Caffeine (1, 3, 7-trimethylxanthine) is a major component of tea, coffee, guarana, and cacao [8]; it is a pharmacologic and psychoactive substance that is consumed frequently worldwide [9]. In sports, caffeine is a well known nutritional supplement, and has been actively studied since the 1970s [10].

Generally, caffeine helps to improve endurance, strength, and power by activating various biologic mechanisms [11]. Caffeine stimulates the sympathetic nervous system and increases the blood epinephrine concentration, promoting lipolysis of adipose and intramuscular triglycerides. These changes conserve stored glycogen and improve endurance performance. This is referred to as the glycogen sparing effect [8]. Potgieter et al. [11] reported that 6 mg/kg body weight (b.w.) of caffeine intake before exercise reduced swim time and time to completion by 3.7% and 1.3%, respectively, in triathletes. Felippe et al. [12] reported that 5 mg/kg b.w. of caffeine intake before exercise resulted in faster 4-km cycling time trials in cyclists.

Caffeine is able to help improve strength and power because it increases the intracellular Ca²⁺ concentration by stimulating the sarcoplasmic reticulum in muscle fibers, while also affecting excitation-contraction coupling by inhibiting uptake [13]. In fact, Diaz-Lara et al. [14] reported that Jiu-jitsu athletes who took caffeine before exercise showed improved hand grip force, countermovement jump height, one-repetition maximum, and mean power during the bench-press exercise, and Glaister et al. [15] reported that pre-exercise caffeine intake significantly improves peak anaerobic power output in well-trained men.

In terms of practical applications for athletic performance, factors such as dose, form, and timing can be considered (Table 1). A caffeine dose of 3-6 mg/kg b.w. is recommended [16,17]. Recently Chia et al. [18] reported that intake of 3-6 mg/kg b.w. of caffeine appears to be a safe ergogenic aid for ball game athletes. However, doses of caffeine over 9 mg/kg b.w. show no beneficial effects, and can actually decrease performance level [19]. It has been reported that caffeine produces a stronger effect when consumed in an anhydrous state (capsule/tablet, powder) compared to coffee [17], but if the dose is sufficient, pre-exercise caffeine intake is effective at improving endurance performance in either state [20,21]. Several studies have reported that caffeine-containing energy drinks help to improve athletes’ performance [22,23]. Although some athletes have tried consuming caffeine in energy drinks, it is important to be aware that one can does not provide the recommended caffeine intake for athletes. For example, the caffeine content in a single can (250 mL) of the well-known energy drink ‘Red Bull’ is only 80 mg. In actual research, athletes consumed 3 mg/kg b.w. of caffeine from energy drinks, not just a single can [22,23]. In order to satisfy the caffeine intake suggested in these studies, the athlete needs to drink several cans, and this requires caution as it could promote weight gain due to unnecessary calorie and carbohydrate intake.

In recent studies, caffeine chewing gum (300 mg of caffeine) was also reported to help improve muscle function, such as vertical jump height and knee extension peak torque [24], as well as exercise tolerance [25]. Meanwhile, with regard to the timing of intake, most studies suggested a protocol for caffeine intake 60 minutes before exercise for optimum absorption [17]. In addition, caffeine is quickly absorbed into the body after ingestion. The peak of the blood concentration is shown to be around 30 to 60 minutes after ingestion [19].

Beta-alanine is a non-essential and non-proteinogenic amino acid synthesized by the liver, and can also be obtained from foods such as pork, chicken, or red meat [26]. The main aim of beta-alanine supplementation is to increase intramuscular carnosine (b-alanyl-L-histidine). Carnosine is a cytoplasmic dipeptide formed when carnosine synthetase catalyzed the formation of peptide bond between beta-alanine and L-histidine; carnosine is found in many tissues in the body, but mostly is present in skeletal muscle [27]. In normal conditions, beta-alanine production is relatively low and serum beta-alanine is undetectable, but when serum beta-alanine increases to detectable levels, intramuscular carnosine levels are known to also increase [28]. Carnosine synthesis is controlled by the rate and amount of beta-alanine absorption into muscle fibers, insufficient serum carnosine synthetase activity, beta-alanine content in food, and hepatic synthesis of amino acids and their transport to skeletal muscles [29,30]. When beta-alanine is supplemented, 1) beta-alanine moves into the blood stream via the gastrointestinal tract, 2) beta-alanine and histidine are transported into the muscle sarcoplasm by amino acid transporters, 3) beta-alanine and histidine are bonded together in the muscle sarcoplasm by carnosine synthetase to produce carnosine [31].

