Inconsistency in the Ergogenic Effect of Caffeine in Athletes Who Regularly Consume Caffeine: Is It Due to the Disparity in the Criteria That Defines Habitual Caffeine Intake?

nutrients

Discussion

Inconsistency in the Ergogenic Eﬀect of Caﬀeine in

Athletes Who Regularly Consume Caﬀeine: Is It Due

to the Disparity in the Criteria That Deﬁnes Habitual

Caﬀeine Intake?

Aleksandra Filip

, Michal Wilk

* , Michal Krzysztoﬁk

and Juan Del Coso

Institute of Sport Sciences, The Jerzy Kukuczka Academy of Physical Education in Katowice,

40-065 Katowice; Poland; a.ﬁ[email protected] (A.F.); m.krzysztoﬁ[email protected] (M.K.)

Centre for Sport Studies, Rey Juan Carlos University, 28942 Fuenlabrada, Spain; [email protected]

* Correspondence: [email protected]; Tel.: +48-32-2075100

Received: 1 April 2020; Accepted: 11 April 2020; Published: 15 April 2020



 

Abstract:

Caﬀeine is the most popular psychoactive substance in the world, and data suggests that it

is widely used by athletes before competition to enhance physical and mental performance.

The high

number of athletes that regularly use caﬀeine suggests the need to investigate the eﬀect of acute

caﬀeine ingestion in athletes habituated to caﬀeine. However, most of the studies supporting this

claim have used samples of athletes who do not consume caﬀeine on a regular basis, and with caﬀeine

intake withdrawal prior to the experiments. A search through 19 databases conducted on habitual

caﬀeine users was performed. The results of the studies regarding the ergogenic eﬀect of caﬀeine in

naïve vs. habitual caﬀeine consumers are contradictory. The diversity of results are likely associated

with the use of diﬀerent thresholds to categorize individuals as naïve or high caﬀeine consumers.

There are no valid and standardized methods to accurately estimate the amount of caﬀeine ingested

per day in athletes. We proposed a classiﬁcation of athletes that habitually consume caﬀeine by

using dietary questionnaires, and ultimately, to reduce the likelihood of discrepancies caused by the

improper qualiﬁcation of daily caﬀeine intake in studies directed at the assessment of acute caﬀeine

intake in habitual caﬀeine consumers.

Keywords: classiﬁcation; habituation; caﬀeine users; daily consumption; ergogenic aids

Nowadays, caﬀeine is the most popular psychoactive substance in the world and data on urine

caﬀeine concentration suggests that it is widely used by athletes before competition to enhance physical

and mental ﬁtness [

]. The research supporting the ergogenic eﬀects of acute caﬀeine intake in

a variety of sporting disciplines and exercise scenarios is extensive, and the International Olympic

Committee has recently classiﬁed caﬀeine as a substance that enhances performance based on this

strong scientiﬁc evidence [

]. However, most of the studies supporting this claim have used samples

of athletes who do not consume caﬀeine on a regular basis and with caﬀeine intake withdrawal

prior to the experiments, in an attempt to enhance the margin of action of this substance on physical

performance [

]. The high number of athletes that regularly use caﬀeine during training/competition

suggests the need to investigate the eﬀect of acute caﬀeine ingestion in athletes habituated to caﬀeine,

because the current guidelines for caﬀeine use in sport may not apply to these athletes. In addition,

it is

vital to determine the minimal dose that enhances performance and if the prevalence of side eﬀects

is substantially modiﬁed in athletes habituated to caﬀeine.

