by the use of a manual racquet to propel an implement (a ball, shuttlecock, etc.) between two or four players with the objective of placing it in a position with no return possibilities for the opponent. There are two different game formats: (a) passing the implement over a net in a divided field (tennis, badminton, paddle and table tennis) or (b) hitting the implement onto a wall in a shared field (squash and racquetball) [1]. Racquet sports are acyclic disciplines with very intense workload cycles, which are interrupted by small pauses that allow for an incomplete recovery. Therefore, metabolic demands in racquet sports alternate between both anaerobic and aerobic energy sources. Anaerobic energy comes from intramuscular ATP and phosphocreatine (PC), as well as from anaerobic glycolysis, the three of which are used during high intensity, short duration points, changes of direction, and hits. On the other hand, Nutrients 2020,12, 2842; doi:10.3390/nu12092842 www.mdpi.com/journal/nutrients Nutrients 2020,12, 2842 2 of 20 the aerobic system is involved during long points of moderate intensity, playing a primary role in delaying fatigue, and indirectly, favoring concentration, technical skills, and maintaining workload during a match [2–5]. As aresultof thisfact, theaverage heartrate (HR) duringamatchreaches up to 60–80% ofHR maximum (HRmax), increasing to 90% of HRmax in high-intensity situations [ 6 – 8 ]. Nonetheless, HRmax does not provide clear information regarding real energy demands or the metabolic pathways involved, since this parameter is affected by dehydration, heat stress, age, and playing techniques [ 9 ]. Measuring blood lactate concentration during a match could report more accurately the energetic pathways used by racquet sports players. Ranges vary from 1.0–4.0 mmol/L to 8.0–12.0 mmol/L during prolonged high-intensity matches [2,10–12], supporting the key role of glycolytic pathways during the match. An ergogenic aid is any training method, mechanical device, nutritional or pharmacological approach, or psychological technique that can improve exercise performance capacity and/or improve training adaptations [ 13 ]. Therefore, a nutritional ergogenic aid (NEA) is defined as those nutritional supplements taken orally containing a nutritional ingredient that intends to complement diet. The objective of these supplements is to improve sports performance without exerting harmful effects on the individual [14]. The consumption of NEAs has been increasing in recent years around the world, which has led to a great variety of research with the aim of estimating their intake and use. In fact, sales of dietary supplements grew 6.1% in 2017, achieving an income of 39.8 billion dollars in the US [ 15 ]. A meta-analysis published in 2015 concluded that elite athletes used many more dietary supplements than non-elite athletes, and the prevalence of use was similar in men and women [ 16 ]. The NEAs most frequently used by high-level tennis players tend to be creatine and caffeine [ 17 ] while among international rank squash players, sodium bicarbonate is also frequently consumed in addition to the two aforementioned NEAs [ 18 ]. Normally, NEA recommendations in high-level racquet sports players are directed by personal trainers, coaches, or sports dietitian–nutritionists. However, proper counseling based on current scientific evidence is required. In this line, several organizations such as the Australian Institute of Sport (AIS) or the World Anti-Doping Agency (WADA) propose classifications of sports supplements grouped into different categories according to effectiveness, legality, and safety. Nevertheless, there are not policies regarding the regulation of alleged benefits and safety claims [ 19 , 20 ]. Thus, athletes find themselves under the influence of companies’ advertising, which claims improved performance and recovery through the consumption of a wide range of products without scientific evidence regarding their effect, dosage, or instructions for use. The main aim of this systematic review was to evaluate the scientific evidence concerning NEAs in the improvement of performance of racquet sports athletes specifically through published RCTs. 2. Materials and Methods The conduct and reporting of the current systematic review conform to the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) [ 21 ]. Five racquet sports were analyzed regarding the effectiveness of certain nutritional ergogenic aids: tennis, badminton, squash, table tennis, and paddle. 