Wake up and smell the coffee: caffeine supplementation and exercise performance—an umbrella review of 21 published meta-analyses

GrgicJ, etal. Br J Sports Med 2019;0:1–9. doi:10.1136/bjsports-2018-100278

Wake up and smell the coffee: caffeine

supplementation and exercise performance—an

umbrella review of 21 publishedmeta-analyses

Jozo Grgic,

1

Ivana Grgic,

Craig Pickering,

3,4

Brad J Schoenfeld,

David J Bishop,

1,6

Zeljko Pedisic

1

Review

To cite: GrgicJ, GrgicI,

PickeringC, etal.

Br J Sports Med Epub ahead

of print: [please include Day

Month Year]. doi:10.1136/

bjsports-2018-100278

► Additional material is

published online only. To view

please visit the journal online

(http:// dx. doi. org/ 10. 1136/

bjsports- 2018- 100278).

Institute for Health and Sport

(IHES), Victoria University,

Melbourne, Australia

County Hospital

Schrobenhausen,

Schrobenhausen, Germany

Institute of Coaching and

Performance, School of Sport

and Wellbeing, University of

Central Lancashire, Preston, UK

Exercise and Nutritional

Genomics Research Centre,

DNAFit Ltd, London, UK

Department of Health Sciences,

Lehman College, Bronx, USA

School of Medical and

Health Sciences, Edith Cowan

University, Joondalup, Australia

Correspondence to

JozoGrgic, Institute for Health

and Sport (IHES), Victoria

University, Melbourne, Australia;

jozo. grgic@ live. vu. edu. au

Accepted 10 March 2019

employer(s)) 2019. No

commercial re-use. See rights

and permissions. Published

by BMJ.

ABSTRACT

Objective To systematically review, summarise and

appraise ﬁndings of published meta-analyses that

examined the effects of caffeine on exercise performance.

Design Umbrella review.

Data sources Twelve databases.

Eligibility criteria for selecting studies Meta-

analyses that examined the effects of caffeine ingestion

on exercise performance.

Results Eleven reviews (with a total of 21 meta-

analyses) were included, all being of moderate or high

methodological quality (assessed using the Assessing

the Methodological Quality of Systematic Reviews 2

checklist). In the meta-analyses, caffeine was ergogenic

for aerobic endurance, muscle strength, muscle

endurance, power, jumping performance and exercise

speed. However, not all analyses provided a deﬁnite

direction for the effect of caffeine when considering

the 95% prediction interval. Using the Grading of

Recommendations Assessment, Development and

Evaluation criteria the quality of evidence was generally

categorised as moderate (with some low to very low

quality of evidence). Most individual studies included

in the published meta-analyses were conducted among

young men.

Summary/conclusion Synthesis of the currently

available meta-analyses suggest that caffeine ingestion

improves exercise performance in a broad range of

exercise tasks. Ergogenic effects of caffeine on muscle

endurance, muscle strength, anaerobic power and

aerobic endurance were substantiated by moderate

quality of evidence coming from moderate-to-high

quality systematic reviews. For other outcomes, we found

moderate quality reviews that presented evidence of

very low or low quality. It seems that the magnitude of

the effect of caffeine is generally greater for aerobic as

compared with anaerobic exercise. More primary studies

should be conducted among women, middle-aged and

older adults to improve the generalisability of these

ﬁndings.

INTRODUCTION

In 2018, the IOC published a consensus statement

regarding the effects of dietary supplements on

exercise performance of athletes.

The consensus

statement placed meta-analyses at the top of the

evidence pyramid.

In sports nutrition research,

meta-analyses provide a method of pooling avail-

able primary studies exploring the efficacy of a given

supplement on a specific outcome (eg, performance

of an exercise test). As such, meta-analyses are used

to support establishing evidence-based guidelines

and decision making for the effective prescription

of nutritional supplements and ergogenic aids.

One supplement with a long history of use for

its ergogenic effects on performance is caffeine.

Caffeine ingestion is highly prevalent among

athletes, especially since 2004, when it was

removed from the World Anti-Doing Agency list

of within-competition banned substances.

For

example, 74% of urine samples collected from

2004 to 2008 and analysed as a part of doping

control contained caffeine.

Given inconsistent

evidence in the primary research that examined the

effects of caffeine on exercise performance, several

research groups explored this area using meta-an-

alytical methods.

4–15

While these meta-analyses

generally report ergogenic effects of caffeine on

exercise performance, even adequately conducted

meta-analyses tend to focus on the ergogenic effects

of caffeine within just a single performance domain.

As an illustration, Grgic and Pickering

only exam-

ined the effects of caffeine ingestion on isokinetic

peak torque.

Given that each meta-analysis is typically focused

only on a specific aspect of exercise performance, it

is challenging to: (1) compare the effects of caffeine

ingestion on different performance domains; (2)

comparatively assess the availability and strength

of evidence for different performance domains;

(3) establish comprehensive recommendations

on the use of caffeine in sports and exercise; and

(4) provide overall recommendations for future

research on the ergogenic effects of caffeine on

exercise performance. Such recommendations may

increase the uptake of evidence-based findings in

the context of supplement prescription and guide

future research in this area.

