Nutrition and Supplementation in Soccer

General nutritional recommendations for peak performance promote strategies to achieve optimal muscle glycogen concentrations through means of high CHO availability. In sports where physical activity duration ranges between 1 h and 2 h, it is believed that athletes should consume 5 to 10 g/kg/day [12]. However, recent research has provided new insights into the interactions of exercise with low CHO availability, whereby the acute and chronic adaptive responses to training or recovery are enhanced in an environment of low exogenous and endogenous CHO stores, since consistently high levels of muscle glycogen seem to attenuate training adaptations [75]. Therefore, researchers are now exploring if a low CHO availability at less relevant moments (such as some training sessions) can potentiate the exercise effects, while maintaining high CHO availability at critical moments (match-play), thereby improving soccer athletes’ performance.

Anderson and colleagues [6] quantified the daily training and accumulative weekly load (reflective of both training and match play) in professional soccer players during a one, two and three match per week schedule and found evidences of training periodization. Specifically, results showed that training load was progressively reduced in the three days prior to match day (one match per week); that daily training load and periodization was similar in a one and two match per week schedule (although total accumulative distance was higher in a two-match week); and that daily training total distance was lower in a three-match week (although accumulative weekly distance was highest and more time was spent in high speed zones). As such, these authors suggested that CHO intake should also be periodized according to training periodization, suggesting high CHO availability on the day before match, on the day of the match and on the day after match on both one match and two match per week schedule, and a reduced CHO availability on the other days. Given the extreme frequency of match play on a three match per week schedule, these researchers do not advise adopting a low CHO availability in these conditions.

CHO periodization has already been implemented to assess its effects on athletes’ performance, although in other sports than soccer. One practical example cycle consists of (1) late-afternoon scheduling of a high-intensity training session undertaken with high glycogen stores; (2) withholding the ingestion of CHO after the session to maintain glycogen depletion during the overnight recovery period; and (3) at low–moderate intensity steady-state exercise session the following morning completed after an overnight fast [76], or in other words, “train high, sleep low, train low”, or more simply, “sleep low”. Marquet and colleagues [77] performed a three-week training-diet intervention comparing the effects of the “sleep low” method to a standard high CHO availability every day (6 g CHO/kg) and found that the short-term periodization of dietary CHO availability around selected training sessions promoted significant improvements in submaximal cycling economy, as well as supramaximal cycling capacity and 10-km running time in trained endurance athletes. The authors proceeded to see if a shorter exposure to this CHO periodization strategy would be successful in inducing metabolic adaptations and performance improvement and concluded that implementing the “sleep-low” strategy for one week improved performance by the same magnitude previously seen in a three-week intervention, without any significant changes in selected markers of metabolism [76]. The effects of CHO periodization in soccer are still unknown but one can expect future developments on the subject, to detect if and which type of periodization can be most successful for soccer athletes’ performance.

Given the relevance of muscle carbohydrate content for exercise performance, every feeding opportunity may matter for achieving the highest values. A pre-exercise/match diet rich in CHO augments muscle glycogen content as previously observed on a 90 min (four-a-side) soccer play study [69]. Here, six male football players competed on two occasions following an exercise and diet with either high or low CHO regimen (65% vs. 30% of total daily energy intake), designed to manipulate muscle glycogen concentrations. Researchers found that pre-match muscle glycogen concentrations following the high CHO diet were significantly higher than following the low CHO diet. Most importantly, players performed significantly more high-intensity exercise in the match played following the high CHO diet, without any observed differences on the evaluated technical variables.

Regarding the pre-event meal, a study [78] evaluated the effects of ingesting a CHO meal (2.5 g CHO/kg) after an overnight fast. Vastus lateralis muscle biopsies were obtained before and 3 h after the meal and revealed that the CHO meal resulted in an approximate 10% increase in muscle glycogen concentration [78]. Furthermore, it was shown that the intake of a substantial amount of CHO (~200–300g) in the 2–4 h before exercise could prolong endurance performance [79,80,81]. As such, the ingestion of 1–4 g/kg of CHO in the last 4h before match may be recommended, depending on players’ tolerance and individual preferences. The last meal should ideally take place 3–4 h before match and include easy-to-digest foods. The meals 4 h before the start of competition (main meals, e.g., lunch) can contain more carbohydrates, with smaller meals (intermediate meals, e.g., afternoon snack) being targeted for occasions when less time is available prior to the start of competition. Within 60 min before the match, usually until warm-up, light snacks containing high CHO (25–30 g) may further increase the availability of CHO before match, thus sparing liver glycogen. However, there are still some concerns about reactive hypoglycemia (due to increases in insulin production) when CHO are consumed within the last hour prior to exercise [82]. Although it is reasonable to consider that higher pre-exercise insulin concentrations may affect exercise performance by glycemic disposal at the start of the exercise, studies have shown that this does not seem to compromise exercise performance [83]. To this respect, we believe that the precise timing and quantity of feeding within 1 h prior to match should be based on individual preference. It is also important to ensure that the pre-event meal is composed of familiar foods to avoid gastrointestinal issues that many athletes experience before big events. Nervousness and unfamiliar food can sometimes lead to stomach cramps, nausea and diarrhea. Keeping meal choices simple and familiar may be the most important concept for successful pre-event fuelling.