2.1. Systematic Search Relevant articles were identified by title and abstract in the electronic databases Medline, Scopus, and EBSCO (since inception to 20 February 2020) using the search strategy in Table 1. The electronic search was supplemented by a manual review of reference lists from relevant publications and reviews to find additional publications on the subject. Nutrients 2020,12, 2842 3 of 20 Table 1. Combined terms used in the search for studies in the database. 1 Mesh terms were used in the search; 2 Term not included in the Mesh search; 3 nutritional ergogenic aid (NEAs) filed in the A group of the Australian Institute of Sport (AIS). Pubmed 1Scopus and EBSCO NEA Sport NEA 3Sport Dietary supplements AND Racquet Sports Dietary supplements AND Racquet Sports Caffeine Tennis Ergogenic aid Tennis Creatine Caffeine Badminton Beta-alanine Creatine Table tennis Sodium Bicarbonate Beta-alanine Squash and sport Ergogenic aid 2Sodium Bicarbonate Paddle Nitrate Beetroot juice Glycerol 2.2. Data Extraction Two reviewers (N.V.-S. and G.S.-S.) independently extracted the following data from each study using a predefined Microsoft Excel data extraction form including the number of participants within each group, participant characteristics, racquet sport discipline, and supplementation intervention characteristics, end points, measurement methods, and results in order to produce an overview table of all eligible studies. 2.3. Study Selection Studies were eligible for inclusion if they met each of the following criteria: (a) not using any doping substances established by the World Anti-Doping Agency (WADA), (b) using a randomized controlled trial (RCT) design that included one group taking supplementation and 1+groups receiving a placebo or not taking supplementation, (c) not including any ergogenic aids classified within group A by the Australian Sports Commission (AIS) because of their high evidence grade [ 22 ], (d) not presenting supplementation as a source of nutrients, such as bars, gels, or drinks rich in carbohydrates and electrolytes, and (e) not being gray literature (abstracts, conference proceedings, or editorials) or reviews. 2.4. Quality Assessment and Publication Bias Characteristics of the retrieved RCTs were evaluated using the ‘risk-of-bias’ assessment tool following the recommendations by the Cochrane Handbook for Systematic Reviews of Interventions [23,24] . This evaluation was carried out by two reviewers (N.V.S. and G.S.S.) working independently in order to present bias comprehensively. The following criteria were analyzed: randomized treatment order and carry-over effect (selection bias), blinding of participants and research staffto group allocation (performance bias), blinding of outcome assessor (detection bias), incomplete outcome data (attrition bias), selective reporting (reporting bias), and other bias (it was assessed if there was controlled diet, exercise use of supplements or drugs, and sport stratification when a mixture of disciplines was analyzed). Then, the retrieved RCTs were classified as being of “high”, “unclear”, or “low” risk of bias. Effect size was calculated using Cohen´s d test. 3. Results 3.1. Included Studies A total of 438 studies were screened by title and abstract, and 377 were assessed for eligibility criteria (full-text screening). From the retrieved articles, twenty-one met all the inclusion criteria and were included in the systematic review (Figure 1, Tables 2and 3). Thirteen RTCs were found Nutrients 2020,12, 2842 4 of 20 in the Medline database (eleven for tennis and two for badminton), seven were found in the Scopus database (three for tennis, one for badminton, two for squash, and one for paddle) where one article was not available despite requesting it from its main author; and none were retrieved from the EBSCO database (because all those found there were repeated). Additionally, one