Increased carnosine can improve performance during exercise by buffering hydrogen ion (H⁺) accumulation, limiting the effects of metabolic acidosis such as reduced phosphofructokinase activity, phosphocreatine resynthesis and glycolysis inhibition, and competitive inhibition of Ca²⁺ at troponin C and delayed Ca²⁺ reuptake to the sarcoplasmic reticulum [32]. Thus, it has also been suggested that the improvements in muscle strength, power, and endurance following beta-alanine supplementation are due to increased muscle fiber sensitivity to Ca²⁺ and the resulting enhancement in excitation-contraction coupling [26]. In practice, Baguet et al. [33] observed increased carnosine concentration in the soleus and gastrocnemius after elite rowers consumed 5 g/day of beta-alanine for 7 weeks, and reported that these changes were positively correlated with rowing performance. Saunders et al. [34] also had soccer players consume 3.2 g/day of beta-alanine for 12 weeks, and reported significant improvements in Yo-Yo Intermittent Recovery test (Yo-Yo IR) times.

In order to significantly increase intramuscular carnosine levels, 4-6 g/day of beta-alanine should be consumed for at least 2-4 weeks, and 4-6 g/day of beta-alanine intake is safe for healthy individuals [26] (Table 1). The optimal timing for beta-alanine intake is not yet well known. With reference to previous studies, beta-alanine has been taken after each meal (morning, lunch, and dinner) [35,36]. Adverse events have not commonly been reported after beta-alanine supplementation, but paresthesia can occur [37]. However, when beta-alanine intake is divided across several small doses (1.6 g/dose) rather than one large dose, paresthesia can be reduced [26].

Bicarbonate (HCO³⁻) is an extracellular anion that plays an important role in maintaining extracellular and intracellular pH [38]. In humans at rest, the blood is slightly alkaline (pH ~7.4) while the muscles are neutral (pH ~7.0), and this acid-base balance in the blood and muscle is important for normal cell metabolism [39]. However, during high-intensity exercise, acidification of cells occurs. This change interferes with muscle contraction through negative effects on myosin ATPase, Ca²⁺ ATPase, and Na⁺-K⁺ ATPase activity, weakens K⁺ efflux through pH-sensitive potassium channels, and impairs the activity of metabolic enzymes [40]. It also decreases energy production by lowering the proton gradient between the mitochondrial matrix and the cell cytoplasm [41].

When bicarbonate is supplemented, bicarbonate loading prevents the accumulation of H⁺, which is over-produced by the anaerobic energy metabolism used in short-duration, high-intensity exercise, which delays fatigue and allows energy production to continue, ultimately contributing to improved exercise performance [42]. In other words, bicarbonate has a similar mechanism and effect to those of beta-alanine, discussed above. Krustrup et al. [40] analyzed the serum pH after giving trained young men bicarbonate supplement, and observed higher serum pH than a control group, accompanied by significantly improved high-intensity intermittent exercise performance. Likewise, Wu et al. [43] reported that bicarbonate supplementation in tennis players resulted in higher blood pH, and helped to maintain service and forehand ground stroke consistency scores. Lopes-Silva et al. [44] reported that, when bicarbonate supplementation was provided to taekwondo athletes, glycolytic energy contribution increased in the first round of simulated taekwondo combat, resulting in improved exercise performance.

Bicarbonate supplementation needs to occur at the appropriate time before exercise, and a loading protocol should be utilized, as this can increase the serum bicarbonate concentration and the buffering capacity. Bicarbonate supplementation can use acute loading or serial loading methods (Table 1). For acute loading, Price & Singh [45] reported that bicarbonate needs to be taken 1-3 hours before exercise in order to significantly increase serum bicarbonate levels, and Lopes-Silva et al. [44] reported that 0.3 g/kg b.w. should typically be taken before exercise. Serial loading, which is the method of taking bicarbonate several times per day for several days, involves 3-4 doses/day of 0.3-0.5 g/kg b.w. for 3-5 days before competition or training [46].