After oral ingestion, caﬀeine is rapidly absorbed, and due to its lipophilic nature, passes through all

biological membranes including the blood–brain barrier [

]. Furthermore, caﬀeine possesses the ability

Nutrients 2020, 12, 1087; doi:10.3390/nu12041087 www.mdpi.com/journal/nutrients

Nutrients 2020, 12, 1087 2 of 6

to inhibit adenosine A

and A

receptors, thus reducing the fatiguing eﬀects of this neurotransmitter

during exercise [

]. Due to the blockage of adenosine receptors, caﬀeine indirectly aﬀects the release

of norepinephrine, dopamine, acetylcholine, serotonin, among others, which ultimately results in

reducing pain, diminishing perceived exertion, and delaying fatigue [

]. However, in animal models,

it has been found that chronic intake of caﬀeine results in more newly created adenosine receptors that

partially reduce the blocking-action of caﬀeine on the central nervous system [

]. In fact, some reports

indicate that habitual caﬀeine intake may change the physiological and cognitive responses to acute

caﬀeine administration, which could negatively impact its ergogenic eﬀect [10,11].

Several cross-sectional studies have compared the ergogenic eﬀect of caﬀeine in naïve vs. habitual

caﬀeine consumers. The results of these studies are contradictory because either naïve caﬀeine users

beneﬁted from 3 to 6 mg/kg of caﬀeine to a similar extent as habitual caﬀeine consumers [

or naïve

consumers experienced a higher ergogenic eﬀect than habitual consumers [

]. The diversity

in the results are likely associated with the use of diverse thresholds to categorize individuals as naïve

or high caﬀeine consumers, along with diﬀerences in the exercise testing protocols. Unfortunately,

evidence from well-controlled situations, in which participants underwent a standardized protocol

of chronic intake of caﬀeine to create a potential tolerance to caﬀeine, is scarce and somewhat

contradictory. To date, it is known that caﬀeine exerts beneﬁts on aerobic and anaerobic exercise

after

20 days

of consecutive ingestion of 3 mg/kg/day [

]. However, the ergogenic eﬀects of caﬀeine

after

20 days

of consecutive ingestion have been found to be lower than the eﬀect found in the ﬁrst

day of ingestion when the participants are not habituated to caﬀeine. This suggests that there is a

progressive reduction in caﬀeine’s ergogenicity in both aerobic and anaerobic exercise along with

continuous ingestion. Interestingly, it has been found that the ergogenic eﬀect of caﬀeine may be totally

removed after 28 days of consecutive caﬀeine ingestion [

]. Despite the diﬀerences in the outcomes,

two investigations [

] suggest that a progressive tolerance to the performance beneﬁts of caﬀeine

develops when this substance is ingested chronically. Interestingly, Pickering et al., [

] suggested

that tolerance to caﬀeine’s ergogenicity could be avoided by using doses greater than the mean daily

intake of caﬀeine. However, there are no valid and standardized methods to accurately estimate

the amount of caﬀeine ingested per day in athletes. In athletes accustomed to daily caﬀeine intake,

the assessment

of daily intake should include dietary sources of caﬀeine as well as caﬀeine ingested as

dietary supplements. Lastly, it is necessary to establish thresholds, in mg of caﬀeine ingested per day,

to discriminate athletes habituated to caﬀeine from those who are naïve or low caﬀeine users.

Following the preferred reporting items for systematic review and meta-analyses (PRISMA)

guidelines [

], we carried out a search for published studies in Medline and SportDiscus on the

eﬀects of caﬀeine on physical performance in subjects habituated to caﬀeine (search performed in

April 2020). After ﬁlters were applied to remove duplicates, reviews or publications with unsuitable

methodology, the search showed a total of 19 original studies that fulﬁlled the objective of the analysis.

These investigations were selected because participants reported habitual caﬀeine use of >100 mg/day,

the experiment contained the measurement of at least one physical performance variable, and there

was a placebo/control situation. There was a restriction on the form of caﬀeine administration, so we

discarded studies that used coﬀee or multi-ingredient supplements to administer caﬀeine in order to

avoid the eﬀect of the co-ingestion of several substances on the results of the analysis. The research

procedures are presented in Figure 1.

In the ﬁnal 19 studies, a total of 200 participants were catalogued as habitual caﬀeine users.

Overall, only 17 women habituated to caﬀeine were included in these investigations. Interestingly,

the largest number of participants habituated to caﬀeine under investigation was in 2019 (Figure 2),

which suggests a progressive, but still insuﬃcient interest in investigating the eﬀects of acute caﬀeine

ingestion in athletes habituated to caﬀeine.