article that was not found through the initial search but was found in a review published in the Medline database was added for full-text analysis. The PRISMA flowchart was applied to illustrate the step-by-step exclusion of unrelated/duplicate retrieved records, leading to the final selection of twenty-one RCTs that met the predefined inclusion criteria (Figure 1). Nutrients 2020, 12, x FOR PEER REVIEW 4 of 25 3. Results 3.1. Included Studies A total of 438 studies were screened by title and abstract, and 377 were assessed for eligibility criteria (full-text screening). From the retrieved articles, twenty-one met all the inclusion criteria and were included in the systematic review (Figure 1, Tables 2 and 3). Thirteen RTCs were found in the Medline database (eleven for tennis and two for badminton), seven were found in the Scopus database (three for tennis, one for badminton, two for squash, and one for paddle) where one article was not available despite requesting it from its main author; and none were retrieved from the EBSCO database (because all those found there were repeated). Additionally, one article that was not found through the initial search but was found in a review published in the Medline database was added for full-text analysis. The PRISMA flowchart was applied to illustrate the step-by-step exclusion of unrelated/duplicate retrieved records, leading to the final selection of twenty-one RCTs that met the predefined inclusion criteria (Figure 1). Figure 1. Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) flow chart [21] of the study selection process. Records identified through database searching (n=438) Records excluded (n=62) Reasons: -Repeated (61) - Not available ( 1 ) Screening Included Eli g ibilit y Full-text articles assessed for eli g ibilit y ( n=377 ) Identification Studies included in qualitative synthesis (Systematized review) (n=21) Full-text articles excluded (n=356) Reasons: - Included doping substances (27) - Included supplements with CHO and/or isotonic drinks (15) - Did not include nutritional ergogenic aids (127) - Did not include racket sports (139) - Other motives (Review, Book Chapter or Congress abstract) (48) Additional records identified through other sources (n=1) Figure 1. Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) flow chart [21] of the study selection process. Nutrients 2020,12, 2842 5 of 20 Table 2. Included studies on nutritional ergogenic aids in tennis. BCAAs: Branched-chain amino acids; FFA: Blood free fatty acids; Glu: Blood glucose; Gly: Blood glycerol; HR: Heart rate; Lac: Blood lactate; LTPT: Leuven Tennis Performance Test; LTST: Loughborough Tennis Skill Test; NO; Nitric oxide; Pl: Placebo; RSA: repeated-sprint ability shuttle test; STPT: Skill Tennis Performance Test; Trp/BCAAs: Blood tryptophan/branched-chain amino acids ratio; u-EPI: Urine epinephrine; u-NE: Urine norepinephrine. ↑ : Significant increase compared to placebo/control group; ↓ : Significant decrease compared to placebo/control group; ↔: without changes compared to placebo/control group. Study NEA Dosage/Time Participants Age (yrs) Level Blinded/Double Blinded Duration Exercise Protocol Measurements Main Outcomes [25]Caffeine - 0.2 (women)–0.25 (men) mg/kg/0 min before match and every 15 min during a match 16 (8 men/ 8 women) 25.4 ±1.9/ 20.4 ±2.8 National ranking (Germany) DB 1 day 3 matches (2 of 75 min/match and 1 of 90 min/match with only rest between match 2 and 3 of 30 min) + Accuracy and sprint test - Lac ↔Lac - Glu ↔Glu - Gly ↔Gly - FFA ↔FFA - u-EPI ↑u-EPI - u-NE ↔u-NE - Sprints ↔Sprints - Accuracy hit ↔Accuracy hit - Perceived exertion ↔ Perceptual training intensity [26]Caffeine - 5 mg/kg/60 min before pre-test. - 0.75 mg/kg/Each 1 h after start pre-test and during protocol 13 men 20.4 ±0.9 National ranking (Belgium) DB 1 day LTPT +Sprint test +Court session (120 min) +LTPT - Sprints ↔Sprints - Serve quality ↔Serve quality - Backhand stroke quality ↑Backhand stroke - Volley errors and fatigue ↑Volley errors and fatigue - HR ↔HR - Perceived exertion ↔ Perceptual training intensity [27]Caffeine - 3 mg/kg/30 min before match 12 men 18.3 ±3.0 National ranking (Australia) B1 day 1 match of 160 