Consistency of meta-analytical findings is often

lacking; even meta-analyses that have examined

the same outcome may produce conflicting find-

ings. For instance, Gonçalves Ribeiro et al

did not

observe significant effects of caffeine ingestion on

power. In contrast, a subsequent meta-analysis by

Grgic

reported that caffeine ingestion is ergogenic

for this outcome. Such conflicting findings hinder

firm evidence-based conclusions from individual

meta-analyses. Ultimately, the methods employed

in a specific meta-analysis (eg, the number of data-

bases searched, the comprehensiveness of the search

syntax and the methods used for analysing the data)

determine the robustness of the pooled results. For

example, a meta-analysis on the effects of caffeine

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Review

supplementation on power conducted by Gonçalves Ribeiro et

included only studies that were published between January

2010 and December 2015. Due to these restrictions, studies

published before 2010 were excluded from consideration, and

the authors provided no rationale for their approach. Only four

studies that assessed power during Wingate tests were included

in their review, and no significant pooled effects were found.

Grgic

conducted a similar meta-analysis without any restric-

tions regarding the year of publication; this analysis included

16 studies and reported significant improvements in both mean

and peak power on the Wingate test following caffeine ingestion.

One proposed method to overcome some of the above,

and other, potential limitations of meta-analyses is to perform

umbrella reviews.

Umbrella reviews (ie, reviews that include

the syntheses and appraisal of existing systematic reviews and

meta-analyses) provide a comprehensive view of the evidence

landscape on a given topic because they encompass larger scale

of evidence.

Such reviews help us to understand the current

strengths and limitations of the entire body of evidence by

comparing and contrasting findings from the entirety of the

published data. Such a treatise on the effects of caffeine on exer-

cise may be a useful resource for researchers, sports nutritionists,

athletes, coaches and others interested in the ergogenic effects

of caffeine on acute exercise performance. To date, there are no

published umbrella reviews focusing on the effects of caffeine on

exercise performance.

The aim of the present paper is threefold: (1) to systematically

review available meta-analytical evidence that has examined the

effects of caffeine on exercise performance; (2) to addresses the

quality, strengths and limitations of the meta-analytical evidence;

and (3) to identify current gaps in the literature and make key

suggestions for future research.

METHODS

Search strategy

This review followed the guidelines set forth by Aromataris and

colleagues.

We systematically searched through 12 different

databases, including: Academic Search Premier, AUSPORT,

CINAHL, Cochrane Library, ERIC, Health Source: Nursing/

Academic Edition, MasterFILE Premier, PsycINFO, PubMed/

MEDLINE, Scopus, SPORTDiscus and Web of Science. The

databases were searched from the inception of indexing until

24 September 2018 using the following search syntax: caffeine

AND (meta-an* OR ‘systematic review’) AND (exercise OR

training OR muscle OR ‘physical performance’). The search

syntax for each database is provided in online supplementary

table S1. Quotation marks and the wildcard symbol were used

to narrow down the search. In each full-text that was read, we

also screened the reference list as a part of a secondary search.

The search was carried out independently by two authors

(JG and IG) to prevent any selection bias. The authors inde-

pendently examined the titles, abstracts and, when applicable,

the full-texts of the identified publications. On examination, the

authors compared their lists of included and excluded papers;

any disagreements were resolved by discussion and agreement

between the authors.

Inclusion criteria

We included reviews coupled with a meta-analysis that examined

the acute effects of caffeine ingestion on any exercise perfor-

mance-related outcome. Both peer-reviewed and conference

papers published in English or other languages were considered.

Meta-analyses that included studies that combined caffeine with

other ergogenic compounds, such as taurine, were excluded as

they do not allow for the differentiation of the effects between

the compounds. However, meta-analyses that included studies

comparing caffeine and carbohydrate ingestion versus caffeine

alone were included as long as the effect of caffeine could be

isolated (ie, two solutions were given to the participants—one

with caffeine and one without). As reported by the Partici-

pant-Intervention-Comparison-Outcome (PICO) process, the

following criteria were followed:

Participants

Apparently healthy individuals of both sexes and all ages.

Interventions

Any acute study examining the effects of caffeine ingestion on

exercise performance.

Comparison group

Placebo (provided that the effects of caffeine could be isolated).

Outcome measures

Any form of exercise performance.

Data extraction

The following data were extracted from the included meta-anal-

yses: (1) the list of authors and year of publication; (2) the

number and type of studies included in the meta-analysis; (3)

the pooled number of participants; (4) the type of exercise test

that was evaluated; (5) the pooled effect size with the 95% CI;

(6) p values; and (7) percent changes and I

values. The same

two authors that carried out searches also conducted the data

extraction process. All data were tabulated to a spreadsheet

predefined for this review. After data extraction, the spread-

sheets were cross-checked between the authors for accuracy.

Methodological quality evaluation

The methodological quality of the included meta-analyses

was assessed using the validated Assessing the Methodolog-

ical Quality of Systematic Reviews 2 (AMSTAR 2) checklist.

