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Nutritional ergogenic aids are substances which
enhance the athletic performance by inuencing
physiological as well as psychological process. Caffeine
is one of the most common ergogenic supplements in
endurance sports1.
Effects of acute supplementation of caffeine on cardiorespiratory
responses during endurance running in a hot & humid climate
Wong Chee Ping, Chen Chee Keong & Amit Bandyopadhyay*
Sports Science Unit, School of Medical Sciences, Universiti Sains Malaysia, 16150, Kubang Kerian, Kelantan
Malaysia & *Department of Physiology, University College of Science & Technology, University of Calcutta
Kolkata, India
Received April 9, 2009
Background & objectives: Athletes in Malaysia need to perform in a hot and humid climate. Chronic
supplementation of caffeine on endurance performance have been studied extensively in different
populations. However, concurrent research on the effects of acute supplementation of caffeine on
cardiorespiratory responses during endurance exercise in the Malaysian context especially in a hot and
humid environment is unavailable.
Methods: Nine heat adapted recreational Malaysian male runners (aged: 25.4 + 6.9 yr) who were nonusers
of caffeine (23.7 + 12.6 mg per day) were recruited in this placebo - controlled double - blind randomized
study. Caffeine (5 mg per kg of body weight) or placebo was ingested in the form of a capsule one hour
prior to the running exercise trial at 70 per cent of VO2max on a motorised treadmill in a heat-controlled
laboratory (31oC, 70% relative humidity). Subjects drank 3 ml of cool water per kg of body weight
every 20 min during the running trials to avoid the adverse effects of dehydration. Heart rate, core body
temperature and rate of perceived exertion (RPE) were recorded at intervals of 10 min, while oxygen
consumption was measured at intervals of 20 min.
Results: Running time to exhaustion was signicantly (P<0.05) higher in the caffeine trial compared to the
placebo trial. Heart rate, core body temperature, oxygen uptake and RPE did not show any signicant
variation between the trials but it increased signicantly during exercise from their respective resting
values in both trials (P< 0.001).
Interpretation & conclusion: Our study showed that ingestion of 5 mg of caffeine per kg of body weight
improved the endurance running performance but did not impose any signicant effect on other individual
cardiorespiratory parameters of heat-acclimated recreational runners in hot and humid conditions.
Key words Acute supplementation - caffeine - endurance - heart rate - RPE - VO2
In a prevalence survey2, 89 per cent of the athletes
were found to use a caffeinated substance in the
2005 Triathlon World Championships. Desbrow and
Leveritt3 reported that 73 per cent of the 140 athletes
at the 2005 Ironman Triathlon World Championships
Indian J Med Res 132, July 2010, pp 36-41
36
believed that caffeine is an ergogenic aid that enhanced
their endurance performance.
Ingestion of 3-6 mg of caffeine per kg of body
weight exerts an equivalent ergogenic effect to its
higher doses4. A dose of 5 mg caffeine per kg of body
weight has been shown to provide most consistent
effect to improve endurance performance5. According
to the Medical Council of International Olympic
Committee (IOC), caffeine is allowed in sports as long
as its urinary excretion level is below 12 µg/ml6. Thus,
it has been recommended that caffeine dose should be
limited to 7 mg per kg of body weight or less to avoid
a positive drug test because there may be individual
variations in caffeine clearance6.
Though, various studies have demonstrated the
ergogenic effects of caffeine on endurance performance7-14,
data on the effect of acute supplementation of caffeine
on endurance performance, especially in the heat are
lacking. Moreover, study on the efcacy of acute
caffeine supplementations on Malaysian population
under a hot and humid environment has not been
reported. Therefore, the present study was conducted to
nd out the effect of acute supplementation of caffeine
on cardiorespiratory responses during endurance
running of male recreational runners in a hot and humid
environment.
Material & Methods
Selection of subjects: Nine (9) male Malaysian
recreational runners (aged: 25.4 + 6.9 yr) were recruited
in this randomized double-blind, placebo-controlled
cross-over study. The study was conducted at the Sprots
Science Unit Laboratory, Universiti Sains Malaysia,
Kelantan, Malaysia. All the subjects were categorized as
nonusers of caffeine (23.7+ 12.6 mg/day). Subjects with
hypertension, asthma, diabetes, bronchitis, anaemia,
heart problems, kidney or liver diseases and or any other
major diseases were excluded from the study. Written
informed consent was taken from each subject. The
experiment protocol was approved by the Research and
Human Ethics Committee of Universiti Sains Malaysia.