min/match - Lac ↔Lac - Glu ↔Glu - CK ↔CK - Prolactin ↔Prolactin - Fluid loss ↔Fluid loss - Serve and stroke velocity ↑Serve velocity in 4th set - Serve kinematics ↔Serve kinematics - Perceptual skills ↔Perceptual skills - HR ↔HR - Perceived exertion ↔ Perceptual training intensity Nutrients 2020,12, 2842 6 of 20 Table 2. Cont. Study NEA Dosage/Time Participants Age (yrs) Level Blinded/Double Blinded Duration Exercise Protocol Measurements Main Outcomes [28]Caffeine - 6 mg/kg/60 min before test 16 (8 men/8 women) 20.7 ±1.7 National ranking (USA) University players (UK) DB 1 day Intermittent treadmill exercise (45 min) +Tennis skills test - Successful shots ↑Total shot successes - HR ↔HR - Perceived exertion ↔ Perceptual training intensity [29] Caffeine - 80 mg/30 min before test 12 (6 men/6 women) 18–22 DB 1 day 3 days of sleep restriction follow a day of accuracy serve test - Accuracy serve ↔Accuracy serve [30]Caffeine - 3 mg/kg/60 min before test 14 (10 men/4 women) 16.4 ±1.2 Elite-level Junior players (Spain) DB 1 day Tennis specific test +Simulated Match of best-of-3-sets system - Handgrip force ↑Handgrip force - Serve velocity ↔Serve velocity - Running speed ↑Only in high intensity - Number of sprints ↑Number of sprints - Distance ↔Distance - HR ↔HR - Sweat rate ↑Sweat rate [31]Caffeine - 6 mg/kg/60 min before test 10 (5 men/5 women) 19.9 ±1.8 National ranking (USA) DB 1 day - Tennis serve trial +Shuttle run sprint +Tennis serve trial - Accuracy serve ↑ Accuracy serve (depending of conditions of time and distance - Shuttle run time ↔Shuttle run time - Likert scale ↔Feelings [32]Creatine - 20 g/day (4 ×5g/day)/ During 5 days before test 8 men 20.4 ±0.9 National ranking (Belgium) DB 5 days LTPT +Shuttle run sprint - Quality of 1st and 2nd service ↔Service quality - Stroke quality ↔Stroke quality - Sprint power ↔Sprint power [33]Creatine - 0.3 g/kg in loading phase (6 days) - 0.03 g/day in maintenance phase (28 days) 36 men 22.5 ± 4.9–28.8 ± 4.8 ITN 3 DB 5 weeks - Service test +Ball machine ground stroke drill + Intermittent sprint test +Strength test - Lac ↔Lac - Serving velocity ↔Serving velocity - Stroke velocity ↔Stroke velocity - Sprinting velocity ↔Sprinting velocity - Strength ↔Strength - HR ↔HR - Perceived exertion ↔ Perceptual training intensity Nutrients 2020,12, 2842 7 of 20 Table 2. Cont. Study NEA Dosage/Time Participants Age (yrs) Level Blinded/Double Blinded Duration Exercise Protocol Measurements Main Outcomes [34]Sodium Bicarbonate - 0.3 g/kg/70 min before test - 0.1 g/kg/During test 9 men 21.8 ±2.4 College Tennis players (Taiwan) DB 1 day - LTST + Simulated match (50 min) +LTST - Lac ↑Lac - pH ↔pH - Serve consistency Keeps serve consistency while Pl ↓ - Stroke consistency Keeps stroke consistency while Pl ↓ - Serve Accuracy ↔Serve Accuracy - Stroke Accuracy ↔Stroke Accuracy - HR ↔HR - Perceived exertion ↔ Perceptual training intensity [35]Sodium Citrate - 0.5 g/kg/120 min before test 10 men 17.0 ±1.0 Junior National Ranking (Brazil) DB 1 day STPT +RSA + Simulated match (60 min) +STPT + RSA - Lac ↑Lac - pH ↑pH - Stroke consistency ↑Stroke consistency - Stroke accuracy ↔Stroke accuracy - Number strokes ↔Number strokes - Time of sprints ↔Time of sprints - Perceived exertion ↔ Perceptual training intensity [36]Beetroot juice - 70 mL (6.4 mmol of NO3−)/3 h before test 13 men 25.4 ±5.1 ATP and National ranking (Spain) DB 1 day - Serve velocity test +Counter movement jump + Isometric handgrip strength +Agility and sprint test - Serve velocity ↔Serve velocity - Jump height ↔Jump height - Handgrip force ↔Handgrip force - Agility ↔Agility - Sprint velocity ↔Sprint velocity - Perceived exertion ↔ Perceptual training intensity [37]Citrullinemalate -8g/60 min before test 17 women 51.0 ±9.0 Masters ranking in USTA (USA) DB 1 day - Isometric handgrip strength +Counter movement jump + Wingate cycling test - Handgrip force ↑Handgrip strength - Peak vertical power ↔Jump power - Anaerobic capacity ↔Anaerobic capacity - Relative peak power ↑Relative peak power - Explosive power ↑Explosive power - Sustained power ↔Sustained power Nutrients 2020,12, 2842 8 of 20 Table 2. Cont. Study NEA Dosage/Time Participants Age (yrs) Level Blinded/Double Blinded Duration