Keywords Ergogenic Aids - Football - Performance - Sports Nutrition
Introduction
Caffeine is a well-established performance-enhancing supplement with benefits from
acute consumption demonstrated across several facets important to sports
performance. The wealth of literature has been summarised in several meta-analyses
which demonstrate small but significant caffeine-induced improvements in aerobic
endurance performance,[1 ]
[2 ]
[3 ]
[4 ] muscular strength and
power,[5 ]
[6 ]
[7 ]
[8 ] anaerobic power,[9 ] and cognitive functions.[10 ] Such outcomes represent important
facets of soccer match play and whilst it might seem reasonable to infer that these
benefits may translate to improved soccer performance, evidence examining the
specific and interacting demands of soccer match play is sparse.
Attributable to what may appear to be overwhelming evidence and the highly
commercialised potential of caffeine in the sports supplement market, caffeine
consumption for the purpose of improved performance in commonplace. Specifically,
a
recent survey of elite soccer teams in England indicated that 97% of teams sampled
provided caffeine to players to improve performance.[11 ] As such, a review of this topic is
particularly pertinent given the prevalence of caffeine use, and apparent limited
translation of caffeine administration procedures advocated in research to
practice.[11 ]
The bioavailability of caffeine is high and nearly all ingested is absorbed in the
stomach (20%) or gastrointestinal tract (80%) before entering the bloodstream.[12 ]
[13 ] Caffeine is both hydrophobic and lipophilic, and as such, easily and
rapidly transverses biological membranes resulting in wide distribution throughout
the body, including diffusion across the blood–brain barrier.[13 ] Mechanisms underpinning the ergogenic
potential of caffeine have been the subject of previous reviews,[13 ]
[14 ]
[15 ] and in the most part
have been attributed to the action of caffeine as a central adenosine receptor
antagonist particularly at A1 and A2a subunits.[16 ] Such effects result in an increase in excitatory
neurotransmitters,[17 ]
[18 ]
[19 ] resulting in increased motor unit firing, improved neuromuscular
function, increased arousal and alertness, suppressed pain and reduced fatigue.[15 ]
[20 ] Furthermore, at physiologically relevant doses, caffeine may act
directly on the muscle, augmenting processes involved with excitation-contraction
coupling, resulting in increased muscle power output.[21 ] These interacting mechanisms likely
account for the influence of caffeine on sports performance.
Soccer is a highly dynamic sport requiring complex interplay between physical,
technical and tactical proficiency which are underpinned by neuromuscular, cognitive
and emotional function.[22 ]
[23 ]
[24 ]
[25 ]
[26 ] Studies evaluating the ergogenic
potential of caffeine for soccer performance considering the complex demands of
match play are limited, and subsequently influence the development of safe and
effective guidelines for use. This review aims to evaluate the current state of the
knowledge regarding the ergogenic potential of acute caffeine consumption
specifically for soccer performance. Specifically, this review will critique the
evidence regarding the effects of caffeine supplementation on physical and cognitive
facets important for soccer performance; evaluate caffeine effects on technical and
tactical outcomes and match play; identify knowledge gaps and limitations in the
application of research findings to soccer-specific practice; and based on the
available evidence, produce recommendations to athletes and practitioners.
Effects of caffeine on soccer-specific physical performance
Effects of caffeine on soccer-specific physical performance
Attributable to the advancement in sports technology, the physical and energetic
demands of soccer match play have been well characterised, and the intermediate
nature of the sport has a complex physiological demand being highly taxing on both
the aerobic and anaerobic energy systems.[23 ] However, given a myriad of factors (e.g., opposition, environment,
formation, and tactics), greater physical performance is not always associated with
match outcome,[27 ]
[28 ] although it is evident that physical
capability is an essential facet of performance that distinguishes between elite and
sub-elite players[29 ] and provides the
basis for execution of the chosen match strategy. Furthermore, several indirect
links between physical performance and facets that may underpin success exist.
Specifically, in the 2007/2008 German Bundesliga, 83% of goals in the latter half
of
the season involved at least one power action from the scoring or assisting player,
defined as sprint, jump or change of direction (CoD) speed.[30 ] Moreover, evidence suggests that
covering greater distance and increased frequency of high-intensity runs when in
ball possession results in more goal chances, shots, shots on target, passing and
passing accuracy, crosses, and a successful high press in elite soccer players.[31 ] As such, small increases in physical
ability induced by caffeine consumption could be beneficial to athletes.
Interestingly, two relatively recent systematic reviews have evaluated the effects
of
caffeine supplementation on physical performance in soccer players and report
conflicting outcomes.[32 ]
[33 ] Such conclusions were based on 16 and
17 studies, respectively, with the most recent review being published in 2021.