Two reviewers (JG and IG) independently assessed the meth-

odological quality of the included reviews using the AMSTAR

2 checklist. This checklist contains 16 items that include ques-

tions regarding the use of the PICO description as a part of the

inclusion criteria, the a priori registration of the review design,

the comprehensiveness of the literature search, the number of

authors that performed that search and data extraction, the

description of included studies, the assessment of the quality of

the included primary studies, reporting of sources of funding in

the primary studies, the use of appropriate statistical methods,

assessments of heterogeneity in the meta-analyses and reporting

of the potential conflicts of interest. Full details on the checklist

can be found in the paper by Shea et al.

Each item on this

checklist is answered with a ‘yes’, ‘no’, ‘cannot answer’ or ‘not

applicable’. Out of these possible answers, only the ‘yes’ answer

counts as a point in the total score for the assessed review. Based

on the summary point scores, the meta-analyses were catego-

rised as high quality (at least 80% of the items were satisfied),

moderate quality (between 40% and 80% of the items were

satisfied) or low quality (less than 40% of the items were satis-

fied), as performed previously.

18 19

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Review

Quality of evidence

To assess the quality of evidence we used the modified Grading

of Recommendations Assessment, Development and Evaluation

(GRADE) principles.

For the purpose of this review, we exam-

ined the following GRADE aspects: (1) risk of bias (determined

by the quality of the primary studies, as assessed in the orig-

inal reviews); (2) inconsistency (determined by variables such as

the variation in the effects across the included studies and the

overlap of the 95% CIs between the studies); (3) indirectness

(determined by the generalisability of the findings while consid-

ering the study populations included in the primary research);

(4) imprecision (determined by the total sample size in the anal-

ysis and the width of the 95% CI of the pooled effect size); and

(5) publication bias (determined if the effect size of the largest

study in each analysis was smaller than the pooled estimate from

the meta-analysis and by examining the asymmetry of the funnel

plot). Based on these criteria, the meta-analytical evidence was

classified as high, moderate, low or very low. The GRADE assess-

ment was conducted independently by two authors (JG and IG),

with discussion and agreement for any differences.

Prediction interval(PI)

Using the number of included studies, the pooled standardised

mean difference, the upper limit of the 95% CI and the

tau-squared values (from each analysis), we calculated 95% PI

for all included meta-analyses (spreadsheet available at: https://

www. meta- analysis. com/ pages/ prediction. php). The 95% PI

represents the range in which the effect size of a future study

conducted on the topic will most likely lie. If the tau-squared

values were not provided in the meta-analysis, these data were

either requested from the authors or recalculated based on the

data presented in the included studies.

RESULTS

Search results

The initial literature search identified 405 search records. Out

of that pool of search results, 18 full-texts were read. Seven

reviews were excluded after reading the full-texts.

7 21–26

The

reasons for their exclusion are provided in online supplementary

table S2. Eleven reviews (with a total of 21 meta-analyses) were

included in this umbrella review.

4–6 8–15

All included reviews

were published in peer-reviewed journals. The flow diagram of

the search process can be found in figure 1.

Characteristics of the meta-analyses

The included meta-analyses were published between the years

2004 and 2018. The median number of studies included per

meta-analysis was 19 (range: 2–44). The prevalence of primary

studies with male-only samples ranged from 72% to 100% across

the meta-analyses. The assessed outcomes in the meta-analyses

included: maximal speed during running, cycling or rowing

(defined as the maximal achieved speed in exercise performance

tests lasting from 45 s to 8 min that had either a fixed duration or

a fixed distance), aerobic endurance (assessed by time-to-exhaus-

tion, time trial and graded exercise tests), peak and mean power

in the 30 s Wingate test, peak torque in an isokinetic strength

assessment, strength in the one repetition maximum (1RM)

test, height in a vertical jump test, muscular endurance (assessed

both using isometric and dynamic tests), duration of time trial

or power during a time trial and maximal voluntary strength

(assessed by pooling isometric, isokinetic and 1RM tests). A

summary of the included meta-analyses can be found in table 1.

Effects of caffeine on exercise performance

The effects of caffeine ingestion on aerobic endurance were

examined in five reviews with a total of nine meta-analyses; the

majority reported ergogenic effects of caffeine (effect size range:

0.22–0.61). The range of included primary studies was from 2

to 44 (average: 23 studies). Doherty and Smith

did not report

significant effects of caffeine on aerobic endurance performance

when considering only graded exercise tests and including six

studies. Gonçalves Ribeiro et al

did not report significant effects

of caffeine on this outcome (analysed using maximum running

distance tests) while including two studies. The 95% PIs for

these analyses are reported in table 1.

Four analyses examined the effects of caffeine on different

measures of muscle strength. In three of these analyses, an ergo-

genic effect of caffeine was observed (effect size range: 0.16–

0.20). The range of included studies was from 3 to 27 (average:

13 studies). In the analysis by Grgic and Pickering,

the 95%

PI was from −0.17 to 0.49. In the analysis by Grgic et al,

the

95% PI was from 0.02 to 0.39, while in Warren et al’s

analysis,

the 95% PI was from −0.18 to 0.56. The 95% PI in the analysis

by Polito et al

(this analysis did not report significant effects of

caffeine on 1RM strength) was from −0.09 to 0.27.