Nutrients 2020, 12, 1087 3 of 6

Nutrients 2020, 12, x FOR PEER REVIEW 3 of 6

Nutrients 2020, 12, x; doi: FOR PEER REVIEW www.mdpi.com/journal/nutrients

Figure 1. Selection of studies.

In the final 19 studies, a total of 200 participants were catalogued as habitual caffeine users.

Overall, only 17 women habituated to caffeine were included in these investigations. Interestingly,

the largest number of participants habituated to caffeine under investigation was in 2019 (Figure 2),

which suggests a progressive, but still insufficient interest in investigating the effects of acute caffeine

ingestion in athletes habituated to caffeine.

Figure 2. Number of participants habituated to caffeine included in research on the ergogenic effects

of acute caffeine intake.

Figure 1. Selection of studies.

Nutrients 2020, 12, x FOR PEER REVIEW 3 of 6

Nutrients 2020, 12, x; doi: FOR PEER REVIEW www.mdpi.com/journal/nutrients

Figure 1. Selection of studies.

In the final 19 studies, a total of 200 participants were catalogued as habitual caffeine users.

Overall, only 17 women habituated to caffeine were included in these investigations. Interestingly,

the largest number of participants habituated to caffeine under investigation was in 2019 (Figure 2),

which suggests a progressive, but still insufficient interest in investigating the effects of acute caffeine

ingestion in athletes habituated to caffeine.

Figure 2. Number of participants habituated to caffeine included in research on the ergogenic effects

of acute caffeine intake.

Figure 2.

Number of participants habituated to caﬀeine included in research on the ergogenic eﬀects of

acute caﬀeine intake.

Most of the studies on participants habituated to caﬀeine have assessed the ergogenic eﬀects of

acute doses of caﬀeine on endurance-like exercise. However, there is a lack of experiments that measure

the impact of acute caﬀeine intake on muscular strength and other anaerobic-like variables.

Only eight

studies were conducted on participants with a daily caﬀeine intake between 100 and

299 mg/day

(Table 1). Investigations within this range of daily caﬀeine intake are particularly interesting because

Nutrients 2020, 12, 1087 4 of 6

this might represent an intake of ~3 mg/kg of caﬀeine per day for athletes with a body mass between

40 and 100 kg, whereas this is typically considered as the minimal dose that gives an ergogenic eﬀect in

naïve/low caﬀeine consumers [3].

Table 1.

Number of participants in investigations aimed at determining the ergogenic eﬀects of caﬀeine

in habitual groups, depending on dose, habitual intake, type of exercise and participant’s ﬁtness level.

Participants

n (Frequency)

Athlete level

Active 117 (58.5%)

Trained 83 (41.5%)

Untrained 0 (0.0%)

Type of exercise

Endurance 127 (60.2%)

Power 33 (15.6%)

Strength 16 (7.6%)

Anaerobic-like 11 (5.2%)

Speed 10 (6.6%)

Other 14 (4.7%)

Habitual intake of caﬀeine

>299 mg/kg/day 123 (61.5%)

161–299 mg/kg/day 54 (27.0.%)

100–160 mg/kg/day 23 (11.5%)

Caﬀeine doses

>6 mg/kg 63 (21.2%)

3–6 mg/kg 224 (74.9%)

<3 mg/kg 12 (4.0%)

Studies in which participants ingested caﬀeine for several days to habituate them to caﬀeine were included in

this analysis. If the experimental procedure consisted of several doses of caﬀeine or in several types of exercises

(e.g., aerobic and anaerobic-like exercise), every experimental situation was used independently to be included in

this table.

The threshold used to classify individuals as high caﬀeine consumers varied from 190 mg/day [

]

to more than 600 mg/day [

]. In addition, only three studies estimated daily caﬀeine intake in relation

to body mass [

–

]. In those investigations that used absolute values to assess the level of habituation

to caﬀeine, the ﬁndings could lead to incorrect conclusions because the same absolute amount of

caﬀeine (in mg/day) would have a diﬀerent impact on subjects with diﬀerent body mass. Only seven

studies, with a total of 95 individuals, analyzed more than seven days of consecutive caﬀeine intake.