Whilst there have been several recent publications that may allow for a more precise
summary, for the most part, studies specific to soccer evaluate acute caffeine
effects in well-established physical performance tests (e.g., vertical jump,
straight-line sprint speed, CoD performance, and repeated sprint performance)
in elite and sub-elite soccer players across different ages.[34 ]
[35 ]
[36 ] There is no reason to
believe soccer players would respond to caffeine differently than athletes in other
sports; therefore, it is prudent to evaluate the ergogenic potential of caffeine on
relevant well-established performance tests more broadly considering the wealth of
published work that has reported effects on these assessments before evaluating
literature employing soccer-specific tests.
Vertical jumps are an indicative measure of lower limb power and have some
sports-specific relevance. The ergogenic effects of caffeine on vertical jump
performance (mainly countermovement jump [CMJ] and squat jump) have been thoroughly
assessed, with the weight of evidence suggesting small but significant benefits
across a range of athletic populations.[37 ] Furthermore, unlike other physical performance measures, there is a
growing body of work demonstrating that caffeine doses lower than 3
mg·kg−1 , and as low as 1 mg·kg−1 , have been shown to
improve CMJ height.[36 ]
[38 ] However, in a study by Ellis, Noon,
Myers and Clarke, [36 ] only 3
mg·kg−1 was effective in improving the CMJ performance of elite youth
soccer players both pre- and post-exercise. Whilst the CMJ challenges biomechanical
constraints that underpin performance in other vertical jumps, it should be
considered that bipedal standing vertical jumps do not represent the more complex
jumping strategies used in soccer match play.
Elite outfield players cover 22–24% of total match distance at high-intensity speeds
greater than 15 km/h and 2–3% at a threshold corresponding or close to maximal
sprint speed.[23 ] As the game has
evolved, more recent data have indicated that high-intensity runs during match play
have increased by ~30%,[39 ] and that
straight-line sprints are the most frequently demonstrated action of both scoring
and assisting players in goal situations.[30 ] The frequency and intensity of high-speed running is influenced by
several factors including formation, playing position, age and playing level;[23 ] however, sprint distances in elite
male and female soccer players are typically less than 10 m.[40 ] The potential for caffeine to improve
lower limb power, inferred by improved vertical jump performance, may lead to
speculation of a transfer effect to straight-line sprint speed. Whilst there is some
evidence, typically administering 3 mg·kg−1 caffeine, demonstrating small
improvements in 20 and 30 m sprint time in both elite and sub-elite
participants,[35 ]
[36 ]
[41 ]
[42 ]
[43 ] fewer studies have considered effects
on ≤10 m sprinting, and although there is some support,[43 ] there is also evidence for a lack of
an effect,[42 ] even when doses of up to 5
mg·kg−1 are administered.[44 ] It should also be considered that studies evaluating sprint ability
typically assess performance from a standing start. Soccer performance also requires
curved running and rapid CoD. Furthermore, the evidence discussed thus far is
specific to a single bout of running and elite soccer players on average will
perform 11 sprint efforts per game.[45 ]
Therefore, it is important to consider the effects of caffeine on repeated sprint
running performance.
There is support from meta-analyses regarding the beneficial effects on anaerobic
power;[9 ] however, studies that have
specifically considered repeated sprint performance are equivocal which is likely
attributable to differences in work:rest ratios and exercise modalities (sprint
running vs. cycling).[46 ] When sprint
running specifically is considered, there is some evidence to support small
improvements in repeated sprint performance;[47 ]
[48 ]
[49 ]
[50 ] however, benefits have been shown to be prevalent for some sprint
efforts and not all, sprint distances are ≥ 20 m, rest durations vary (30–60 s in
the cited studies), and studies typically consider effects of higher doses of
caffeine 5–6 mg·kg−1 . Whilst there may be effects at lower doses (e.g.,
200 mg), these effects may be specific to low habitual consumers.[51 ] Furthermore, beyond a lack of
consistency in published findings, there is a need for caution regarding the direct
translation of these results to soccer given the lack of studies that consider
soccer-specific work:rest ratios and that sprint running is interspersed with varied
intensity of submaximal running and walking. As outlined, across the course of a 90+
minute match, players on average will perform 11 sprints [45 ] where evidence in female soccer
players has demonstrated a time of ~87 s between sprint bouts (>19 km/h),
resulting in a work:rest of ~1:43.[40 ]
Numerous studies have considered the ergogenic potential of caffeine on CoD
performance. In comparison to repeated sprint running, positive outcomes are much
more consistently reported and are prevalent across 1–6 mg·kg−1
doses.[36 ]
[52 ]
[53 ]
[54 ] Effects have been
demonstrated across several CoD tests (e.g., arrowhead agility, 5-0-5,
Illinois agility run), many of which are used for soccer athlete profiling.