Exercise Protocol Measurements Main Outcomes [38] BCAAs + Arginine + Citrulline - 0.17 g/kg BCAAs (Leu–Ile–Val = 10:7:3) +0.05 g/kg Arginine + 0.05 g/kg Citrulline/80 min before test 9 men 25.6 ±0.7 National Ranking (Taiwan) B1 day Perceptual-motor performance test (LTST modified) + Simulated match (120 min) + Perceptual-motor performance test (LTST modified) - Lac ↔Lac - Gly ↔Gly - Glu ↔Glu - FFA ↔FFA - NO ↑NO - Trp/BCAAs ↓Trp/BCAAs - HR ↓HR - Stroke Accuracy Prevents a high decrease in stroke accuracy compared with Pl - Stroke consistency Keeps stroke consistency while Pl ↓ - Stroke velocity Keeps stroke velocity while Pl ↓ - Perceived exertion ↓Perceptual training intensity [39]Glycerol -1g/kg/150 min before test - 0.5 g/kg/15 min after test 11 men 27.0 ±2.0 Ranking 4–5 in USTA (USA) DB 1 day Tennis specific test +Simulated match (75 min) +Tennis specific test - Change in body ↑Body weight vs. Pl Weight - Plasma osmolality ↑Plasma osmolality vs. Pl (only preand post-exercise) - Change in plasma ↑Plasma volume vs. Pl (only preand post-exercise) volume - Electrolytes ↔Electrolytes - Urine volume ↓Urine volume - Sprint velocity ↔Sprint velocity - Agility ↔Agility - Stroke accuracy ↔Stroke accuracy - Serve accuracy ↔Serve accuracy Nutrients 2020,12, 2842 9 of 20 Table 3. Included studies on nutritional ergogenic aids in badminton, squash, and paddle. Glu: Blood glucose; HR: Heart rate; Lac: Blood lactate. ↑ : Significant increase compared to placebo/control group; ↓: Significant decrease compared to placebo/control group; ↔: without changes compared to placebo/control group. Badminton Study NEA Dosage/Time Participants Age (yrs) Level Blinded/Double Blinded Duration Exercise Protocol Measurements Main Outcomes [40]Caffeine - 3 mg/kg/60 min before test 16 men 25.4 ±7.3 National ranking (Spain) DB 1 day Handgrip force +Jump tests + Agility Test + Simulated match (45 min) - Handgrip maximal force ↔Handgrip force - Smash jump - Squat jump ↔Smash jump - Countermovement ↑Squat jump height/power Jump (CJ) ↑CJ height/power - Agility ↔Agility - Number of impacts ↑Number of impacts - HR ↔HR - Perceived exertion ↔ Perceptual training intensity [41]Caffeine - 4 mg/kg/60 min before exercise - 4 mg/kg /during 2nd Badminton specific test 12 men 28 ±9 National ranking (United Kingdom) DB 1 day Badminton specific test + Fatigue protocol (33 min) + Badminton specific test - Lac ↔Lac - Glu ↔Glu - Errors in anticipation ↓Errors in anticipation - Accuracy serve ↔Accuracy serve - Reaction time ↓Reaction time - Time sprints ↓Time sprints - HR ↔HR - Perceived exertion ↓Perceptual training intensity [42]Sodium bicarbonate - 300 mg/kg/90 min before test 30 men 21 Student players (Indonesia) ?1 day Treadmill testing to exhaustion - pH ↑pH - Lac ↑Lac - Time to exhaustion ↑Time to Exhaustion [42]Sodium citrate - 300 mg/kg/90 min before test 30 men 21 Student players (Indonesia) ?1 day Treadmill testing to exhaustion - pH ↓pH - Lac ↑Lac - Time to exhaustion ↑Time to Exhaustion Nutrients 2020,12, 2842 16 of 20 power or jump power [ 37 ]. Due to the lack of a washing time between conditions and control of the consumption of other stimulant substances, the risk of bias is moderate. Further studies are necessary to analyze the role of citrulline supplementation in the performance of younger racquet sports players. Yang et al. (2017) [ 38 ] showed improvements regarding the prevention of a decrease in stroke accuracy and keeping stroke consistency and velocity (as opposed to a worsening in the placebo group) using 0.05/kg citrulline +0.05 g/kg arginine +0.17 g/kg branched-chain amino acids (BCAAs). The study presented a low risk of bias. Additionally, perceived exertion after the test decreased significantly. These results appear to be due to a lower plasma tryptophan/BCAAs ratio than placebo, since theoretically, BCAAs compete for the same tryptophan transporter across the blood–brain barrier, avoiding serotonin formation and, consequently, central fatigue instauration [ 63 ]. It is common to use a mixture of several NEAs in one product with the objective to obtain a synergic effect, but further studies are necessary in order to verify the true effects of citrulline or arginine