Test procedure:
Preparation of subjects - To minimise variation
in pre-exercise muscle glycogen status, subjects were
requested to record their food intake for 3 consecutive
days prior to the rst trial and repeat the same diet over
3 days prior to consecutive trials. Subjects were asked
to refrain from heavy exercise for 24 h before all the
experimental trials.
Pre-trial and trial procedure - Each subject came
to the laboratory ve times, rst 3 visits for pre-trial
tests and remaining 2 visits for experimental trials,
respectively (Fig.). First pre-trial visit involved the
measurement of oxygen consumption at various sub-
maximal running speeds while the second pre-trial visit
involved the measurement of VO2max. These two trials
were conducted to determine the exact treadmill speed
that corresponded to the 70 per cent of the subject’s
VO2max which was set as the exercise trial speed.
Subjects were familiarized with the endurance testing
protocol in the heat (310C, 70% relative humidity)
on their third pre-trial visit. At least 3 days gap was
maintained between the consecutive trials. For the
experimental trials, each subject visited the laboratory
twice with at least seven days gap to nullify the effect
of the supplement, i.e., caffeine. Subjects reported to
the laboratory after an overnight fast of 10 h. All the
experimental trials were conducted at 0800 h.
Measurement of oxygen uptake at submaximal
running speeds - After the warm up, subject was
asked to wear a mouthpiece, a nose clip and heart rate
sensor (Sport Tester PE3000, Polar, Finland). A head
gear was tted to support a two-way non-re-breathing
valve (Hans Rudolph 2700 series, USA) attached to the
mouthpiece. The subjects ran on a motorized treadmill
(Quinton 18-60, USA) for four minutes each at four
different speeds (7, 8, 9 and 10 km/h). The speed was
increased by 1 km/h
after every 4 min. Expired air during
the test was passed through a mixing chamber where
sensors to the pre-calibrated paramagnetic oxygen
and infrared carbon dioxide analyzers (Sensormedic
2900, USA) were used to determine the percentages of
oxygen and carbon dioxide respectively in the expired
air. Both analysers were calibrated using two nitrogen
based calibration gases (26% oxygen in nitrogen
Fig. Experimental design of the study.
WONG et al: CAFFEINE SUPPLEMENTATION & ENDURANCE PERFORMANCE 37
38 INDIAN J MED RES, JULY 2010
mixture, and 4 per cent carbon dioxide and 16 per cent
oxygen in nitrogen mixture). The outputs from the
gas analyser were processed using a computer for the
calculation of oxygen consumption (VO2) and carbon
dioxide production (VCO2). Expired gas was measured
every twenty seconds by the analyser. Heart rate and
rate of perceived exertion (RPE) were measured during
the nal min of each 4 min of the speed increment.
Measurement of maximum oxygen optake (VO2max)
- Maximum oxygen uptake was determined by using
a modied Astrand protocol15. This test required
the subjects to run to volitional exhaustion during a
continuous incremental run on a motorized treadmill.
Subjects were initially allowed to warm up for 5 min at
a low speed (6-7 km/h). After the warm up, the subjects
were tted with the headgear, mouthpiece, nose clip
and heart rate sensor as in the sub-maximal test. An
appropriate speed (8-12 km/h) was selected and the test
began with a grade of 0 per cent for 3 min. Thereafter,
the grade was increased by 2½ per cent every 2 min and
the subject was encouraged to run until exhaustion.
Expired air samples and heart rate responses were
measured at the end of each 2 min stage. The maximum
oxygen uptake (VO2max) value was accepted when three
of the following criteria were met16 :
(i) A plateau in oxygen uptake despite increasing
workload.
(ii) A heart beat within 10 beats/min of age-predicted
maximum heart rate: 220 beats/min - age.
(iii) A respiratory exchange ratio of > 1.15.
The 1st pre-trial visit was conducted to determine
the association between oxygen uptake and running
speed. The VO2max was determined in the 2nd pre-
trial visit. The data from these 2 trials were used to
determine the running speed that would elicit 70 per
cent of their respective VO2max during the experimental
trials.