Performance-enhancing effects have also been shown to occur in what might be
considered more ecologically valid reactive CoD tasks [46 ] and in team sports athletes in both
fresh and fatigued states.[55 ]
Caffeine may evoke more pronounced effects for endurance activity,[56 ] where aerobic performance is an
important discriminator between elite and sub-elite soccer players.[57 ] Whilst the effects of caffeine on
aerobic function have been determined across several exercise modalities consisting
of time trial and fixed duration activity,[1 ]
[2 ]
[3 ]
[4 ] it is important in the context of this review to consider assessment
methods that best represent the physiological demands of soccer, which is dependent
on both aerobic and anaerobic energy contribution. One assessment that has been
particularly advocated in this regard,[58 ] and has received attention in the literature examining the ergogenic
potential of caffeine, is the Yo–Yo intermittent shuttle run (Yo–Yo IR). Several
studies support improved performance in variants of the Yo–Yo IR, where evidence is
much less equivocal than other modes of exercise and there appears to be little
benefit of higher doses (6 mg·kg−1 ) over lower doses (200 mg absolute
dose).[59 ] Further to this in
professional and semi-professional players, 6 mg·kg−1 caffeine has been
shown to increase time to exhaustion running performance at 70% VO2max ,
following the completion of three 22.5 minute bouts of a treadmill running protocol
designed to replicate the physiological demands of soccer.[60 ] Such effects were prevalent
independent of cardiorespiratory or neuromuscular fitness level.
Whilst it might appear caffeine may produce favourable effects on the outlined
physical performance facets, when studies evaluate multiple assessments in the same
study, it is rare to see beneficial effects in all measured outcomes.[34 ]
[61 ]
[62 ] However, it should be
noted that the interpretation of the results from studies focusing on multiple
outcomes is somewhat limited by the small sample sizes and intraindividual variation
in performance measures.[63 ] Furthermore,
for each of the listed assessments, contradictory results can be found,[61 ]
[62 ]
[64 ]
[65 ]
[66 ] which have been attributed to several potential moderators of
caffeine’s effects. Specifically, dose, habituation, time of day, athletic level and
difference in gene polymorphisms responsible for caffeine metabolism and sensitivity
may influence effects.[67 ] These
potentially interacting moderators likely explain interindividual caffeine responses
and infer that an individualised supplementation strategy is needed to maximise
effects.
Effects of caffeine on soccer-specific cognitive functions
Effects of caffeine on soccer-specific cognitive functions
Soccer has a substantial cognitive demand where players are required to maintain high
levels of attention and make rapid decisions in a dynamic environment.[68 ] As such, soccer has been suggested to
elicit cognitive fatigue,[68 ] where in
such circumstances caffeine may offer superior effects.[69 ] Executive functions represent an
important cognitive function subgroup within this context and can be divided into
core (working memory, cognitive flexibility and inhibitory control) and high-level
(reasoning, problem solving, and planning) executive functions.[70 ] Effective executive functions are
particularly relevant given their association with technical proficiency.[71 ]
Caffeine has been shown to elicit beneficial effects on self-report energy and mood
as well as low-order cognitive functions such as attention, vigilance, reaction time
and memory which may be beneficial for improved soccer performance.[10 ]
[72 ] Positive effects have been shown at doses as low as 1 mg
kg−1 ,[72 ] and caffeine’s
effect on cognition may be specific to lower doses. For example, Zhang et al.[73 ] demonstrated that only 3 mg
kg−1 caffeine was effective in improving both reaction time and brain
activation compared to higher doses (6 or 9 mg kg−1 ). Where increased
anxiety and tension that may occur because of higher dose supplementation may result
in attentional narrowing, and as a result, less effective information
processing.[72 ]
Whilst it might seem intuitive that such effects may translate to soccer performance,
studies specific to the cognitive demands of soccer are limited and results are
inconclusive. Jafari et al. [74 ]
demonstrated that 3 mg kg−1 caffeine impaired decision-making in a
computerised task where participants were challenged in evaluating the most
appropriate outcome in simulated pre-recorded soccer events. Similarly, whilst
small-sided games have been shown to improve response time in the Stroop test, in
a
sample of professional male soccer players, 5 mg kg−1 caffeine reduced
both post-exercise reaction time and accuracy.[75 ] Field et al. [76 ]
demonstrated that 200 mg caffeinated chewing gum administered 90 min following
soccer-specific exercise attenuated the declines in reaction time but reduced
composure when assessed during the extra-time period of simulated soccer match play
using a soccer-specific virtual reality simulator.[76 ]
One key aspect for future studies wishing to determine the impact of caffeine on
cognitive performance specific to soccer is in the use of ecologically valid
assessments alongside representative research designs. While there is some evidence
that caffeine may modulate cognitive responses in some cognitive functions related
to soccer performance, the research designs employed to date are limited, with a
tendency to examine cognition pre-post some form of soccer perfor.ance (e.g., ref.
[75 ]), or in simulated match play
(e.g.,ref. [77 ]). Likewise, in studies
that have employed a technical motor skill task (e.g.,ref. [74 ]), this does not really effectively
assess soccer-specific cognition. There are emerging research designs that could be
employed in future work which better explore applied aspects of cognition in a
soccer-specific manner such as work assessing soccer-specific divergent thinking and
creativity, determined via performance analysis in game play.[78 ] To date such designs have not been
considered in the context of caffeine ingestion, or other nutritional ergogenic
aids, but for progress in relation to caffeine’s effects on soccer performance, such
representative designs and assessments are needed.