Two analyses examined the effects of caffeine on muscular

endurance. Both reported ergogenic effects of caffeine (effect

size range: 0.28–0.38). Polito et al

included 16, while Warren

et al

included 23 studies. The 95% PI was from 0.02 to 0.74

and from −0.29 to 0.85 for the analyses by Polito et al

and

Warren et al,

respectively.

Anaerobic power was examined in three analyses. In a

meta-analysis including four studies, Gonçalves Ribeiro et al

did not report significant ergogenic effects of caffeine on power.

The 95% PI in this analysis was from −0.65 to 1.01. In an anal-

ysis including 16 studies, Grgic

reported ergogenic effects of

caffeine on both mean and peak power (effect size range: 0.18

to 0.27). In the analysis for peak power, the 95% PI was from

−0.35 to 0.89, while in the analysis for mean power, the 95% PI

was from 0.04 to 0.32.

One meta-analysis, including 10 studies, examined the effects

of caffeine on vertical jump height and reported an ergogenic

effect of caffeine (effect size: 0.17).

The 95% PI was from

−0.03 to 0.37.

Figure 1 Flow diagram of the search process.

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Review

Table 1 Summary of the meta-analyses included in the review

Reference Included studies

Number of included studies

(sample size) Performance test(s)

Effect size (95% CI) and

pvalue* 95% PI Percent change I

(95% CI)

Christensen et al

Single or double-

blind crossover study

designs.

Nine studies (n=97). Speed during running, cycling or

rowing.†

0.41 (0.15to 0.68); p=0.002. 0.41 (0.09to 0.73). ~2 0% (0%to 35%).

Conger et al

Crossover study

designs.

Carbohydrate versus

caffeine+carbohydrate: 21

studies (n=333).

Caffeine versus placebo: 36

studies (n=352).

Any form of aerobic exercise

if it was 10 min or longer in

duration.

Carbohydrate versus

caffeine+carbohydrate: 0.26

(0.15to 0.38); p<0.001.

Caffeine versus placebo: 0.51

(0.41to 0.62); p<0.001.

Carbohydrate versus

caffeine+carbohydrate: 0.26

(−0.18to0.70).

Caffeine versus placebo: 0.51

(−0.06to 1.08).

Carbohydrate versus

caffeine+carbohydrate:+6%.

Caffeine versus placebo:+16%.

Carbohydrate versus

caffeine+carbohydrate: 7% (0%to

42%).

Caffeine versus placebo: 24% (0%to

50%).

Doherty and Smith

Double-blind

crossover study

designs.

24 studies (n=217) for aerobic

exercise, 6 studies for graded

exercise tests (n=62) and 12

studies for short-term high-

intensity exercise (n=127).

Exercise testing divided to

aerobic exercise, graded exercise

tests and short-term high-

intensity exercise.

Aerobic exercise: 0.63 (0.50to

0.77).

Graded exercise tests: 0.17

(−0.02to 0.36).

Short-term high-intensity

exercise: 0.16 (0.01to 0.31).‡

Aerobic exercise: 0.63 (0.06to 1.20).

Graded exercise tests: 0.17 (−0.09to

0.44).

Short-term high-intensity exercise:

0.16 (−0.18to 0.50).

+12% across all exercise tests. Aerobic exercise: 4% (0%to 52%).

Graded exercise tests: 0% (0%to 42%).

Short-term high-intensity exercise: 0%

(0%to 24%).

Gonçalves Ribeiro

et al

Crossover study

designs.

Seven studies (n=91) for

timetrial duration, four

studies (n=52) for power

and two studies (n=31) for

running distance.

Timetrial duration, power and

running distance.

Timetrial duration: 0.40 (0.11to

0.70); p=0.007.

Power: 0.18 (−0.21to 0.56);

p=0.366.

Running distance: 0.38

(−0.13to 0.88); p=0.142.

Timetrial duration: 0.40 (0.01to

0.79).

Power: 0.18 (−0.65, 1.01).

Running distance: unable to

determine.

Timetrial duration:+2%.

Power:+4%.

Running distance:+11%.

Time trial duration: 0% (0%to 19%).

Power: 0% (0%to 43%).

Running distance: 0% (unable to

determine).

Grgic

Crossover study

designs.

16 studies (n=246). Peak and mean power in the

30 s Wingate test.

Peak power: 0.27 (0.08to 0.47);

p=0.006.

Mean power: 0.18 (0.05to

0.31); p=0.005.

Peak power: 0.27 (−0.35to 0.89).

Mean power: 0.18 (0.04to 0.32).

Peak power:+4%.

Mean power:+3%.

Peak power: 7% (0%to 44%).

Mean power: 0% (0%to 28%).

Grgic and Pickering

Crossover study

designs.

10 studies (n=133). Peak torque in an isokinetic

strength assessment.

0.16 (0.06to 0.26); p=0.003. 0.16 (−0.17to 0.49). +5%. 15% (0%to 57%).

Grgic et al

Single or double-

blind crossover study

designs.

10 studies (n=149) for

strength and 10 studies

(n=145) for vertical jump.

Strength in the 1RM test and

height in a vertical jump test.

1RM: 0.20 (0.03to 0.36);

p=0.023.

Vertical jump: 0.17 (0.00to

0.34); p=0.047.

1RM: 0.20 (0.02to 0.39).

Vertical jump: 0.17 (−0.03, 0.37).