Experimental trial protocol:
On arrival to laboratory - Subject’s body weight
(after emptying their bladder), body height, and pre-
exercise heart rate were measured. Body fat percentage
was also determined by bio-electrical impedance
analysis. A rectal probe was inserted to a depth of 10 cm
beyond the anal sphincter for the determination of core
body temperature that was recorded on a temperature
monitor (Libra Medical ET 300R, USA) throughout
the trial.
A heart rate monitor (Sport Tester PE3000, Polar,
Finland) was placed on the subject’s chest to monitor
the heart rate. Water (500 ml) and a piece of bread (89
kcal) was given to the subject in comfortable sitting
posture 1 h before starting the exercise trial. Subject
ingested capsule containing caffeine or placebo
in random sampling and double blinded method.
Fredholm et al17 established that caffeine concentration
in the blood reached its maximum level after one hour
of the caffeine ingestion. That is why, in the present
study, caffeine was supplemented one hour before the
exercise trials to nd out the optimum effect of the
ingested caffeine.
During the trials - Heart rate, core body
temperature, room temperature, relative humidity and
rate of perceived exertion or RPE18 were monitored at
intervals of 10 min. Subjects were allowed to drink
cool water (4-8o C) at the rate of 3 ml per kg of body
weight at an interval of 20 min to avoid the adverse
effects of dehydration19,20. Subjects ran on the treadmill
at 70% of VO2max “to voluntary exhaustion” that was
determined as the point when they indicated that they
could no longer run at the required speed4. At the point
of exhaustion and to ensure that the subjects were truly
fatigued, the running speed was reduced to elicit 60
per cent VO2max for 2 min. Thereafter, the speed was
returned to the prescribed speed (70% VO2max) and the
subjects were encouraged to run as long as possible.
Statistical analyses: Statistical Package for Social
Sciences (SPSS) version 14.0 (SPSS Incorp, United
States) was used for statistical analysis of the data.
Shapiro-Wilk Test was used to check the normality of
the population. Repeated measure ANOVA was used
followed by Bonferroni post-hoc analysis to observe
the signicant difference at P<0.05 level in heart rate,
oxygen uptake and core body temperature. Paired-t test
was used to compare the difference between means
of endurance time in two trials. Ratings of perceived
exertion (RPE) were analysed using Wilcoxon signed
rank test.
Results & Discussion
The mean body mass index (20.2 + 1.9 kg/m2)
and body fat percentage (15.6 + 2.8 %) indicated that
the subjects were non-obese and non-overweight.
Mean VO2max (51.0 + 8.2 ml/kg/min) reected that the
subjects had good cardiorespiratory tness (Table I).
Average room temperature and relative humidity
in the two trials (placebo and caffeine) were 31 +
0.1 oC; 68.0 + 1.4 per cent; and 31.0 + 0.2 oC; 70.0 +
0.8 per cent; respectively. Mean exhaustion time for
placebo (P) and caffeine (C) trials were 83.6 + 21.4
min and 110.1 + 29.6 min, respectively. Exhaustion
time for C was 31.6 per cent higher than that of the
P trial (P<0.05). This nding indicated that the acute
supplementation of 5 mg of caffeine per kg of body
weight has an ergogenic effect among nonusers of
caffeine and heat-acclimatized recreational runners in a
hot and humid environment. The ndings corroborated
with previous studies on sportspersons and sedentary
individuals5,8,21,22. Caffeine trial demonstrated signicant
improvement in endurance time among rowers, well
trained runners, recreational runners, as well as in 70
yr old male subjects10-12.
There was a gradual increase in heart rate during
exercise in both the trials. The exercise heart rates did
not show any signicant variation between the trails.
However, in both the trials the peak heart rate was
signicantly higher (P<0.001) than the corresponding
resting heart rate (Table II). Such gradual increase
in heart rate over time was to meet the increasing
requirement of the body during the endurance exercise.
There was no signicant difference in the working
heart rate between the two running trials.
The oxygen uptake was gradually increased in
both the trials. Oxygen uptake increased signicantly
(P<0.001) than the corresponding resting values in both
the exercise trials (Table III). There was no signicant
difference in the oxygen uptake during the exercise
between the two trials. This nding corroborated
with previous nding that caffeine sustains the
cardiovascular and respiratory function during cycling
in hot and humid environment23.