Effects of caffeine on technical, tactical performance and match play
Effects of caffeine on technical, tactical performance and match play
Technical proficiency is an important determinant of success and relies on the
interaction of motor control, perception and cognitive functioning.[79 ] However, studies examining the effect
of caffeine on technical skill performance are lacking. Current work appears
specific to passing performance where 3 mg·kg−1 has been demonstrated to
improve both short and long passing accuracy, but in the same study, reduced
performance in the more demanding Loughborough soccer passing test (LSPT).[74 ] Conversely, Foskett et al. [77 ] demonstrated improved LSPT performance
following 6 mg·kg−1 ingestion when assessed several times across 90
minutes of the Loughborough Intermittent Shuttle Test. Mor et al. [53 ] further demonstrated no effect of
caffeine on ball kicking speed, but this was measured independent of accuracy. With
respect to understanding the effects of caffeine on technical proficiency,
researchers should carefully evaluate the between-session reliability of the chosen
assessments as some of the cruder measures of performance (e.g., distance of the
ball from a target in a passing task over a relatively small number of attempts) may
lack the sensitivity to accurately detect what are typically small effects. This
idea has been discussed in a recent publication by our group.[63 ]
A more representative examination of the effects of caffeine on soccer-specific
performance might be to elucidate the ergogenic potential during match play. A small
number of studies have taken this approach and there is some support for an increase
in total distance covered, high-speed running distance covered, and number of
sprints, in both male and female athletes following 3 mg·kg−1
consumption.[35 ]
[80 ] Similar outcomes have been
demonstrated in other sports.[81 ]
Interestingly, Pettersen et al. [82 ]
demonstrated no effect using a similar approach in elite adolescent athletes
following 6 mg·kg−1 caffeine. Whilst such studies have been valuable to
enhancing the understanding of the performance-enhancing potential of caffeine for
soccer match play, they should be approached with caution given that the physical
demands of match play are heavily influenced by opponents, environmental conditions
and tactics, factors that are difficult to control.
The effect of caffeine on tactical performance has been scarcely considered. Recent
work by de Almeida et al. [83 ]
demonstrated that acute caffeine ingestion resulted in a reduction of defensive
errors and an increase in ball possession during a repeated small-sided games
protocol with professional soccer players. However, the impact of caffeine on
tactical performance was heterogeneous, with some positive effects on offensive
decision-making but a decrease in total defensive success in specific scenarios.
This highlights the complexity of tactical analysis, which involves various outcomes
(i.e., distinct offensive and defensive actions). Although speculative, one
hypothesis is that different levels of arousal might be required for different
tactical actions during the game, and the effects of caffeine may vary according to
this relationship. This is an area of interest for future studies. Although, as
previously indicated, caution is needed in the interpretation of these results given
that the improved ecological validity of such study design results in impaired
experimental control, which in the case of the current work could be improved by
more rigorous consideration of the between-session reliability of the outcome
measures. In conclusion, research evaluating the effect of caffeine on soccer on
technical, tactical performance and match play is sparse, and whilst caffeine may
benefit gross physical function, effects on tactical and technical outcomes range
from negative to positive. However, it is difficult to exert full confidence in
these findings.
Knowledge gaps and limitations to practice
Knowledge gaps and limitations to practice
Several areas of future work are needed to provide more robust evidence-based
guidance for optimised caffeine use in soccer. In fact, a translational gap between
science and practice specific to the use of caffeine supplementation in soccer has
been previously highlighted.[11 ]
Priorities for caffeine-focused research for sport and exercise science applications
have been the subject of extensive recent reviews,[67 ]
[84 ] and whilst several of the points identified are relevant, here we
will provide a brief account of priorities specific to caffeine use in soccer.
Ecologically Valid Tests: As outlined throughout this review,
understanding the potential efficacy of caffeine as a performance-enhancing
supplement is somewhat limited by the lack of ecologically valid
assessments. Regarding isolated assessment of the physical components
important for performance, specific focus is needed on replicating the
sprinting and repeated sprint demands of match play. Similarly, while there
is evidence supporting the beneficial effects on cognitive performance, the
application of isolated assessments of cognitive functions to match play
remains somewhat limited. Consequently, there is a clear need for future
research to explore the interaction of football-specific cognitive functions
within scenarios that more closely reflect actual match play. This approach
should also consider cognitive fatigue and multitasking processes, including
their synergistic interaction with physical demands. Furthermore, work is
needed to better understand caffeine effects on tactical and technical
performance using assessments where the between-session reliability of the
outcomes is established, as well as consideration of position-specific
demands, where in particular goalkeepers are poorly represented. These
considerations should extend to evaluation of the impact of both mood as
well as physical and cognitive fatigue, which influence caffeine
erogenicity.[63 ] In general,
the translation of research findings to enhanced performance is limited by
study designs representative of the demands of soccer, and future work
should be considered with this in mind.