The main reported side effect of bicarbonate is gastrointestinal distress including nausea, stomach pain, diarrhea and vomiting [42]. In order to reduce these side-effects, methods such as multiday ingestion, chronic progressive-dose, and split-dose protocols have been suggested [47,48]. For example, recently, Durkalec-Michalski et al. [47] provided progressive supplementation of bicarbonate to wrestlers for 10 days, starting from a dose of 25 mg/kg b.w. up to a dose of 100 mg/kg b.w., and reported that this was effective at eliminating side effects, including gastrointestinal symptoms.

Beetroot (Beta vulgaris) juice is a natural food and nutritional supplement that has recently been receiving much attention from athletes and sports scientists in the field of sports nutrition. Beetroot is known to be abundant in antioxidants and micronutrients such as potassium, betaine, sodium, magnesium, and vitamin C [49]. Most importantly, beetroot is a source of nitrate [50]. Nitrate is a naturally existing inorganic polyatomic anion [49]. When nitrate enters the mouth, it is actively extracted and concentrated from the saliva and reduced to nitrite by bacteria; in the stomach, nitrite is further reduced, and eventually converted into nitric oxide (NO) in the muscle [51]. Therefore, nitrate that is supplemented through beetroot juice can be considered a precursor of NO, and NO contributes to various physiological functions, including vasodilation, mitochondrial respiration, biogenesis, muscle glucose uptake, angiogenesis, and sarcoplasmic reticulum calcium handling [52]. Furthermore, NO can improve exercise tolerance while reducing muscle metabolic perturbation (reduction of phosphocreatine and accumulation of adenosine diphosphate and Pi) that occurs during high-intensity exercise [53].

Several studies have reported that beetroot juice produces significant changes in athletic performance [54-57]. Lansley et al. [55] reported that beetroot juice intake significantly improved power output and 16.1-km cycling time trials in competitive male cyclists, and Cuenca et al. [54] reported that beetroot juice intake significant improved 30-second cycling sprint performance in resistance-trained men. Thompson et al. [56] reported that nitrate supplementation by beetroot juice improved sprint split times and also Yo-Yo IR performance in team-sport players. Wylie et al. [57] reported that nitrate-rich beetroot juice intake helped recreational team-sport players to better maintain muscle glucose uptake and muscle excitability while improving Yo-Yo IR performance. In summary, beetroot juice intake has been found to be effective at improving power exercise, high-intensity intermittent exercise, and endurance performance.

In order for athletes to benefit from the effects of beetroot juice, several protocols need to be considered from previous study (Table 1). First, the appropriate time for beetroot juice intake is approximately 90 minutes before competition. The peak nitrate concentration occurs 2-3 hours after beetroot juice intake [50]. It is recommended to consume 500 mL/day (approximately 2 cups) of beetroot juice containing 5-8.4 mmol nitrate [55,58]. According to previous studies, a single dose of beetroot juice produces an acute effect [55,59], and consuming beetroot juice for 6-7 days also has a positive effect on exercise performance [60,61]. Finally, when consuming beetroot juice, oral antiseptic rinses should be avoided. This is because oral antiseptic rinses inhibit the conversion of nitrate from beetroot juice to nitrite in the mouth [50].

Creatine is an amino acid that is synthesized in the liver, kidney, and pancreas, can be found in high levels in the musculoskeletal system, and can also be obtained naturally by consuming fish or meat [62]. Creatine is currently the most effective ergogenic nutritional supplement that can be used to increase high-intensity exercise ability and lean mass [63]. In many studies, creatine supplementation has been reported to increase intramuscular phosphocreatine concentration and high-energy phosphate metabolism, helping to improve exercise performance, including muscle strength [64-66]. In addition, creatine increases calcium re-uptake into the sarcoplasmic reticulum, enabling rapid detachment of actin-myosin cross-bridges and augmenting the latent force-generating capacity [67]. Creatine also increases water retention by cells [68]. Increased cell size due to creatine-induced water retention is associated with up-regulation of signaling pathways that mediate protein synthesis, as well as mechanistic target of rapamycin (mTOR)-mediated signaling [69,70]. These changes help muscle hypertrophy by both stimulating muscle protein synthesis and reducing breakdown [71].