Another strategy used in endurance sports to maximize muscle glucose content before an exercise event is glycogen supercompensation. This can be interesting in isolated events, occurring from time to time, but its effects are less known in intermittent high-intensity sports like soccer, where maximum performance is needed frequently, often more than once a week. McInerney and colleagues [84] tested six trained athletes on three exercise trials, with each exercise bout separated by 48 h. Twenty-four hours before day one, subjects consumed a moderate (6 g/kg) CHO diet, followed by five days of a high (12 g/kg) CHO diet. After each exercise session, subjects were fed a high-CHO meal and monitored during the subsequent 3 h of recovery. Before and immediately upon completion of each of these three exercise bouts, a muscle biopsy was taken. As a result of this protocol, researchers found that well-trained men cannot repeatedly supercompensate muscle glycogen content after a glycogen-depleting exercise and two days of a high-CHO diet, suggesting that the mechanisms responsible for glycogen accumulation are attenuated as a consequence of successive days of glycogen-depleting exercise. Furthermore, exercise performance was similar on days three and five despite the lack of glycogen accumulation on day five. Therefore, the intake of extremely high doses of CHO (i.e., 12 g/kg) may not be a useful nutritional strategy for soccer players. The intake of 6–10 g CHO/kg is generally recommended during the 24 h period before a match [6].

The benefits of ingesting CHO during endurance exercise are well established, and general recommendations for sports where exercise duration ranges from 1 to 2.5 h advocate an ingestion of 30–60 g/h [12]. It is known that athletes’ performance tends to drop over the course of a soccer match [85] and thus, an adequate CHO supply during exercise may attenuate the progression of players’ fatigue. In fact, CHO intake at a rate of 30–60 g/h has been associated with a consistent beneficial effect on performance in soccer [73]. Nicholas and colleagues [86] undertook a study in which they provided soccer players with either a 6.5% CHO solution or a taste- and color-matched placebo solution between the 15-min activity blocks of the Loughborough Intermittent Shuttle Running Test (LIST—designed to simulate the activity pattern characteristic of soccer and other stop-start sports). After performing 75 min of the LIST, players performed alternated 20-m sprints with jogging recoveries to fatigue. Researchers found that intake of the CHO solution allowed athletes a 33% greater running time, than when they ingested the placebo [87]. More recently, others [86] found that when adjusting for body mass, ingesting a 7% CHO intake (49 g) was associated with improved time to fatigue during a simulated soccer match.

Overall performance in soccer is not only dependent on physical power. Motor skills and cognitive performance also play a crucial role on the performance of soccer players and there is a tendency for players’ skills and cognitive performance to decline during the latter stages of a match. CHO has been shown to attenuate or even eliminate this detrimental effect over the course of a match [88,89,90]. As Russell and Kingsley [70] recently reported in a systematic review, six out of eight studies found that the ingestion of 30–60 g CHO/h (via a 6–8% solution of glucose, sucrose, or maltodextrin) was associated with an enhancement of at least one aspect of soccer skill performance.

Adding a small amount of protein to a CHO supplement may further enhance performance as found by Alghannam [91]. In this case, a beverage containing 4.8% CHO plus 2.1% protein ingested prior to and during a 75-min football-specific intermittent exercise, allowed for a post-exercise longer run time to fatigue and lower ratings of perceived exertion than an isolated 6.9% CHO supplement.

Finally, a simple carbohydrate mouth rinse has also shown to be able to produce similar effects, with improvements typically between 2% and 3% during exercise lasting approximately 1 h [92,93]. The mechanisms associated with this phenomenon are yet to be perfectly understood but it is already known that they are specific to carbohydrates and independent of taste [94]. These results suggest that it may not be necessary to ingest large amounts of carbohydrate when an athlete is going to exercise/play for approximately 30 min to 1 h. It is also noteworthy to mention that most studies have been conducted on a cycle ergometer and that some did not produce beneficial results [95]. Nevertheless, an actual CHO ingestion is recommended in most situations, except when an eventual gastrointestinal discomfort is suspected to outweigh the potential performance benefits.