Genetic Variation: Polymorphism in genes responsible for caffeine
metabolism (Cytochrome P450 1A2 [CYP1A2]) and the adenosine A2A receptor
(ADORA2A) have been highlighted as potential moderators to caffeine’s
ergogenic effects and may explain interindividual responses.[67 ] Whilst there is some
supporting evidence that carriers of the TT allele of ADORA2A [85 ] and the AA allele of CYP1A2
may result in superior effects,[86 ] there is an equal number of studies that indicate no
influence of these gene polymorphisms.[87 ]
[88 ] There is some
suggestion that genotype effects may be specific to exercise modalities and
influenced by dose.[87 ]
[88 ] Furthermore, slow caffeine
metabolisers may in fact initially see a reduction in performance, and an
improvement that takes longer than the 45–60-minute ingestion period
typically used in previous work.[89 ] The effect of gene polymorphisms that may influence
caffeine’s erogenicity is yet to be considered in a soccer-specific context.
One way to address this would be to stratify individuals into groups based
on multiple potentially influential genetic polymorphisms and then compare
the ergogenic potential of caffeine on measures of match-play performance
between these groups. However, this approach presents significant logistical
and financial challenges, given the large number of participants required to
conduct a study of this design.
Effects For Female Athletes: The wealth of evidence examining the
ergogenic effects of caffeine is weighted towards male participants and
there is a need to extend this understanding to female athletes. Oral
contraception can increase the time for caffeine clearance,[90 ] and differences in body
composition and hormonal function have provided a theoretical basis for
sex-specific effects.[91 ]
However, recent meta-analyses specific to female athletes demonstrate
improved muscular strength, strength endurance and vertical jump performance
following acute caffeine ingestion.[37 ]
[92 ] Interestingly,
the caffeine-induced increase in vertical jump performance was prevalent
across different menstrual phase cycles but was greatest in the follicular
phase.[37 ] A meta-analysis
summarising the small number of studies that have directly compared the
sex-specific effects of acute caffeine consumption indicates a similar level
of benefit for aerobic-based activity; however a greater effect for males
completing anaerobic activity.[91 ] This ambiguity in published findings and lack of studies
specific to soccer highlight a need for further investigation.
Time Course Effects & Repeated Dosing: Repeated dosing appears
to be common practice in professional soccer;[11 ] however, studies examining the
time course of caffeine-induced effects and the value of repeat dosing are
lacking.[67 ] Five mg
kg−1 caffeine has been shown to improve endurance cycling
performance when consumed 6 h prior to exercise; however, in habitual
caffeine users, effects were only prevalent 1 and 3 h prior to
exercise.[93 ] Furthermore, a
divided caffeine dose consumed prior to and during endurance cycling
elicited a similar benefit to a bolus dose administered prior to task
completion.[94 ] However, such
effects may be influenced by exercise modality. In a simulated wrestling
tournament consisting of five matches over 6 h, 10 mg kg−1
improved performance in the first bout, and a 2 mg kg−1 divided
dose elicited beneficial effects in both the third and fourth matches. An
individual dose based on performance decrement elicited the longest time
course of effects improving performance in matches three to five.[95 ] The current lack of research
specific to the time course effects of caffeine for soccer performance
somewhat limit recommendations regarding the effectiveness of repeated
dosing and is an important area for future investigation.
Recommendations for athletes & practitioners
Recommendations for athletes & practitioners
Despite a need for future work, the presented review of the literature indicates the
potential for a small but significant caffeine-induced improvement in several facets
important to soccer performance. However, there remain challenges in realising and
maximising this potential. Guided by the available evidence, advice for athletes and
practitioners is presented and summarised in [Fig. 1 ].
Fig. 1 A guide to caffeine supplementation in soccer: recommendations
for athletes & practitioners.
Understand Caffeine Consumption Habits
: Before administering
caffeine for the purpose of improved performance, it is first important to
monitor an athlete’s typical caffeine consumption habits. This information
is essential for mitigating adverse effects, managing potential effects of
habituation, and ensuring that the chosen supplementation strategy is not
influenced by prior or post-caffeine intake. Whilst this is somewhat
challenging given the varying caffeine doses in different batches of the
same product,[96 ] there is no
standardised approach for determining caffeine consumption habits [97 ] and several monitoring tools
exist which lack commonality.[98 ]
However, from an application perspective, there is a need to balance rigour
with timeliness, particularly in team-based sports such as soccer. Whilst
weighted food records and food diaries offer greater accuracy,[99 ] several caffeine-specific
consumption surveys exist. Whilst some of these surveys focus on quantifying
“typical” caffeine consumption,[100 ] the Caffeine Consumption Questionnaire (CCQ) [100 ] and the questionnaire and
calculation programme developed by Bühler, Lachenmeier, Schlegel and Winkler
[101 ] are relatively simple
and have been used readily in the scientific literature. Such surveys may be
particularly useful in the context of soccer given that both capture
caffeine consumed from a range of products and consider the time-specific
nature of these habits across a day. Monitoring this over a 24-hour period
is recommended to understand how consumption habits are influenced at
specific points over a competitive week. It is recommended that such surveys
are used in conjunction with databases such as Caffeine Informer
(https://www.caffeineinformer.com/) to improve the accuracy of estimation.