by themselves, without the presence of the BCAAs being able to distort them. 4.5. Effects of Glycerol Supplementation in Racquet Sports Finally, glycerol is a naturally occurring metabolite that acts as a plasma expander and could help athletes prevent dehydration and improve thermoregulatory and cardiovascular changes [ 14 ]. Until 2018, the World Anti-Doping Agency (WADA) considered glycerol a banned substance, since it was hypothesized that it may alter athlete biological passport [ 64 ]. In any case, the results of its supplementation are mixed both in endurance and anaerobic disciplines [ 14 ]. In intermittent sports such as tennis, 1.0 g/kg glycerol before followed by 0.5 g/kg after 75 min of simulated match, in environmental conditions in the range of 29–38 ◦ C and 50–90% relative humidity (emulating conditions of important tennis tournaments such as The Australian Open Grand Slam or Miami ATP Masters 1000), was not capable of improving accuracy in serves or strokes, sprint velocity, or agility, in spite of its effect increasing preand post-exercise plasma volume and osmolality [ 39 ]. This study has a moderate risk of bias, since its randomized method, carry-over effect, blinding method and control of diet, and other supplementation and drug consumption were poorly controlled. More research is needed to determine glycerol’s supposed potential efficacy in racquet sports during more time-prolonged matches or during several matches on the same day or on consecutive days in hot conditions. 5. Conclusions Caffeine is the NEA showing clearer evidence of benefits for racquet sport players. Acute dosages (3–6 mg/kg) 30–60 min before a match may improve specific skills and accuracy but may not contribute to improve perceived exertion. Even though some evidence concludes that other NEAs, such as creatine, sodium bicarbonate, sodium citrate, beetroot juice, citrulline and glycerol, could play an interesting role in improving performance, more studies are needed to strengthen the evidence (Table 5). Nutrients 2020,12, 2842 17 of 20 Table 5. NEA recommendations from current evidence. Green: High level of recommendation due to the high number and quality of studies and the effects produced; Orange: Low level of recommendation due to the low number and/or quality of studies and the effects produced; Red: Not recommended due to the low number and quality of studies and contradictory or low effects. NEA Effects Posology Caffeine - Improves specific racquet sports skills - Improves sprints and jumps - Improves mental performance and maybe accuracy 3–6 mg/kg 30–60 min before competition Creatine - May improve sprints 0.3 g/kg for 5 days Sodium Bicarbonate - May improve specific racquet sports skills - May hold up time to exhaustion 0.3 g /kg 70–90 min before competition Sodium Citrate - May improve specific racquet sports skills - May hold up time to exhaustion 0.3–0.5 g/kg 90–120 min before competition Beetroot juice - No effects 6.4 mmol 3 h before competition Citrulline-malate - May improve handgrip strength - May improve peak power 8 g 60 min before competition Glycerol - No effects 1 g/kg 150 min before competition and 0.5 g/kg 15 min after it. Author Contributions: Conceptualization, N.V.-S. and G.S.-S.; methodology, N.V.-S. and G.S.-S.; protocol drafting, N.V.-S. and G.S.-S.; risk of bias, N.V.-S. and G.S.-S.; quality assessment, N.V.-S. and G.S.-S.; data extraction, N.V.-S. and G.S.-S.; literature search, N.V.-S. and G.S.-S.; search flowchart, N.V.-S.; writing—original draft preparation, N.V.-S. and E.R.; writing—review and editing, N.V.-S., G.S.-S. and E.R. All authors have read and agreed to the published version of the manuscript. Funding: This study was supported by the official funding agency for biomedical research of the Spanish government, Institute of Health Carlos III (ISCIII) through CIBEROBN CB12/03/30038), which is co-funded by the European Regional Development Fund. Acknowledgments: CIBEROBN is an initiative of Instituto de Salud Carlos III, Spain. Conflicts of Interest: The authors declare no conflict of interest. References 1. Lees, A. Science and the major racket sports: A review. J. 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