1RM:+3%.

Vertical jump:+3%.

1RM: 0% (0%to 37%).

Vertical jump: 0% (0%to 47%).

Polito et al

Double-blind

crossover study

designs.

16 studies (n=239) for

muscular endurance and

three studies (n=46) for the

1RM test.

Muscular endurance (assessed

by repetitions to fatigue) and

strength in the 1RM test.

Muscular endurance: 0.38

(0.29to 0.48); p<0.001.

1RM: 0.09 (−0.07to 0.25);

p=0.25.

Muscular endurance: 0.38 (0.02to

0.74).

1RM: 0.09 (−0.09to 0.27).

Muscular endurance:+6%.

1RM:+2%.

Muscular endurance: 24% (0to 56%).

1RM: 0% (0%to 43%).

Shen et al

Crossover study

designs.

40 studies (n=582). Any form of aerobic exercise if it

was 5 min or longer in duration.

0.33 (0.21to 0.45).‡ 0.33 (0.21to 0.45). +3%. 0% (0%to 14%).

Southward et al

Crossover study

designs.

44 studies (n=639 for time

trial duration and n=350 for

timetrial power).

Duration of the time trial or

power during a time trial.

Time trial duration: 0.28 (0.17to

0.40); p<0.0001.

Timetrial power: 0.22 (0.07to

0.37); p=0.004.

Time trial duration: 0.28 (0.17to

0.40).

Time trial power: 0.22 (0.06to 0.38).

Time trial duration:+2%.

Time trial power:+3%.

Time trial duration: 0% (0%to −56%).

Time trial power: 0% (0%to 14%).

Warren et al

Crossover

andbetween

group study designs.

27 studies (n=576) for MVC

and 23 studies (n=388) for

muscular endurance.

MVC and muscular endurance. MVC: 0.19 (0.09to 0.29);

p<0.001.

Muscular endurance: 0.28

(0.14to 0.42); p<0.001.

MVC: 0.19 (−0.18to 0.56).

Muscular endurance: 0.28 (−0.29to

0.85).

MVC:+4%.

Muscular endurance:+14%.

MVC: 44% (13%to 65%).

Muscular endurance: 12% (0%to 46%).

*Positive effect sizes and percentages show favouring of caffeine over placebo.

†Deﬁned as the maximal achieved speed in exercise performance tests lasting from 45 s to 8 min that had either a ﬁxed duration or a ﬁxed distance.

‡Pvalues were not provided.

1RM,one repetition maximum;MVC, maximal voluntary contraction; PI, prediction interval.

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Review

One meta-analysis, included nine studies, examined speed

during running, cycling or rowing and reported ergogenic effects

of caffeine (effect size: 0.41).

The 95% PI was from 0.09 to

0.73.

One meta-analysis examined various forms of ‘short-term

high-intensity exercise’ while pooling the effects of caffeine

on: (1) time to exhaustion in various high-intensity short-term

cycling and running efforts; (2) mean power, peak power output

and total work during high-intensity short-term cycling; and (3)

time trial time during 2000 m rowing.

This analysis included 16

studies and reported ergogenic effects of caffeine of 0.16; the

95% PI was −0.18 to 0.50.

Besides the main analysis (presented in figure 2), several

reviews also conducted additional subgroup analyses (eg, for

trained vs untrained individuals, for upper vs lower body muscu-

lature) and these results are summarised in online supplementary

table S3.

Methodological quality evaluation

The methodological quality of the 11 included reviews is

summarised in table 2. The reviews scored from 44% to 88% of

the maximum 16 points. Three reviews were classified as being

of high quality, while eight were classified as being of moderate

methodological quality. None of the reviews were considered to

be of poor methodological quality. Several criteria on AMSTAR

2 checklist were under-reported in the analysed reviews: (1)

none provided an a priori design (ie, registration of the review

methods in advance); (2) in four and five analyses the number of

authors conducting the search and data extraction was not clear,

respectively; (3) the list of excluded studies was not provided in

any of the included reviews; and (4) sources of funding for the

studies included in a given review were discussed only in three

reviews.

Quality of the evidence

Based on the GRADE assessment, the included analyses were

considered as providing very low (3 meta-analyses), low (7

meta-analyses) or moderate quality of evidence (11 meta-anal-

yses). For risk of bias, several reviews did not assess the quality

of the included studies and thus were given ‘unclear’ on this

criterion. The meta-analyses were considered as not having

serious inconsistency but were considered as having serious indi-

rectness. The analyses were mostly considered as being ‘precise’

on the imprecision GRADE item. Finally, three meta-analyses

were considered as ‘strongly suspected’ on the publication bias

GRADE item. The results for each analysis are presented in

online supplementary table S4.

DISCUSSION

Based on the 11 included reviews, it can be concluded that

caffeine is ergogenic for different components of exercise perfor-

mance including aerobic endurance, muscle strength, muscle

endurance, power, jumping performance and exercise speed.

Ergogenic effects of caffeine on muscle endurance, muscle

strength, anaerobic power and aerobic endurance were substan-

tiated by moderate quality of evidence coming from moderate-

to-high quality systematic reviews (online supplementary table

S5). For other outcomes, we found moderate quality reviews

that presented evidence of very low or low quality. In addition,

Figure 2 Summary of the effect sizes, 95% CIs (presented in the black lines), and 95% prediction intervals (95% PIs; presented in the grey lines)

from the included meta-analyses. If there is no 95% PI presented, it was the same as the 95% CI.