Whilst it is recommended to initially understand the interindividual
reliability of caffeine consumption habits, such information may not need to
become part of regular athlete monitoring but could be conducted
periodically. This information should inform player education regarding the
use of caffeine and form the basis of an individualised supplementation
strategy.
Player & Coach Education : Player and coach education is
important for safe and effective supplementation and should extend beyond
advice regarding dose, timing and modality. This should be expanded to
several further factors to provide holistic guidance regarding caffeine
practices. Specifically, players should be educated on the effects of acute
caffeine on facets important to soccer match play, how to manage caffeine
consumption prior to and following caffeine prescribed to elicit improved
performance, scenarios where consumption may evoke the greatest benefit,
synergistic effects with other supplements, adverse effects and implications
of misuse, and factors that might preclude safe consumption.
Player education should be informed by knowledge of caffeine consumption
habits and perception of its effects. Whilst evaluating athlete caffeine
consumption provides important information about the quantity and frequency
of caffeine use, it reveals little regarding motivation for consumption,
expectation regarding performance-enhancing effects, and potential
dependence or abuse. Such information is not only important for tailored
education but also for guiding safe and effective practice. Moreover,
several studies indicate that instilling positive caffeine expectancy may
evoke improved cognition and physical performance in athletic assessments in
a magnitude similar to that evoked as a result of the pharmacological
effects induced by ingestion.[102 ] As such, measuring and manipulating caffeine expectancy may
represent and effective strategy to induce improved soccer match play.
However, it should be noted that positive caffeine expectancy is influenced
by previous caffeine experience, the quality of the education provided, and
the method and credibility of the individual delivering the
information.[103 ]
[104 ]
[105 ] When instilling expectancy,
care should be taken that this is not approached in a way that evokes risk
and is in line with safe caffeine consumption practices.
With this in mind, both the Motives for Caffeine Consumption Questionnaire
(MCCQ) or Caffeine Expectancy Questionnaire (CaffEQ) may be appropriate. The
MCCQ is a 34-item Likert scale survey that evaluates motivational factors
related to alertness, habit, mood, social, taste and symptom
management.[106 ] However,
some of the specific questions that make up these factors are specific to
coffee consumption or do not well represent competitive sporting
environments. Although not interchangeable in the constructs assessed, the
CaffEQ may represent and appropriate alternative providing information
regarding positive and negative valanced expectancies related to (a)
withdrawal/dependence, (b) energy/work enhancement, (c) appetite
suppression, (d) social/mood enhancement, (e) physical performance
enhancement, (f) anxiety/negative physical effects, and (g) sleep
disturbances.[107 ] In a large
sample of adults with varied caffeine consumption habits, the CaffEQ has
been demonstrated to be valid for the seven psychometric properties assessed
and to have good test-retest reliability. To overcome the time constraints
of administering this survey, the 20-item Brief-CaffEQ (B-CaffEQ) could be
used and has shown acceptable performance compared to the original 47-item
scale [108 ]
Supplementation For Match Play : An individualised caffeine
supplementation strategy is advised based on the potential for an
interindividual response, habitual caffeine consumption and caffeine
expectancy. However, the available evidence would indicate a dose of ≤3 mg
kg−1 prior to match play might represent an appropriate
balance between performance-enhancing effects across the different facets
important for match play, whilst limiting potential adverse effects. Whilst
doses lower than 3 mg kg−1 may elicit some task-specific
benefits, evidence supporting the ergogenic potential of low-dose caffeine
across a broader range of physical performance facets remains limited.
Conversely, evidence supporting the potential of 3 mg kg−1 dose
is more compelling.[67 ] Although
there is little support for a dose-response effect, higher doses might be
needed to recognise performance improvements in some facets of physical
function, in particular maximal strength,[8 ]
[109 ]
[110 ] and in the function of upper
body musculature.[111 ] However,
improved performance in measures of soccer-specific endurance performance,
jump height, sprint speed, CoD performance, and high-speed running during
match play has been demonstrated at a dose of 3 mg kg−1 .[4 ]
[36 ]
[59 ]
[80 ] Doses greater than this
increase the risk of evoking adverse effects that may influence cognitive
function and other aspects of performance.[112 ] Furthermore, a dose of ≤3 mg
kg−1 may be more suited to balancing effects on both physical
and cognitive function. As an example, Waer et al. [113 ] demonstrated that low-dose
caffeine (100 mg) improved simple reaction time but had no effect on the
physical function of healthy middle-aged females. However, the reverse was
true when a higher dose (400 mg) was consumed.