All this suggests that, to date, determining the existence and the magnitude of the ergogenic

eﬀect of caﬀeine on habitual caﬀeine consumers is not feasible, particularly in women. In addition,

the threshold of daily caﬀeine intake used to correctly categorize an individual/athlete as a habitual

caﬀeine user has not been properly established. We conclude that the general guidelines for acute

caﬀeine intake [

], which are based on individuals with no habituation to caﬀeine, are not applicable

to habitual caﬀeine consumers because of the existence of tolerance to caﬀeine ergogenicity [

Before creating new caﬀeine guidelines for habitual users, the establishment of common and uniﬁed

norms to classify individuals who ingest caﬀeine on a daily basis is required. The authors propose

the following norms to classify individuals according to their habitual intake of caﬀeine (Table 2).

This classiﬁcation

combines the most common doses of caﬀeine used in previous studies related to the

acute eﬀect of caﬀeine in elite and amateur athletes [

]. To correctly classify an individual, a period

of at least four weeks of stable daily consumption is necessary [

], which should computed from

both dietary sources of caﬀeine and from caﬀeine-containing dietary supplements. Therefore, the aim

of this classiﬁcation is to standardize the categorization of athletes that habitually consume caﬀeine by

using dietary questionnaires, and ultimately, to reduce the likelihood of discrepancies caused by the

improper qualiﬁcation of daily caﬀeine intake in studies that aim to assess the eﬀect of acute caﬀeine

intake in habitual caﬀeine consumers. We hope that this proposal allows for a better understanding of

Nutrients 2020, 12, 1087 5 of 6

tolerance to the performance beneﬁts of caﬀeine, and allows for more precise recommendations of

caﬀeine intake in habitual caﬀeine users.

Table 2.

Proposed thresholds for classifying individuals in sport performance research according to

their habitual caﬀeine consumption.

Habitual Intake of CAF Caﬀeine Dose (>4wks)

Naïve consumer <25 mg/day

Low consumer From 25 mg/day to 0.99 mg/kg/day

Mild consumer 1.00–2.99 mg/kg/day

Moderate consumer 3.00–5.99 mg/kg/day

High consumer 6.00–8.99 mg/kg/day

Very high consumer >9.00 mg/kg/day

The current perspective has some limitations. Studies on habitual caﬀeine consumption have

categorized individuals as caﬀeine consumers by using questionnaires about dietary intake of caﬀeine.

However, it is diﬃcult to accurately quantify caﬀeine intake through dietary records due to the

variations in the caﬀeine content in the same food products. In addition, no previous investigation

has validated the data obtained through dietary questionnaires against caﬀeine concentration in body

ﬂuid specimens (plasma, urine or saliva). Future research that aims to validate new questionnaires

for assessing habitual caﬀeine intake should consider assessing its validity against urine caﬀeine

concentration (e.g., for a 24-h collection period).

Author Contributions:

Conceptualization, A.F. and M.K.; Methodology, A.F. and M.K.; Software, A.F.; Formal

Analysis, A.F and M.W.; Investigation, A.F., M.K and J.D.C.; Resources, A.F.; Data Curation, A.F., M.W., and M.K.;

Writing-Original Draft Preparation, A.F. and M.W.; Writing-Review & Editing, A.F., M.W and J.D.C.; Supervision

M.W. and J.D.C. All authors have read and agreed to the published version of the manuscript.

Funding:

This study would not have been possible without our participants’ commitment, time and eﬀort.

The study

was supported and funded by the statutory research of the Jerzy Kukuczka Academy of Physical

Education in Katowice, Poland.

Conﬂicts of Interest: The authors declare no conﬂict of interest.

References

Del Coso, J.; Muñoz, G.; Muñoz-Guerra, J. Prevalence of caﬀeine use in elite athletes following its removal

from the World Anti-Doping Agency list of banned substances. Appl. Physiol. Nutr. Metab.

2011

, 36, 555–561.