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Review

not all analyses provided a definite direction for the effect of

caffeine when considering the 95% PI. Several important aspects

that refer to the generalisability of the meta-analytical findings

as well as the spread of summary effects need to be considered

when interpreting these findings from a practical standpoint.

Generalisability of the results

Based on the GRADE assessment for directness of evidence, the

included reviews were rated as having serious indirectness given

that the evidence is not direct enough to apply to all popula-

tions. Scrutiny of the meta-analyses included in this umbrella

review highlights that primary studies conducted among women

are lacking. Specifically, in all of the included meta-analyses,

72%–100% of the pooled sample participants were men. Women

may metabolise caffeine differently than men given that changes

in circulating steroid hormones during phases of the menstrual

cycle can impact caffeine elimination in women,

27 28

which might

also impact the ergogenic effect of caffeine on exercise perfor-

mance in this population. When conducting studies in women,

the differences in caffeine metabolism across the follicular and

luteal phase of the menstrual cycle may increase the complexity

of the study design, which might partially explain why studies in

this population are lacking. While there are studies conducted

in both sexes that suggest both men and women may experience

similar acute effects of caffeine ingestion on exercise perfor-

mance,

29 30

the generalisability of the meta-analytic findings is,

however, limited mostly to men.

The majority of the primary studies were conducted in young

individuals, and therefore, several meta-analyses are limited

exclusively to young individuals.

4 8 9 11–14

This may be relevant to

highlight given that in animal models, with ageing, there appears

to be a reduced ergogenic effect of caffeine.

Caffeine has been

shown to elicit positive effects on mood and cognitive perfor-

mance in older adults.

If caffeine also increases exercise perfor-

mance in older adults, it might also enhance performance during

activities of daily living in these individuals. This is particularly

important from a public health point of view, given that reduced

physical functioning (eg, in terms of reduced strength) may

impact the quality of life in this population group.

Although

some of the studies conducted in older adults showed an ergo-

genic effect of caffeine on exercise performance,

34 35

additional

studies that directly compare the effects of caffeine between

young versus older individuals are needed to explore if the

effects of caffeine differ between age groups.

Methodological quality

While the meta-analyses included in the present umbrella review

show that caffeine ingestion may indeed be ergogenic across a

large range of exercise tasks, some additional considerations may

help to improve future meta-analyses on this topic. Several of

the included reviews did not adhere to the Preferred Reporting

Items for Systematic Reviews and Meta-Analyses (PRISMA)

guidelines, which currently represent a widely accepted standard

for reporting meta-analyses. It should be taken into account that

the PRISMA guidelines were published in 2009, which is 5 years

after the review by Doherty and Smith.

Nonetheless, several

meta-analyses that did not follow the guidelines were published

following the release of the PRISMA statement.

4 5 8 15

None of the 11 reviewes registered their protocol for a review

and thus did not receive a point on item 2 of the AMSTAR 2

checklist. Protocols of systematic reviews can be registered in

the PROSPERO database. If registered, such protocols can help

reduce the risk of wasteful duplication of reviews by independent

Table 2 Results of the Assessing the Methodological Quality of Systematic Reviews 2 (AMSTAR 2) quality assessment

Reference

AMSTAR items

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 Score

Christensen et al

Yes No Yes Yes Yes Cannot answer No Yes Yes Yes Yes Yes Yes Yes No Yes 75% moderate

Conger et al

Yes No Yes Yes Cannot answer Cannot answer No Yes No No Yes Not applicable Not applicable Yes Yes Yes 50% moderate

Doherty and Smith

Yes No Yes Yes Cannot answer Cannot answer No Yes No No Yes Not applicable Not applicable Yes Yes No 44% moderate

Gonçalves Ribeiro et al

Yes No Yes No Yes Yes No Yes Yes No Yes Yes Yes Yes No No 63% moderate

Grgic

Yes No Yes Yes No No No Yes Yes No Yes Yes Yes Yes Yes Yes 69% moderate

Grgic and Pickering

Yes No Yes Yes Yes No No Yes Yes No Yes Yes Yes Yes Yes Yes 75% moderate

Grgic et al

Yes No Yes Yes Yes Yes No Yes Yes Yes Yes Yes Yes Yes Yes Yes 88% high

Polito et al

Yes No Yes Yes Yes Yes No Yes Yes No Yes Yes Yes Yes Yes Yes 81% high

Shen et al

Yes No Yes Yes Yes Yes No Yes Yes No Yes Yes Yes Yes Yes Yes 81% high

Southward et al

Yes No Yes Yes Cannot answer Cannot answer No Yes Yes No Yes Yes Yes Yes Yes Yes 69% moderate

Warren et al

Yes No Yes Yes Cannot answer Cannot answer No Yes Yes Yes Yes Yes Yes Yes Yes Yes 75% moderate

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research groups. However, the PROSPERO database is primarily

focused on health outcomes and not exercise performance. As

stated on their website, ‘PROSPERO includes protocol details

for systematic reviews relevant to health and social care, welfare,

public health, education, crime, justice, and international devel-

opment, where there is a health related outcome’. The authors

are not aware of any registries that focus on the publishing of

protocols for systematic reviews in the sport and exercise field.