Muscle glycogen depletion induces muscle glycogen resynthesis, even in the absence of post-exercise CHO intake [96]. Nevertheless, ingestion of post-exercise CHO further stimulates muscle and liver glycogen synthesis, up to a 10-fold increased rate, in comparison to a post exercise non-fed state [74]. Considering a typical mean glycogen storage rate of 5–6 mmol/kg wet weight/h, roughly 24 h of recovery may be needed to normalize muscle glycogen levels. During periods of high frequency of competitive matches and/or training, athletes may see their optimal muscle glycogen recovery compromised, which can ultimately lead to lower overall individual performances. Most importantly, and since higher muscle glycogen synthesis rates occur immediately after the exercise bout, special attention must be placed in this early-phase of recovery. Since the interval between training sessions can be shorter than 8 h, CHO intake, in the form of solids or liquids, should start as soon as possible after the first session to maximize the effective recovery time [97]. In these scenarios, recommendations estimate an ingestion of ≥1 g/kg/h [74], during the first few hours after exercise. The type of CHO that is consumed seems to be of lesser relevance, provided it produces a large glycemic and insulinemic response [74], although it should be considered that lower glycemic index CHO generally have higher fiber content and result in higher levels of satiation, so more food has to actively be consumed to match the amount of CHO generally found in higher glycemic index foods. Nevertheless, research has shown that individuals may respond to the glycemic index of foods in many different ways. In one interesting paper, researchers continuously monitored week-long glucose levels in an 800-person cohort, measured responses to 46,898 meals, and found high variability in the response to identical meals [98]. In response to the ingestion of a given food, some participants could see a significant elevation on their glycemic levels, while others saw only a slight increase or even none at all, suggesting that universal dietary recommendations may have limited utility. To optimize recovery, athletes’ reaction to different foods or CHO should be tested in order to select the ones that guarantee a rapid and large glycemic and insulinemic response. Nonetheless, athletes should also select foods (or drinks) based on their personal preference and experience.

When CHO intake is adequate (e.g., >1 g/kg/h), the co-ingestion of protein seems to provide no further effect on glycogen synthesis [99]. Nevertheless, in specific circumstances such as low daily energy intake or when the amount of carbohydrates is insufficient (especially in the first 4 h after exercise), the addition of ≥0.3 g/kg/h of protein to a carbohydrate supplement may accelerate muscle glycogen resynthesis via a synergistic increase in insulin secretion and muscle glucose uptake [74]. In soccer, the combination of glycogen and protein in a post-exercise meal may be especially relevant during pre-season, where weight management is generally regarded as a critical goal. Finally, we recall that a reduced total energy intake (below requirements) may affect CHO kinetics and compromise an adequate muscle glycogen repletion [100].

For the purpose of maximal enhancement of MPS, players should ingest 0.25–0.4 g/kg of intact proteins in the meal before exercise [71]. As stated above, special attention should be given to CHO before matches, and protein may not be the primary nutritional concern. Nevertheless, protein ingestion before exercise allows post-exercise MPS rates to be elevated more rapidly during the early stages of recovery due to a greater amino acid availability [113]. Since a soccer match leads to extended muscle damage [114], pre-match protein ingestion should also be a priority, especially on a multiple match per week schedule.

Although there are some studies [115,116] showing positive effects of ingesting protein during exercise, its benefits are not yet consensual [113,117]. Given the duration of a typical exercise training and match (<3 h), football might not be the modality that most benefits from protein ingestion during exercise [113].

In order to enhance protein synthesis and to allow repair, remodeling and adaptation, players should ingest 0.25–0.4 g/kg in the early recovery period [71,105,118]. Moreover, there is evidence that supports ingestion of 30–40 g casein before sleep for maximal stimulation of MPS at night after a strength exercise session [111,112].

Fat is generally recommended to remain at low levels in the pre-exercise meals, to avoid interfering with gastric emptying or causing gastrointestinal problems [12].

The use of medium chain triglycerides (MCTs) has been hypothesized to benefit sport performance due to their enhanced digestibility and metabolization when compared to other fats [125]; however, it is unlikely that the provision of MCTs during exercise can promote a sufficiently large increase in rates of fat oxidation to support glycogen sparing and performance benefits, and there is a substantial risk that gastrointestinal disturbances may actually impair performance [16]. Therefore, the consumption of fats during training sessions and matches is not advised for soccer players.

The minimal available evidence suggests that the ingestion of low to moderate amounts of fat do not impair glycogen [126,127] nor muscle protein resynthesis [128], if adequate amounts of carbohydrates and protein are consumed. Excessive fat may, however, displace carbohydrate foods within the athlete’s energy requirements and gastric comfort, thereby indirectly interfering with glycogen storage.