Core body temperature increased signicantly
(P<0.001) after the exercise than the corresponding
resting values in both the trials. However, no
signicant difference was observed in the core body
temperature between the placebo and experimental
trials (Table III). There was no signicant association
between the time and supplements on core body
temperature during the trials. This nding was in
agreement with Roti et al24 who reported insignicant
alteration in body’s thermoregulatory responses
during endurance exercise following chronic
supplementation of caffeine in a hot and humid
environment. Millard-Stafford et al23 concluded
that caffeine or caffeinated sports drink maintains
hydration during endurance performance in the
heat. Similar studies with 10mg/kg body weight
of caffeine intake did not impose any signicant
effect on VO2 peak, heart rate, sweat rate and rectal
temperature during an exercise heat tolerance test in
hot and humid environmental condition25,26.
During the exercise, subjects expressed their
feelings on a numerical scale i.e., Borg’s scale18
to indicate the fatigue level or RPE (Table IV). C
trial helped the subjects to sustain the exercise with
lesser exertion. There was signicant difference in
RPE between two trials (P<0.05) as also previously
reported among swimmers and recreational college
runners1-27. Rate of perceived exertion increased
signicantly (P<0.001) from 10 min to the end of test
in the two trials, respectively. The lower RPE during
C trial was probably due to the effect of caffeine
ingestion towards exertion of positive inuence on
nerve impulse transmission as well as analgesic effect
and psychological effect1,12,28-30. This was perhaps
the reason why subjects could sustain the treadmill
Table I. Anthropometric and physical characteristics of subjects
Age (yr) 25.4 + 6.9
Body mass (kg) 57.6 + 8.4
Height (cm) 168.3 + 7.6
Body fat (%) 15.6 + 2.8
Body mass index (BMI) (kg/m2) 20.2 + 1.9
Heart rate max (beats/min) 197 + 6.5
VO 2max (ml/kg/min) 51.0 + 8.3
Values are means + SD (n=9)
Table II. Heart rate of the subjects in placebo and caffeine trials
Heart rate (beats/min)
Trial Resting Exercise (min) Exhaustion
10 20 30 40 50
Placebo 67 + 5.7 157 + 18.3 162 + 16.8 165 + 16.8 167 + 15.2 170 + 13.7 172 + 9.6#
Caffeine 68 + 5.0 158 + 17.9 164 + 13.9 166 + 11.4 168 +11.1 173 + 11.2 182 + 9.9#
Value are mean + SD (n=9)
#P<0.001 between resting and exhaustion values in the respective trial
WONG et al: CAFFEINE SUPPLEMENTATION & ENDURANCE PERFORMANCE 39
running for signicantly (P<0.05) longer duration in
the C trial in comparison with P trial.
In conclusion, our results showed that ingestion
of 5 mg of caffeine per kg of body weight one hour
before the exercise improved the endurance running
performance among non-users of caffeine in a hot and
humid condition but did not impose any signicant
effect on other individual cardiorespiratoy parameters
in heat-adapted recreational runners.
Acknowledgment
Authors wish to thank the subjects for their participation and
valuable co-operation in the research. The nancial assistance
provided by the Universiti Sains Malaysia (vide short term grant
no. 304/PPSP/6131556) is acknowledged.
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Trial VO2
(ml/kg/min)
Core body temperature
(oC)
Resting Peak Resting Peak
Placebo 7.3 + 1.3 35.8+ 5.0#36.5+0.3 39.2+0.3#
Caffeine 7.5 + 1.5 36.9+3.8# 36.6+0.4 39.8+0.5#
Value are mean + SD (n=9)
# P<0.001 between resting and peak values in the same trial
Table IV. Rate of perceived exertion of the subjects during exercise
in both the trial
Time (min) Placebo Caffeine
10 10.6 + 2.4 9.6 + 2.0
20 12.0 + 2.3 11.0 + 2.2
30 13.0 + 2.6 11.7 + 2.1*
40 14.2 + 2.5 12.4 + 1.1*
50 15.3 + 2.7 13.5 +1.8*
60 16.7 + 2.6 14.1 + 2.2*
End of the trial 19.6 + 0.5#19.6 + 0.2#
*P<0.05 between trials
#P<0.001 between resting and peak values in the same trial
40 INDIAN J MED RES, JULY 2010
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Reprint requests: Dr Amit Bandyopadhyay, Lecturer, Department of Physiology, University of Calcutta, University College of Science &
Technology, 92, A. P. C. Road, Kolkata 700 009, India
e-mail: bamit74@yahoo.co.in
WONG et al: CAFFEINE SUPPLEMENTATION & ENDURANCE PERFORMANCE 41