Given that the number of published systematic reviews has

increased over the last years, the formation of such a register for

this line of research appears warranted.

Publication bias, as highlighted by Borenstein et al,

can

occur because studies that report higher (and significant) effect

sizes are more likely to be published than those with low or

non-significant effect sizes (ie, the file drawer problem).

Therefore, the inclusion of only published studies in a given

meta-analysis can lead to publication bias and may be a concern

for the validity of the results. Four meta-analyses included in

this umbrella review also examined unpublished literature in

the form of master’s theses and doctoral dissertations.

5 11 12 15

In the meta-analysis by Conger et al,

the effect size of the

unpublished studies was 0.13 (95% CI –0.08 to 0.33), while the

effect size of the published studies was 0.32 (95% CI 0.19 to

0.46). These results might indeed suggest that studies with

smaller effect sizes tend to remain unpublished and to avoid

publication bias future meta-analyses should consider including

unpublished results as well. The reviews that included unpub-

lished literature highlight that, in many cases, such unpublished

documents may be of equal or even greater methodological

quality as those found in peer-reviewed journals. The influence

of unpublished results can be examined by conducting a sensi-

tivity analysis in which the pooled results are inspected after

the exclusion of these studies. In this context, journal editors

and reviewers are also encouraged to facilitate greater accep-

tance and publication of studies with results that would appear

to be ‘less favourable’ (or statistically non-significant) to truly

progress this area of work.

The spread of summary effects

Based on the GRADE assessment of inconsistency, the reviews

were classified as not possessing serious inconsistencies. Indeed,

the effect sizes across individual studies indicate that the studies

rarely show a negative effect of caffeine supplementation on

exercise performance. The effects in the primary studies were

either positive or around the null value. In addition, the 95% CI

from the primary studies largely overlap.

One interesting aspect refers to the spread of summary

effects. Historically, caffeine ingestion has been suggested

to predominantly provide a performance-enhancing effect

on aerobic exercise performance.

As shown both here and

by others,

38 39

it is evident that caffeine ingestion enhances

performance in anaerobic exercise tasks as well. However, it is

possible that the magnitude of the effect of caffeine is greater

for aerobic as compared with anaerobic exercise. The effect

sizes for meta-analyses that focused on aerobic tests of perfor-

mance are generally higher than those that used anaerobic tests

of performance (figure 2). Future studies may consider inves-

tigating the effects of caffeine ingestion on both aerobic and

anaerobic tests of performance in the same sample to further

explore whether the effect size magnitude differs between

tasks that rely on predominantly oxidative or predominantly

non-oxidative energy pathways.

The optimal dose of caffeine

While the included meta-analyses report that caffeine ingestion

may be ergogenic across a broad range of exercise activities,

the ‘optimal’ dose of caffeine remains elusive. If we observe the

dosage used in the primary studies (across all of the included

meta-analyses), it is clear that most of the studies used a single

dose of caffeine (most commonly 6 mg/kg). Warren et al

exam-

ined the dose–response effects between the amount of caffeine

ingested and its ergogenic effect on muscular endurance. This

analysis found that for an increase in caffeine dose by 1 mg/

kg, the relative effect size for muscular endurance increased by

0.10. However, these results should be interpreted with caution

given that the dosage explained only 16% of the between-study

variance. To explore the optimal doses of caffeine for exercise

performance future dose–response studies are needed. The

optimal doses may differ based on the source of caffeine,

exer-

cise test,

41–44

muscle action type

and between individuals,

46 47

which needs to be taken into account when prescribing caffeine

supplementation.

Is coffee a good way to consume caffeine?

While the results of this umbrella review suggest that caffeine is

ergogenic in the majority of exercise situations, it is important to

keep in mind that the majority of studies use caffeine anhydrous

(highly concentrated caffeine powder) as the caffeine source,

with a smaller group of studies utilising caffeine-containing

supplements such as energy drinks, bars and gels. Coffee—the

most widely used method of caffeine ingestion globally—is rela-

tively underexplored as a pre-exercise performance enhancer.

Hodgson and colleagues

reported that caffeine anhydrous and

coffee, standardised to deliver a caffeine dose of 5 mg/kg, were

similarly effective in enhancing aerobic endurance performance.

Similar results have been reported for resistance and sprint exer-

cise.

49 50

As a result, coffee is likely an effective ergogenic aid; the

main issue here is a practical one. To be ergogenic, the caffeine

dose from coffee likely has to fall within the 3–6 mg/kg range.

The caffeine dose received from coffee depends on many factors,

including bean type, preparation method and size of the cup;

there are large differences in caffeine concentrations between

different coffee brands and flavours and within the same brand

across time.

51–53

As a result, while the ‘average’ cup of coffee

contains around 100 mg of caffeine—meaning that two cups

would deliver ~200 mg, representing ~3 mg/kg for a 70 kg indi-

vidual—this amount is hard to quantify in the specific cup of

coffee that person is drinking.