Creatine is typically taken in a loading phase (0.3 g/kg b.w. or 20-30 g/day, divided into 3-4 doses/day, for 3-5 days) and a maintain phase (3-5 g/day after the loading phase) [72] (Table 1). This method is called ‘creatine loading’, and is the optimal method to increase the intramuscular creatine capacity [73]. Creatine can be taken in small amounts (3-5 g/day) from the start, without a loading phase, but the initial enhancement of exercise performance is better if a loading phase is included [63]. In some studies, long-term intake for at least 3 months could cause downregulation of creatine transporter [74,75], and so it has been suggested that a washout phase is necessary. However, there is still a shortage of clear evidence for this hypothesis, and further research is required. Alongside creatine loading, it is recommended that creatine should be taken in a mixture with carbohydrates or carbohydrates and proteins [76], since this can promote insulin stimulation [77].

In terms of the timing of creatine intake, post-exercise is recommended more than pre-exercise [78,79]. Antonio & Ciccone [78] reported that when creatine was given before or after resistance exercise, post-exercise supplementation had greater positive effects on body composition and strength; Candow et al. [79] also reported that creatine supplementation after resistance exercise resulted in greater augmentation of muscle accretion and lean tissue mass. One possible reason why post-exercise creatine intake could be more effective is because creatine transport and accumulation is increased due to higher muscle blood glow and Na⁺-K⁺ pump function [80,81].

On the other hand, creatine is not a suitable nutritional supplement for athletes trying to improve aerobic performance [82]. This is because aerobic metabolism is affected by triglycerides rather than PCR [83].

Glycerol (1, 2, 3-propanetriol) is produced from glucose, protein, pyruvate, triacylglycerols and other glycerolipid metabolic pathways [84]. The function of glycerol is to maintain hydration status by increasing water compartments in the body. The aim of glycerol supplementation is to create a state of hyperhydration pre-exercise, delays fluid loss and dehydration during exercise, and improve the rate of rehydration and total fluid retention post-exercise [85]. Although maintaining hydration status pre-exercise is important to improve athletic performance, it is difficult to drink a large volume of water at once, and this can cause side effects such as frequent diuresis [86]. Glycerol can be supplemented to reduce these issues. Compared to water intake only, glycerol supplementation increased fluid retention by 400-1,000 mL [87]. For this reason glycerol is currently sold as a sports supplement for water or sports drink consumption [88].

Glycerol supplementation can be especially effective when exercising in high-temperature environments [89-91]. According to a study by van Rosendal et al. [90], when endurance-trained men received intravenous fluids and glycerol supplementation, plasma volume restoration increased. Wingo et al. [91], administered a water and glycerol mixture to male mountain bikers, and observed less dehydration compared in a high-temperature environment compared to water alone, lower pre-exercise urine volume, and lower post-exercise environmental symptoms questionnaire scores. Lyons et al. [89] also administered a mixture of water and glycerol, and reported increased sweat rate and decreased rectal temperature during exercise, resulting in reduced thermal burden in a high-temperature environment. This changes are ultimately linked to improvements in exercise performance, especially endurance performance. van Rosendal et al. [85] reported that glycerol supplementation increased endurance time to exhaustion up to 25%. During exercise after glycerol supplementation, fluid retention helps to reduce dehydration and cardiovascular strain while improving endurance performance. According to a study by Montner et al. [92], pre-exercise glycerol-enhanced hyperhydration reduced heart rate and increased endurance time.

Generally, glycerol is used before and during exercise [87,92,93] (Table 1). Pre-exercise, it is recommended to take 1.2 g/kg b.w. of glycerol together with 26 mL/kg b.w. of fluid in 60 minutes [86]. During exercise, the glycerol supplementation method is as follows: if glycerol was taken pre-exercise, 0.125 g/kg b.w. of glycerol should be taken with 5 mL/kg b.w. of fluid; if glycerol was not take pre-exercise, athletes should take 0.4 g/kg b.w. of glycerol with fluid during each of the first 4 hours of exercise [85]. Although rare, side-effects during glycerol supplementation can include nausea, gastrointestinal discomfort, dizziness, and headaches [94]. If an athlete develops these symptoms, they should stop taking glycerol or reduce the dose [85].