Before exercise, the goal is to start the physical activity euhydrated and with normal plasma electrolyte levels [149]. The prehydration program will help ensure that any previously incurred fluid electrolyte deficit is corrected prior to initiating the exercise task. When hydrating prior to exercise the individual should slowly drink beverages (for example, ~5–7 mL·kg⁻¹ per body weight) at least 4 h before the exercise task. If the individual does not produce urine, or the urine is dark or highly concentrated, he should slowly drink more (for example, another ~3–5 mL·kg⁻¹) about 2 h before the event. By hydrating several hours prior to exercise there is sufficient time for urine output to return towards normal before starting the event [174]. Another small bolus in the end of warm-up is recommended to replenish sweat losses during this period [174].

Consuming beverages with sodium (20–50 mEq·L⁻¹) and/or small amounts of salted snacks or sodium containing foods at meals will help to stimulate thirst and retain the consumed fluids [175,176].

Enhancing palatability of the ingested fluid is one way to help promote fluid consumption, before, during, or after exercise. The preferred water temperature is often between 15 and 21 °C, but this factor, as well as flavor preference varies greatly between individuals and cultures [72].

The amount and rate of fluid replacement depends upon the individual sweating rate, exercise duration, and opportunities to drink. Individuals should periodically drink (as opportunities allow) during exercise whenever possible, if it is expected they will become excessively dehydrated [72]. It is recommended that individuals should monitor body weight changes during training/competition to estimate their sweat lost during a particular exercise task with respect to the weather conditions. This allows customized fluid replacement programs to be developed for each person’s particular needs [72].

Both soccer rules and gastric tolerance do not allow suitable hydration for soccer players [177]. During the match, it is difficult to ingest fluids because there are no breaks for this specific purpose, hence the player’s fluid intake has to be a priority during half-time [136]. Its ingestion should be enough to replace loss from sweat, with volume and contents known by players and based in individual requirements and preferences [178]. These beverages should have a carbohydrate concentration of 6% to 8%, and should be provided at a temperature of 15 to 20 °C at every 15–20 min, in a volume of 150 to 300 mL, to provide substratum, but not to limit rate of gastric emptying [72,136].

During training sessions, the need to include carbohydrates and electrolytes will depend on the specific exercise task (e.g., intensity and duration) and weather conditions. The sodium and potassium help to replace sweat electrolyte losses, while sodium also helps to stimulate thirst, and carbohydrate provides energy. These components also can be consumed by non-fluid sources such as gels, energy bars, gums and other foods.

After exercise, the goal is to fully replace any fluid and electrolyte deficit [72]. If players have accrued a body mass deficit, they should aim to completely replace fluid and electrolyte losses prior to the start of the next training session or match. If dehydration is severe (>5% of body mass) or rapid rehydration is needed (e.g., <24 h before next training session or match) the recommendation is to drink ~1.5 L of fluid for each 1 kg of body mass deficit [179]. The additional volume is needed to compensate for the increased urine production accompanying the rapid consumption of large volumes of fluid [176]. Therefore, when possible, fluids should be consumed over time (and with sufficient electrolytes) rather than being ingested in large boluses to maximize fluid retention [180,181]. If recovery time and opportunities permit, consumption of normal meals and snacks with a sufficient volume of plain water and sodium will restore euhydration [149,182].

Dehydration is not the only problem caused by inadequate intake of fluids, because hyperhydration can also occur. Hyperhydration can be achieved by overdrinking combined with an agent that “binds” water within the body, such as glycerol [183,184]. Simple overdrinking will usually stimulate urine production [149,185] and body water will rapidly return to euhydration within hours, [182,186], however, this compensatory mechanism (urine production) is less effective during exercise and there is a risk of dilutional hyponatremia [185].

Iron deficiency, with or without anemia, can impair muscle function and limit work capacity, leading to compromised training adaptation and athletic performance [12]. This is especially important for soccer, due to its heavy reliance in aerobic metabolism [189].

Reinke and colleagues [190] observed a significant proportion (approximately 30%) of absolute (serum ferritin <30 μg/L) and functional (normal ferritin with transferrin saturation <20%) iron deficiency in professional soccer players (n = 10) at the end of season. Although holiday period led to increased ferritin levels in these players, this was not sufficient to fully restore an optimal iron status [190]. Other observations [191,192] also support a tendency for iron status disturbances during a soccer season. It was speculated that accumulated fatigue and inadequate recovery time during a competitive in-season period may predispose football athletes to iron status disturbances [190]. Others [193], albeit detecting a significant (approximately 31%) prevalence of iron deficiency/depletion (serum ferritin <30 μg/L) in professional soccer athletes, did not observe significant changes in the iron status during a competitive season.