While keeping those caveats in

mind, as a broad rule of thumb, two cups of coffee, consumed

around 60 min before exercise, should exert an ergogenic effect

in most individuals.

Suggestions for future research

Subgroup analyses conducted in the included meta-analyses

in most cases are based on a low number of included studies

(or effect sizes), which limits any definitive conclusions. Many

areas remain unclear when it comes to caffeine supplementation.

Some of these areas include:

The effects of caffeine habituation

Does habituation to caffeine reduce (or eliminate) its ergogenic

effect following acute caffeine supplementation? The included

meta-analyses could not explore the differences in effects

between low and high habitual caffeine users as currently there

is a lack of primary studies investigating this topic. The body of

research is limited and equivocal, with some studies suggesting

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Review

that low habitual caffeine users experience greater ergogenic

effects than the high habitual users, while others report similar

acute responses to caffeine ingestion in terms of exercise perfor-

mance regardless of habituation.

54 55

Pickering and Kiely

suggested the possibility that the response may be dose depen-

dent, which may be an interesting aspect to explore in future

studies.

Optimal timing of caffeine ingestion

Most studies provided caffeine supplementation 60 min pre

exercise; therefore, it remains unclear if smaller/greater effects

of caffeine would be observed with shorter/longer wait time

from ingestion to exercise. This area needs further exploration,

and there is potential that different timing may be required for

different doses

or genotypes.

Effects of different sources of caffeine

Most of the included studies in the meta-analyses used the

capsule form of caffeine. It remains unclear if comparable

results can also be seen with alternate sources of caffeine, such

as caffeine mouth rinsing, caffeine gels and chewing gums. A

detailed review on the topic of alternate forms of caffeine can be

found elsewhere.

Effects of caffeine among trained versus untrained individuals

While it has been suggested that trained individuals might

respond better to caffeine ingestion, the current evidence on

this topic is scarce and conflicting.

59–61

The meta-analyses that

have tried to explore this matter were commonly performed

on a limited number of studies. For example, Grgic et al

only

included seven and four studies for their subgroup analysis of

the effects of caffeine among trained and untrained individuals,

respectively. The majority of the studies pooled in the mentioned

subgroup analysis only examined the effects of caffeine on

strength performance in either trained or untrained individ-

uals. The only study included in the review by Grgic et al

that

directly compared the effects of caffeine between trained and

untrained individuals reported ergogenic effects of caffeine in

untrained but not in trained individuals.

These results are in

contrast to the common belief about greater responsiveness to

caffeine in trained individuals. Future work is needed on this

topic (for additional discussion on this topic see the reviews by

Tallis et al

and Burke

Chronic effects of caffeine on exercise adaptations

While many studies have examined the acute effects of

caffeine supplementation on exercise performance, it remains

unclear whether these acute increases in performance also

impact chronic adaptations to training and in which way. Ulti-

mately, individuals interested in the acute performance-en-

hancing effects of caffeine are likely candidates to continue

to use caffeine supplementation over the long term. Aspects

of long-term supplementation that refer to habituation and

to the attenuation of caffeine’s effects, as well as the effects

of chronic caffeine supplementation on training adaptations,

need to be further investigated.

We hope that highlighting some of these areas will help

catalyse future high-quality research.

CONCLUSIONS

Caffeine ingestion may be ergogenic for a broad range of exer-

cise tasks. The performance-enhancing effects of caffeine on:

(A) muscle endurance, (B) muscle strength, (C) anaerobic power

and (D) aerobic endurance were supported by moderate-to-high

quality reviews and moderate quality of evidence. For other

outcomes, even though the reviews were of moderate quality,

the evidence was of very low or low quality. The magnitude of

the effect of caffeine is generally greater for aerobic as compared

with anaerobic exercise.

What is already known

► Given the often narrow scope (ie, focus on only one test of

performance) of a meta-analysis, the credibility of this type of

evidence for the effects of caffeine on exercise performance

across the totality of the evidence is unclear.

► Caffeine is ergogenic for exercise performance; it remains

unclear if the effect of caffeine differs between various

exercise tests/tasks.

What are the new ﬁndings

► Of the 11 included reviews, all report signiﬁcant

improvements in at least one component of exercise

performance following caffeine ingestion with the effect size

magnitude ranging from small to moderate.

► The effect sizes for meta-analyses that focused on aerobic

tests of performance are generally higher than those that

used anaerobic tests of performance.

► The meta-analytic ﬁndings apply mostly to men and young

individuals.

► Caffeine ingestion (as studied here) does not translate readily

to ‘coffee’—the drink, but begs an obvious question. As a

broad rule of thumb, two cups of coffee, consumed around

60 min before exercise, should exert an ergogenic effect in

most individuals.

Contributors JG and ZP conceived the idea for thereview. JG and IG conducted

the study selection the data extraction and qualityassessment. ZP contributed to

data extraction and conceptualisation of qualityassessment. JG drafted the initial

manuscript. CP, ZP, IG, BJS, and DJBcontributed to writing the manuscript.

Funding The authors have not declared a speciﬁc grant for this research from any

funding agency in the public, commercial or not-for-proﬁt sectors.

Competing interests None declared.

Patient consent for publication Not required.

Provenance and peer review Not commissioned; externally peer reviewed.

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