- Journal of Science and Medicine in Sport

Journal of Science and Medicine in Sport (2008) 11, 72—79
ORIGINAL PAPER
Double blind carbohydrate ingestion
does not improve exercise duration
in warm humid conditions
Camila Nassif a,b, Ana Paula Araujo Ferreira b, Aline Regina Gomes b,
Luciana De Martin Silva b, Emerson Silami Garcia b, Frank E. Marino a,∗
a
School of Human Movement Studies, Charles Sturt University, Australia
School of Physical Education, Physiotherapy and Occupational Therapy,
Federal University of Minas Gerais, Brazil
b
Received 22 August 2006 ; received in revised form 16 August 2007; accepted 24 August 2007
KEYWORDS
Carbohydrate;
Exercise;
Heat stress;
Metabolism;
Placebo;
Rectal temperature
Summary
The positive effects of carbohydrate (CHO) supplementation on
endurance exercise are well documented but the placebo (PLAc ) effect can make the
ergogenic qualities of substances more difficult to determine. Therefore, this study
tested the effect of double blind ingestion of PLAc and CHOc in capsules versus known
capsule (CHOk ) ingestion on prolonged exercise heat stress. Nine well trained male
volunteers (mean ± S.D.: 23 ± 3 years; 62.4 ± 6.5 kg and 65.8 ± 5.2 mL kg−1 min−1
peak oxygen consumption) exercised at 60% of maximum power output until volitional exhaustion (TTE) in the three different conditions. Capsules were ingested
with 252 ± 39 mL of water. Blood glucose in CHOc and CHOk was similar but higher
(p < 0.05) than PLAc from 45 min to end of exercise. There were no differences in
TTE between PLAc (125.2 ± 37.1 min) or CHOc (138.8 ± 47.0 min) or between CHOc
and CHOk (155.8 ± 54.2 min). Time to volitional exhaustion was different between
PLAc and CHOk (p < 0.05). Increased TTE resulted when participants and researchers
knew the capsule content, but not in the double blind condition. The difference
could be related to a combined effect of CHO ingestion and knowledge of what was
ingested possibly acting as a potent psychological motivator.
© 2007 Sports Medicine Australia. Published by Elsevier Ltd. All rights reserved.
Introduction
The positive effects of carbohydrate (CHO) supplementation on exercise duration are well docu∗
Corresponding author.
E-mail address: [email protected] (F.E. Marino).
mented. Most of these studies have predominantly
used beverages ranging from 2 to 7% CHO.1—4
Conversely, other studies suggest that CHO supplementation is not as effective as previously thought
and exercise performance is not enhanced.5—7 The
mechanism/mechanisms by which carbohydrate
ingestion might improve exercise performance
1440-2440/$ — see front matter © 2007 Sports Medicine Australia. Published by Elsevier Ltd. All rights reserved.
doi:10.1016/j.jsams.2007.08.015
Carbohydrate ingestion and exercise duration
remains debatable partly because of the various
environmental conditions, as well as a variety of
experimental designs and placebo (PLA) controls.
For instance, some studies have focused on warm
environments although controversy regarding the
benefits of ingestion of CHO in these conditions still
exists.8
Although some researchers have questioned the
methods used in CHO studies with respect to
the control of the placebo group,9,10 others have
recently confirmed the placebo effect as a potent
factor that can have a significant effect on performance or on the sense of effort.11 Therefore,
it would be prudent to account for this possibility
when studying the ingestion of carbohydrates as the
expectations of participants could determine the
intrinsic feedback as shown in the classic study of
Arile and Saville.12 As researchers are interested
in distinguishing between the effect and the real
responses of substances being tested, the placebo
effect can make the ergogenic qualities of different substances more difficult to determine. For
example, Clark et al.10 showed an improved performance when participants believed they were
ingesting CHO but were actually consuming a PLA.