It is recommended that caffeine be administered in anhydrous form; however,
there is growing evidence supporting administration via caffeinated chewing
gum, which has logistical benefits and results in a faster onset of effects
due to more rapid absorption. The increased onset of effects has been
attributed to absorption into the bloodstream via the highly vascularised
buccal mucosa,[114 ] and/or
activation of bitter taste receptors [115 ] and antagonistic effects on adenosine receptors in the oral
cavity.[116 ] Benefits to
physical performance have been shown 15 minutes post-maceration,[117 ] with caffeine in blood plasma
peaking once at this time and again 50 minutes later as a result of
absorption through the gut.[118 ]
Presently, there is little evidence to support caffeine administered via
other modes, including the use of caffeine mouth rinsing,[119 ] and therefore, this practice
is not recommended.
Caffeine anhydrous should ideally be administered in capsule form rather than
dissolved in large volumes of liquid or carbonated drinks, to minimise the
risk of gastrointestinal discomfort. Delivering caffeine anhydrous in
capsule form also reduces the likelihood of dosing errors due to
inaccuracies in weighing. Another way to mitigate this risk is to use
commercially available, “off-the-shelf” caffeine-only tablets; however,
careful attention must be paid to the absolute dose provided by each tablet.
This consideration can also be extended to caffeinated chewing gum and
energy drinks/shots. Given the potential risk of dosing errors with caffeine
anhydrous, appropriate weighing and preparation methods should form an
important part of player and coach education. Irrespective of method of
consumption, where possible, it is advisable to use batch-tested
products.
Post-ingestion, caffeine in the blood plasma has typically been suggested to
peak after ~60 minutes,[72 ]
although it may take as long as 80–120 minutes in some individuals.[120 ]
[121 ] It should be noted that peak
plasma concentration is highly individual and at least in part is
attributable to polymorphisms in genes that encode enzymes involved with
caffeine digestion.[89 ] Given the
duration of match play and that caffeine effects are unlikely to be specific
to the time of peak plasma concentration, ingestion of caffeine anhydrous 30
minutes prior to match play would mean that for most athletes, peak plasma
concentration would occur prior to the final 45 minutes of the first half
when athletes are likely to experience greater fatigue. However, it may be
desirable to shift the ingestion time to coincide with tactics.
Practitioners and athletes should be aware that caffeine has a half-life of
1.5–9.5 hours,[122 ] and therefore
repeat dosing in high concentrations, which appears to be prevalent in
English professional soccer,[11 ]
is not advised. Given the suggestion in previous work that a divided dose
may offer more sustained benefits,[95 ] a suitable approach may be half time supplementation with a
low dose (e.g., ~1 mg kg−1 ) of caffeinated chewing gum, which may
be suitable for maintaining caffeine blood plasma levels close to the peak
for the remainder of the match. However, it should be noted that there is
limited evidence to determine if peak blood plasma concentration corresponds
with peak performance-enhancing effects and further work is needed to
evaluate the suitability of this approach.
It should also be noted that consumption of caffeine following a high
carbohydrate meal has been shown to reduce the speed of digestion and
prevalence in the blood plasma [123 ] highlighting a need to consider caffeine supplementation
strategy holistically within the wider nutritional programme.
Supplementation For Training: Adopting a similar caffeine
consumption strategy to that recommended for match play may also be suitable
to enhance training performance. However, the importance of improved
performance in training should first be considered. Whilst it may be
conceivable that a caffeine-induced increase in training intensity may
result in improved adaptation if apparent across a training block, evidence
examining the effect of chronic supplementation for this purpose in the most
part demonstrates this not to be the case.[124 ]
[125 ] Although a point of
contention, there is a long-standing paradigm that regular caffeine
consumption may evoke habituation to its effects. However, this idea has
been somewhat discredited based on the results of a recent
meta-analysis,[126 ] although
conclusions are largely drawn from comparing studies evaluating acute
effects in participants with different habitual caffeine consumption levels.
Studies examining chronic caffeine supplementation (i.e., repeated doses in
the same population) on athletic and cognitive function are limited, but
there is some evidence to suggest a reduction, and in some cases, a blunted
response.[127 ]
[128 ]
Practitioners should also be mindful that caffeine may mask the time course
of recovery assessed via objective evaluation of neuromuscular fatigue,[129 ] as well as influence outcomes
in physical performance profiling assessments. Data from a survey of
professional soccer clubs in the top four English leagues indicates a high
prevalence of caffeine use prior to training,[11 ] where from a practitioner
perspective there appears to be little advantage to administering caffeine
to enhance training performance. However, given that performance in training
may influence selection, caffeine supplementation may be perceived as
desirable from an athlete’s perspective.
Monitor Adverse Effects: Athletes and practitioners should be aware
of the impact of caffeine on sleep which may exacerbate detrimental effects
on sleep quality that occur post-evening match play [130 ] and as a result of
international travel.[131 ] In
fact, insomnia was the most commonly reported side effect from caffeine
supplementation in professional soccer.[11 ] The effects of caffeine on sleep are well documented, with
reduced total sleep time, increased time it takes to fall asleep, amount of
time awake during the night, light sleep, and decreased deep sleep duration
and proportion all reported.[132 ]
Specifically, recent evidence demonstrates that for moderate users, 400 mg
can result in sleep disruption when consumed within 12 hours of bedtime,
with adverse effects increasing closer to bedtime.[133 ] However, such effects may not
be prevalent when lower amounts (100 mg) are consumed.[133 ] Importantly, reduced sleep
has been shown to impair processes involved with exercise recovery, reduce
mental wellbeing, as well as physical and cognitive functions.[134 ]
[135 ]
[136 ]
[137 ] Although caffeine has been
shown to be effective in mitigating impaired physical and cognitive function
as well as sport specific skill execution as a result of sleep
deprivation,[138 ]
[139 ] the impact of caffeine on
sleep should be monitored, particularly during intense periods of the
season.