[CrossRef] [PubMed]

Aguilar-Navarro, M.; Muñoz, G.; Salinero, J.; Muñoz-Guerra, J.; Fern

ndez-

lvarez, M.; Plata, M.D.M.;

Del Coso

, J. Urine caﬀeine concentration in doping control samples from 2004 to 2015. Nutrients

2019

11, 286.

[CrossRef] [PubMed]

Maughan, R.J.; Burke, L.M.; Dvorak, J.; Larson-Meyer, D.E.; Peeling, P.; Phillips, S.M.; Rawson, E.S.;

Walsh, N.P.; Garthe, I.; Geyer, H.; et al. IOC consensus statement: Dietary supplements and the

high-performance athlete. Br. J. Sports Med. 2018, 52, 439–455. [CrossRef] [PubMed]

Del Coso, J.; Muñoz-Fern

ndez, V.E.; Muñoz, G.; Fern

ndez-El

as, V.E.; Ortega, J.F.; Hamoutim, N.;

Barbero, J.C.; Muñoz-Guerra, J. Eﬀects of a caﬀeine-containing energy drink on simulated soccer performance.

PLoS ONE 2012, 7, e31380. [CrossRef] [PubMed]

Del Coso, J.; Salinero, J.J.; Gonz

lez-Mill

n, C.; Abi

n-Vic

n, J.; P

rez-Gonz

lez, B. Dose response eﬀects

of a caﬀeine-containing energy drink on muscle performance: A repeated measures design. J. Int. Soc.

Sports Nutr. 2012, 9, 21. [CrossRef] [PubMed]

McCall, A.L.; Millington, W.R.; Wurtman, R.J. Blood-brain barrier transport of caﬀeine: Dose-related

restriction of adenine transport. Life Sci. 1982, 31, 2709–2715. [CrossRef]

Davis, J.M.; Zhao, Z.; Stock, H.S.; Mehl, K.A.; Buggy, J.; Hand, G.A. Central nervous system eﬀects of caﬀeine

and adenosine on fatigue. Am. J. Physiol. Regul. Integr. Comp. Physiol. 2003, 284, R399–R404. [CrossRef]

Nutrients 2020, 12, 1087 6 of 6

Goldstein, E.R.; Ziegenfuss, T.; Kalman, D.; Kreider, R.; Campbell, B.; Wilborn, C.; Taylor, L.; Willoughby, D.;

Stout, J.; Graves, B.S.; et al. International society of sports nutrition position stand: Caﬀeine and performance.

J. Int. Soc. Sports Nutr. 2010, 7, 5. [CrossRef]

Fredholm, B.B. Are methylxanthine eﬀects due to antagonism of endogenous adenosine? Trends Pharmacol.

Sci. 1979, 1, 129–132. [CrossRef]

10.

Fredholm, B.B.; Bättig, K.; Holm

n, J.; Nehlig, A.; Zvartau, E.E. Actions of caﬀeine in the brain with special

reference to factors that contribute to its widespread use. Pharmacol. Rev. 1999, 51, 83–133.

11.

Svenningsson, P.; Nomikos, G.G.; Fredholm, B.B. The stimulatory action and the development of tolerance

to caﬀeine is associated with alterations in gene expression in speciﬁc brain regions. J. Neurosci.

1999

, 19,

4011–4022. [CrossRef] [PubMed]

12.

Dodd, S.L.; Brooks, E.; Powers, S.K.; Tulley, R. The eﬀects of caﬀeine on graded exercise performance in

caﬀeine naive versus habituated subjects. Eur. J. Appl. Physiol. Occup. Physiol.

1991

, 62, 424–429. [CrossRef]

[PubMed]

13.

Jordan, B.J.; Farley, R.S.; Caputo, J.L. Caﬀeine and sprint performance in habitual and caﬀeine naïve

participants. Int. J. Exerc. Sci. 2012, 5, 50–59.

14.

Bell, D.G.; McLellan, M.T. Exercise endurance 1, 3, and 6 h after caﬀeine ingestion in caﬀeine users and

nonusers. J. Appl. Physiol. 2002, 93, 1227–1234. [CrossRef]

15.