Soccer players, especially those at higher risk for deficiency, should aim for an iron intake equal or greater than their RDA (i.e., >8 mg/day for men) [12]. When iron deficiency anemia (IDA) is detected, clinical follow must ensue. For those who have iron deficiency (e.g., low ferritin) without anemia, eating strategies that promote an increased intake of food sources of well-absorbed iron (e.g., heme iron from animal sources, non-heme iron from vegetables plus vitamin C foods) should be promoted as the first line intervention [12]. Oral supplementation with iron tablets (i.e., ferrous sulfate providing 80 mg of elemental iron) may also be useful to correct low ferritin levels [194]. Of note, the intake of iron supplements in the period immediately after strenuous exercise is contraindicated since there is the potential for elevated hepcidin levels to interfere with iron absorption [195].

It is long recognized that vitamin D regulates calcium and phosphorus absorption and metabolism, and plays a key role in maintaining bone health. Emerging research, however, also highlights the important role of vitamin D for non-skeletal functions including skeletal muscle growth, immune function, inflammatory modulation and athletic performance [196].

Soccer players with low levels of 25(OH)D (<30 ng/mL or <75 nmol/L) may be more likely to have musculoskeletal injuries and stress fractures [197]. While it has also been suggested that lower levels of vitamin D may lead to reduced muscle strength [198,199], two studies performed in professional soccer have failed to find a consistent association between vitamin D status and muscle strength variables [200,201]. Several studies assessed vitamin D status in soccer players living in different regions and at different time of the year [200,202,203,204,205]. Vitamin D insufficiency (25(OH)D < 30 ng/mL or 75 nmol/L) was relatively common in players living in countries at higher latitudes (> 35th parallel), especially in winter months [202,203,204]. However, insufficient vitamin D levels were also detected in a large proportion (84%) of players living in Qatar (latitude < 35th) during summer, highlighting the relevance of other factors such as exposure practices, clothing, sunscreen use, skin pigmentation and timing of training sessions (e.g., after sunset) [200].

Although the human requirement can be met entirely through synthesis in the skin upon exposure to sunlight [206], dietary vitamin D may also contribute to promote an adequate status. Vitamin D is found in the diet from foods such as fatty fish and egg yolks but also fortified foods (e.g., milk, yogurt, ready-to-eat cereal), and is well absorbed in association with dietary lipids [196]. The RDA for vitamin D varies according to region, with recommendations ranging from 200 IU in Australia and New Zealand to 600 IU in USA and Canada [207]. Where sensible sun exposure is not possible or desired, athletes with insufficient status require supplementation with at least 1500–2000 IU/day vitamin D to keep blood vitamin D concentration in the sufficient range [208].

Routine screening of vitamin D status may be useful in the athlete [196]. If routine screening is not possible, athletes with low exposure to UVB, history of stress fracture, frequent illness, bone and joint injury, skeletal pain or weakness, or signs of overtraining should be prioritized for assessment [12]. Vitamin D blood levels from 30–32 ng/mL (80 nmol/L) and up to 40 ng/mL (100 nmol/L) to 50 ng/L (125 nmol/mL) have been recognized as prudent goals for optimal training induced adaptation [12].

Several studies to date have shown inconsistent results indicating either positive or negative effects of antioxidant supplementation combined with training [209,210,211,212,213]. Briefly, the most commonly used arguments to support antioxidant supplementation are: (i) the fact that exercise leads to an increase in ROS production and that increased levels of antioxidants could counteract the ROS, preventing or reducing damage and, therefore, muscle pain [214], (ii) that some antioxidants shown to improve endurance performance [215] and to delay fatigue, and (iii) that some athletes may not achieve the nutritional recommendations for antioxidant intake just with food [216,217,218]. On the other hand, some arguments have been used against antioxidant supplementation, namely: (i) the fact that regular exercise leads to an increase in enzymatic and non-enzymatic antioxidants in muscle fibers [219]; (ii) that antioxidant supplementation may impair muscle function or delay some adaptations induced by exercise [220,221], by interfering with cell signaling functions of ROS, affecting muscular performance [222]; (iii) that antioxidant supplementation does not seem to lead to better outcomes, compared with placebo, regarding muscle function, inflammation [223] and redox status [224] after eccentric exercise; (iv) that antioxidant supplementation may contribute to increase muscle damage and oxidative stress [225], and (v) that some studies do not support the concept that antioxidant supplementation is beneficial to human health [226] and doubts have been placed about the chronic intake effects on performance of antioxidant supplementation in high doses [222,227]. Moreover, it has been reported that the protective effect of a diet, with natural sources of antioxidants, is not equivalent to the protective effect of supplementation [228]. Given these facts, it is currently suggested [209,229] that due to the limited evidence to recommend antioxidant supplements, athletes should rather focus on consuming a well-balanced and energetically adequate diet, which can provide antioxidant-rich foods. For detailed information regarding the impact of natural-present antioxidants on exercise, other review paper is recommended [118]. Nevertheless, new approaches regarding the acutely intake of some antioxidants such as vitamin E and N-acetylcysteine seem promising and are pointing towards an acute performance benefit, but further research is needed [227].