It is also important to be aware of other side effects of caffeine that can
occur relatively acutely and, in the days, following consumption. Side
effects specific to caffeine consumption in professional and amateur
athletes have been recently summarised, where in addition to insomnia,
headaches, anxiety, irritability, and gastrointestinal problems have been
shown to occur.[112 ] The
incidence of such effects is relatively low following exposure to doses ≤3
mg kg−1 and increase with quantity consumed. As such, it is
important to monitor adverse effects and adapt the supplementation strategy
accordingly.
There has also been recent interest in health effects associated with
caffeine consumption. Evidence suggests that ≤400 mg/day in adults is not
associated with overt, adverse effects,[140 ] and within typical levels caffeine may evoke health
benefits.[141 ] That said
caffeine supplementation should not be recommended for athletes with
cardiovascular illness, unmedicated hypertension or isolated systolic
hypertension, sleep or anxiety disorders, those prescribed certain
medications, and individuals with low bone mineral density.[142 ]
[143 ]
[144 ]
[145 ] This is not an exhaustive
list and should be considered based on appropriate health screening, and
where relevant, advice sought from a medical professional. Although ≤2.5 mg
kg−1 per day in children and adolescents is suggested to not
result in adverse health effects,[140 ] caffeine supplementation is not recommended for these age
groups.
Genetic Testing May Be Useful, but Evidence is Not Robust: Genetic
testing is becoming more commercialised and easily accessible,[146 ] with a specific market for
elite sports clubs. A survey of UK elite athletes and sports staff indicates
a willingness to engage in genetic testing to improve sports
performance,[147 ] an opinion
shared by key stakeholders in professional soccer.[148 ] Whilst genetic testing for
ADORA2A, HTR2A and particularly CYP1A2 polymorphisms may offer some insight
that may inform caffeine susceptibility and supplementation strategy,
evidence is not conclusive and the influence on the integrating facets
important to soccer match play has not yet been considered which currently
preclude the cost/benefit of this analysis.
Time of Day Effects: Athletes and practitioners should also be aware
that caffeine effects may be influenced by time of day. Across several
facets of physical function, performance in the afternoon has been
demonstrated to be superior to that in the morning. Caffeine may be most
effective in alleviating the morning performance decline, with a number of
studies indicating benefits of acute ingestion are reduced or not prevalent
in the afternoon when compared to the morning.[42 ]
[149 ]
[150 ]
[151 ] Whilst the mechanism
unpinning this has not been fully elucidated, this difference is unlikely
attributable to time-of-day differences in plasma concentration which have
been shown to be similar in the morning and afternoon.[149 ] One explanation may be the
action of Cytochrome P450 1A2, the enzyme responsible for caffeine
metabolism, which has been suggested to have higher activity directly after
waking compared to the rest of the day.[150 ] However, it is important to note that these studies fail to
consider the complex demands of soccer match play and interindividual
responses. Given this evidence, it is advisable to determine the
effectiveness of the selected dosing strategy for individuals at different
times of the day, given the potential adverse effects of caffeine on sleep.
Such work is particularly important given that soccer match kick-off times
often range from midday to evening. As such, the potential benefits of
caffeine supplementation may need to be evaluated on a game-by-game
basis.
Co-ingestion With Other Ergogenic Aids
: Research evaluating
the synergistic effect of caffeine and co-ingestion with other ergogenic
aids is sparse [67 ] and available
evidence fails to robustly support an additive response. Bicarbonate loading
(extracellular buffer) is effective in inducing improved anaerobic
performance,[152 ] but in a
recent review, only one (specific to judo performance) out of eight studies
demonstrated and additive effect of co-ingestion with caffeine.[153 ] There is also limited support
for additive effects of co-ingestion with creatine [154 ] and nitrate/beetroot
juice.[155 ] In fact, caffeine
ingested with bicarbonate or creatine may increase gastrointestinal
discomfort [156 ]
[157 ] which may impair performance,
and in the case of creatine, caffeine may hinder the loading phase.[158 ] There is evidence to suggest
that co-ingestion of caffeine in combination with a 6–9% of carbohydrate
solution may incur synergistic performance benefits or attenuate the effects
of fatigue,[159 ]
[160 ]
[161 ]
[162 ] however, the application to
soccer is yet to be elucidated. Collectively, the evidence indicates that
the combination of performance-enhancing supplements needs to be carefully
considered and should be advised and administered by professionals.