Lara, B.; Ruiz-Moreno, C.; Salinero, J.J.; Del Coso, J. Time course of tolerance to the performance beneﬁts of

caﬀeine. PLoS ONE 2019, 14, e0210275. [CrossRef]

16.

Beaumont, R.; Cordery, P.; Funnell, M.; Mears, S.; James, L.; Watson, P. Chronic ingestion of a low dose of

caﬀeine induces tolerance to the performance beneﬁts of caﬀeine. J. Sports Sci.

2017

, 35, 1920–1927. [CrossRef]

17.

Pickering, C.; Kiely, J. Are the current guidelines on caﬀeine use in sport optimal for everyone? Inter-individual

variation in caﬀeine ergogenicity, and a move towards personalised sports nutrition. Sports Med.

2018

48, 7–16. [CrossRef]

18.

Moher, D.; Liberati, A.; Tetzlaﬀ, J.; Altman, D.G.; The PRISMA Group. Preferred reporting items for

systematic reviews and meta-analyses: The PRISMA statement. PLoS Med. 2009, 6, e1000097. [CrossRef]

19.

Gonçalves, L.S.; Painelli, V.S.; Yamaguchi, G.; Oliveira, L.F.; Saunders, B.; da Silva, R.P.; Maciel, E.; Artioli, G.G.;

Roschel, H.; Gualano, B. Dispelling the myth that habitual caﬀeine consumption inﬂuences the performance

response to acute caﬀeine supplementation. J. Appl. Physiol. 2017, 123, 213–220. [CrossRef]

20.

Fisher, S.; McMurray, R.; Berry, M.; Mar, M.; Forsythe, W. Inﬂuence of caﬀeine on exercise performance in

habitual caﬀeine users. Int. J. Sports Med. 1986, 7, 276–280. [CrossRef]

21.

Wilk, M.; Krzysztoﬁk, M.; Filip, A.; Zajac, A.; Del Coso, J. The eﬀects of high doses of caﬀeine on maximal

strength and muscular endurance in athletes habituated to caﬀeine. Nutrients

2019

, 11, 1912. [CrossRef]

[PubMed]

22.

Wilk, M.; Filip, A.; Krzysztoﬁk, M.; Maszczyk, A.; Zajac, A. The acute eﬀect of various doses of caﬀeine

on power output and velocity during the bench press exercise among athletes habitually using caﬀeine.

Nutrients 2019, 11, 1465. [CrossRef] [PubMed]

23.

Wilk, M.; Filip, A.; Krzysztoﬁk, M.; Gepfert, M.; Zajac, A.; Del Coso, J. Acute caﬀeine intake enhances mean

power output and bar velocity during the bench press throw in athletes habituated to caﬀeine. Nutrients

2020, 12, 406. [CrossRef] [PubMed]

24.

Baltazar-Martins, J.G.; Brito de Souza, D.; Aguilar, M.; Grgic, J.; Del Coso, J. Infographic. The road to the

ergogenic eﬀect of caﬀeine on exercise performance. Br. J. Sports Med.

2019

. bjsports-2019-101018. [CrossRef]

25.

Grgic, J.; Grgic, I.; Pickering, C.; Schoenfeld, B.J.; Bishop, D.J.; Pedisic, Z. Wake up and smell the coﬀee:

Caﬀeine supplementation and exercise performance—An umbrella review of 21 published meta-analyses.

Br. J. Sports Med. 2019. bjsports-2018-100278. [CrossRef]

26.

Varani, K.; Portaluppi, F.; Gessi, S.; Merighi, S.; Ongini, E.; Belardinelli, L.; Borea, P.A. Dose and time eﬀects of

caﬀeine intake on human platelet adenosine A

receptors: Functional and biochemical aspects. Circulation

2000, 102, 285–289. [CrossRef]

2020 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access

article distributed under the terms and conditions of the Creative Commons Attribution

(CC BY) license (http://creativecommons.org/licenses/by/4.0/).