Beta-alanine supplementation has become a common practice among competitive athletes participating in a range of different sports [250]. Among other roles [251], beta-alanine supplementation augments intramuscular carnosine content, leading to an increase in muscle buffering capacity, as carnosine readily accepts protons during contraction-induced acidosis [252]. As such, a delay in the onset of muscular fatigue and a facilitated recovery during repeated bouts of high-intensity exercise have also been reported as positive effects of beta-alanine supplementation [250]. As muscle is unable to synthesize the two carnosine precursors, l-histidine and b-alanine, the concentration of intracellular carnosine is largely dependent on the uptake of these amino acids from the bloodstream. Of the two, beta-alanine has been identified as the rate-limiting precursor to carnosine [253]. Reviews on the effects of its supplementation have been published [242,254].

In a classic study by Hoffman et colleagues [255] college football players ingested 4.5 g of beta-alanine or placebo for 30 days. Beta-alanine supplementation began three weeks before preseason training camp and continued for an additional nine days during camp. Anaerobic performance, training volume, and ratings of soreness and fatigue were assessed pre- and post-intervention and included a 60-s Wingate anaerobic power test and three line drills. At the end of the 30-day investigative period, only the beta-alanine group showed a trend toward lower fatigue rates during the anaerobic performance test. More interestingly for team sports, beta-alanine supplementation allowed for higher training volumes and lower subjective feelings of fatigue, indicated that as duration of supplementation continued, the efficacy of beta-alanine supplementation in highly trained athletes became apparent (training logs were used to record resistance training volumes). Earlier, the effects of creatine and beta-alanine supplementation on performance and endocrine responses in strength/power athletes had been analyzed and it was found that beta-alanine + creatine did not produce further strength increments when compared to creatine alone but the co-supplementation elicited greater changes in lean body mass and percent body fat [256].

Studies conducted on the effects of beta-alanine on soccer related performance variables are scarce. Saunders and colleagues gave either a placebo or a beta-alanine supplement (3.2 g/day for 12 weeks) to 17 amateur soccer players and evaluated their effect on the YoYo Intermittent Recovery Test Level 2 [257]. These researchers found that while participants on the placebo group saw a decrease on their performance (−7.3%), participants on the supplemented group achieved the opposite (+34.3%). Also of note, only two participants on the placebo group improved their performance, but eight out of the nine supplemented participants achieved better results, while the remaining participant saw no modification on his performance.

Another interesting study aimed to determine if beta-alanine could help to improve sprinting performance at the end of an endurance competition. This is interesting for team sports, as performance detriments tend to increase by the end of a match and the ability to perform a maximal intensity exercise by then may be decisive for the match result. Therefore, subjects performed a 10-min time trial and a 30-s isokinetic sprint (100 rpm) after a 110-min simulated cycling race and it was found that during the final sprint after the time trial, beta-alanine supplementation resulted on an average increased peak power output by 11.4% and a mean power output increased by 5.0% [258].

A recent position stand of the International Society of Sports Nutrition [243] states that: (1) daily supplementation (4–6 g/day) significantly augments muscle carnosine concentrations; (2) beta-alanine supplementation currently appears to be safe in healthy populations at recommended doses; (3) paresthesia (tingling) may occur as a side effect, but studies indicate this can be attenuated by using divided lower doses (1.6 g) or using a sustained-release formula; (4) daily supplementation (4–6 g/day) for at least two to four weeks has been shown to improve exercise performance, with more pronounced effects in open end-point tasks/time trials lasting 1 to 4 min in duration; (5) beta-alanine attenuates neuromuscular fatigue, particularly in older subjects, and preliminary evidence indicates that beta-alanine may improve tactical performance; (6) combining beta-alanine with other single or multi-ingredient supplements may be advantageous when supplementation of beta-alanine is high enough (4–6 g daily) and long enough (minimum four weeks); (7) more research is needed to determine the effects of beta-alanine on strength, endurance performance beyond 25 min in duration, and other health-related benefits associated with carnosine. In summary, the body of scientific data indicate that athletes may not only be using beta-alanine supplementation to enhance sports performance but also as a training aid to augment bouts of high-intensity training, to decrease rates of perceived fatigue and to perform higher training volumes in team-sport athletes, which may allow for greater overload and superior adaptations compared with training alone [250]. Nonetheless, more studies involving soccer players are needed to fully comprehend the interactions between beta-alanine supplementation and performance in soccer.

The ergogenic effect of caffeine, one of the most popular dietary supplements, is relatively well-documented [259]. The International Society of Sports Nutrition considered that caffeine supplementation is beneficial for intermittent exercise within a period of prolonged duration, including team sports such as soccer [260].

Froskett and collaborators [261] showed that the ingestion of 6 mg/kg of caffeine 60 min before a simulated soccer activity could improve players’ passing accuracy and jump performance without any detrimental effects on other performance parameters. In another study [262], male soccer players performed a 90-min intermittent shuttle-running trial 1 h after ingesting a carbohydrate-electrolyte solution containing 3.7 mg/kg of caffeine. The solution was also ingested every 15 min during the exercise protocol. Compared to placebo, the addition of caffeine to the carbohydrate-electrolyte solution improved sprinting performance, countermovement jumping and ratings of pleasure. Based on this two studies, a recent review [70] concluded that caffeine in doses of 6 mg/kg has the potential to preserve skills performed under conditions that induce soccer-specific fatigue, but its effects are not strongly conclusive due to the small number of studies.

More recently, 6 mg/kg of caffeine ingested 1 h before a 90-min intermittent treadmill-running protocol enhanced the ratings of pleasure and arousal during the exercise protocol and increased vigor compared to placebo [244]. In another study [263], also with female players, the ingestion of 3 mg/kg of caffeine in the form of an energy drink 1 h before a countermovement jump and a 7 × 30 m sprint test followed by a simulated soccer match (2 × 40 min) increased the countermovement jump height and the average peak running speed during the sprint test. During the simulated match, the ingestion of caffeine increased the total running distance, the number of sprints and the running distance covered at >18 km/h. Nevertheless, it is worthy to mention that not all studies investigating the use of caffeine on soccer have found positive effects [264,265].

Creatine is an endogenous compound synthesized from arginine, glycine and methionine and is mostly (95%) stored in skeletal muscle where it can be found as a free form or bound to a phosphate molecule (phosphocreatine) [245]. The most well know physiology effect of creatine is its role on the maintenance of intracellular levels of adenosine triphosphate (ATP), whereby phosphocreatine promptly donates its phosphate molecule to adenosine diphosphate (ADP) to (re-)form ATP, and therefore, maintaining the highest power output. However, this energy system can be almost completely depleted after only a few seconds of maximal exercise [266]. In a single 6-s sprint, glycogen degradation (glycogenolysis) contributes 50% of the ATP production, whereas phosphocreatine contributes 48% and the remaining 2% is provided by the muscle’s small store of ATP [267]. In even shorter maximal sprint durations, phosphocreatine is the primary energy source for ATP production. Along with power and strength, creatine has traditionally been praised for its effects on muscle mass increase and many other beneficial outcomes, from age-related disease prevention to improved brain performance [245,268], and is a popular supplement among elite soccer players [230].

The effects of acute creatine supplementation have been investigated. Ostojic and colleagues found that creatine supplementation for seven days (3 × 10 g/day) improved performance in a soccer-specific battery of tests, including a dribble test, a sprint-power test, an endurance test, and a vertical jump test [269]. Another study revealed that six days of creatine supplementation (4 × 5 g/day) improved repeated sprint performance and jumping ability after an intermittent exercise test in 17 highly trained male soccer players [270]. The same creatine supplementation protocol produced improved results on repeated sprint and agility tasks in elite female soccer players [271].

Longer creatine supplementation protocols have also been implemented. A 13 weeks of creatine supplementation (2 × 7.5 g/day in the first week and 5 g/day throughout the rest of the protocol) improved the muscle strength of collegiate female soccer players [272]. Claudino and colleagues found that creatine monohydrate supplementation prevented the decrement in lower-limb muscle power in elite soccer players during a 7-week pre-season progressive training [273]. Here, subjects from the creatine group received 20 g/day of creatine monohydrate for 1 week divided into four equal doses (loading phase), followed by single daily doses of 5 g for the next six weeks (maintenance phase). More recently, Ramirez-Campillo and colleagues investigated the effects of a six-week plyometric training and creatine supplementation intervention (4 × 5 g/day in the first week followed 5 g/day in the next five weeks) on maximal-intensity and endurance performance in female soccer players during in-season training and found that creatine produced improved results in the jumps and repeated sprinting performance tests [270].

In events where a supercompensation protocol is possible or desirable (i.e., competitions that take place in just one or a few days), creatine ingestion (20 g/day for six days) can augment dietary carbohydrate mediated muscle glycogen supercompensation during the initial 24 h of recovery (sustaining it on the next five days) following prolonged exhaustive exercise [274].

Nevertheless, creatine does not always produce improved performance results, whether using an acute [275] or a chronic supplementation protocol [276]. Non-responders typically have higher preload levels of creatine and phosphocreatine, less type II muscle fibers, small preload muscle cross-sectional area, and lower fat-free mass [277].

On the methodology of creatine supplementation there are various issues that should be addressed. For example, creatine supplementation seems to be most effective when taken after exercise, both on healthy older adults or healthy young adults, at a dose of 0.1 g·kg⁻¹ or around 5 g [278,279]. The ideal frequency of weekly ingestion is not known but on physically active university students there was no difference between a two or a three day/week intake schedule [280]. Nevertheless, professional soccer players often train/play 5–6 days per week and should supplement accordingly, or at least after high intensity training or every match. Creatine ingested in combination with CHO substantially increased muscle creatine accumulation compared with the ingestion of creatine alone [281] but a combination of 50 g of CHO and 50 g of protein seem to be as effective as an isolated dose of 100 g of CHO [282]. Otherwise, a recent review on creatine supplementation and lower limb strength performance found that this supplement is effective for exercise with a duration of less than 3 min, independent of population characteristic, training protocols (including the non-necessity of a loading phase) and supplementary doses and duration [283]. The same study states that creatine monohydrate is still the most used form of creatine supplementation. Finally, long-term creatine use does not appear to result in adverse health effects, at least while consuming the most popular creatine forms [246,284].

Dietary nitrate supplementation is known for its capacity for reducing the oxygen cost of submaximal exercise [285]. The ingestion of nitrate leads to an increment of plasma nitrite concentrations and, consequently, to an increased production of nitric oxide [286]. Nitric oxide has several metabolic and vascular effects that seem to contribute to a better exercise efficiency [286]. Dietary nitrate is available in the form of beetroot shots, nitrate-containing gels and bars [15]. The ingestion of nitrate-rich foods such as spinach, lettuce and arugula might be an alternative to supplementation [287].

Although the use of nitrate became popular on soccer, few studies have investigated the effectiveness of nitrates specifically on soccer performance. Wylie and collaborators [247] investigated the intake of 490 mL of nitrate-rich beetroot juice concentrated or placebo over ≈30 h preceding the completion of a Yo-Yo intermittent recovery level 1 test (IR1) by male recreational team-sport players. The supplementation protocol consisted of the ingestion of 2 × 70 mL (2 × ≈4.1 mmol of nitrate) in the morning and 2 × 70 mL in the evening. On each experimental day, subjects consumed a further 2 × 70 mL 2.5 h prior to and 1 × 70 mL 1.5 h prior to the start of the exercise protocol. Performance in the Yo-Yo IR1 test was 4.2% greater with the beetroot juice compared to placebo. More recently, Thompson and collaborators [248] studied the intake of 70 mL beetroot juice/day (≈6.4 mmol of nitrate) or placebo for five days by male team-sport players. On day 5, subjects completed a series of maximal 20-m sprints followed by the Yo-To IR1; cognitive tasks were completed prior to, during and immediately following the Yo-Yo IR1. The distance covered in the Yo-Yo IR1 test improved by 3.9% compared to placebo. The reaction time to the cognitive tasks was 4.7% shorter in the beetroot juice group than in the placebo group at rest but not during the Yo-Yo IR1. In another study, [288], improvements in total work done during prolonged intermittent exercise and in reaction time of response to cognitive tasks in the second half of the intermittent exercise protocol were also achieved with the ingestion of a larger daily dosage (140 mL beetroot juice; 12.8 mmol of nitrate) during a larger period of time (7 days).

Nitrate seems to be a promising dietary supplement in the context of sport. Nevertheless, further research on high-intensity intermittent team sports such as soccer with elite athletes is necessary.

Although there is a rationale to use alkalizing agents such as sodium bicarbonate on soccer, to our knowledge, few studies have been performed regarding this issue. Saunders and collaborators [289] investigated the acute supplementation of sodium bicarbonate (0.3 g/kg–0.2 g/kg 4 h before exercise and 0.1 g/kg 2 h before exercise) or placebo on three sets of 5 × 6 s repeated sprints performed during a football specific intermittent treadmill protocol performed in hypoxia (15.5% O₂). In this study, the supplementation with sodium bicarbonate did not improve repeated sprints performance. In another study, [290] with male rugby players, the ingestion of 0.3 g/kg of sodium bicarbonate 65 min before a 25-min warm-up followed by 9 min of high-intensity rugby-specific training followed by a rugby-specific repeated-sprint test, increased blood HCO_3⁻ concentration and attenuated the decline in blood pH compared with placebo but did not significantly improve exercise performance. Contrarily, in a recent study [291], high-intensity intermittent exercise performance was improved by the intake of 0.4 g/kg of sodium bicarbonate before the Yo-Yo intermittent recovery test level 2. The sodium bicarbonate was evenly distributed in ~25 gelatin capsules, with one fifth taken at 90, 80, 70, 60 and 50 min prior to exercise in order to avoid stomach discomfort.

Nevertheless, a review from Bishop on the dietary supplements for team-sport performance [249] concluded that based on several studies on running and cycling sprint performances, the ingestion of sodium bicarbonate is likely to improve both repeated- and intermittent-sprint performance. More research is needed to confirm the possible ergogenic effect of sodium bicarbonate on soccer.