In this particular study, 43 participants completed
2 × 40 km time trials; one trial to establish baseline
performance and a second trial where the participants ingested a CHO or a PLA fluid. These two
groups were further subdivided into three groups
where participants either knew they were ingesting CHO or they knew they were ingesting a PLA
or to a group not knowing either. Interestingly, the
group that ingested PLA but was told to be having
CHO had a better performance than the group that
was told and actually had ingested CHO.
In a further attempt to examine the effect of
CHO versus a PLA beverage, Timmons et al.13 using
gelatin capsules for the ingestion of CHO, exercised athletes at 100% VO2peak . These authors did
not observe an enhancement in anaerobic performance, although exercise duration was ∼105.4 s
in the CHO trial compared to ∼103.2 s in the PLA
trial. Furthermore, in a study where glucose (CHO)
and saline (PLA) infusions were used to asses 1 h
Table 1
cycle time-trial performance in a thermoneutral
environment, no improvement in performance was
reported.14 This group15 also investigated the possible effect of CHO receptors in the mouth on
performance, and concluded that an enhancement
in performance is more related to an increase in
central drive or motivation than a metabolic effect.
This study shows the importance in developing
methods that do not allow participants to recognize what is being administered since four of the
nine participants in this study detected they were
ingesting CHO, and of these four, three performed
better when ingesting CHO. Therefore, infusion and
capsules might be a better method to verify the real
effects of a substance and discriminate between
the effect of CHO and PLA on exercise performance.
The purpose of this study was to examine the
effect of ingesting capsules as either 6% CHO or
PLA with the added effect of participants knowing
in advance what they were ingesting. A further aim
was to evaluate the CHO—PLA effect on exercise
duration in heat stress conditions given that controversy still exists as to the role of CHO ingestion
in these conditions.
Methods
The mean ± S.D. characteristics of the participants
are given in Table 1. Nine well trained male participants (5 cyclists, 3 mountain bikers and 1
triathlete) took part in the study after completing a
health screening questionnaire and being released
by a physician. Each participant signed a letter of
informed consent approved by the Ethics in Human
Research Committee of the University.
The preliminary tests included skinfold measurements where standard regression equations
were used for the estimation of body density and
percent body fat.16,17 Peak oxygen consumption
(VO2peak ) was determined from peak power output
(PPO) according to the American College of Sports
Medicine18 protocol in a thermoneutral environment of 21 ± 1 ◦ C and 67 ± 6% relative humidity.
The participants performed the PPO test and all
The mean ± S.D. physical and physiological characteristics of participants (n = 9)
Mean ± S.D.
73
Age (years)
Body mass
(kg)
Height (cm)
23
3
62.4
6.5
171
8
SF (mm)
59.4
2.3
%BF
PPO (W)
VO2peak
(mL kg−1 min−1 )
5.9
2.1
325
33
65.8
5.2
SF is the sum of nine skinfolds (bicep, tricep, subscapular, pectoral, mid-axilla, abdominal, supraspinale, thigh, calf), %BF the
percent body fat derived from Katch and McArdle,17 PPO the peak power output and VO2peak is the estimated peak oxygen uptake.
74
experimental trials using a friction-braked cycle
ergometer (Monark® 824 E Ergomedic, Varberg,
Sweden) with their own bicycle shoes using a
Shimano Pedaling System (SPD). The PPO test commenced at a workload of 50 W which participants
were required to maintain at 50 rpm whilst the load
was increased by 25 W every 2 min until voluntary
exhaustion. Throughout the test, participants were
required to remain in a seated position.
To allow for recovery of muscle glycogen, for
2 days preceding each trial participants were
required to follow a diet with 60—70% CHO,
1.2—1.7 g of protein kg−1 of body mass and 20—25%
fats, prescribed individually by a dietitian.
The hydration status was assessed by urine specific gravity (Uridens® Inlab, Sao Paulo, Brazil)
measured before and after the trials. In addition,
participants were required to maintain their usual
training routine but abstain totally from training or
any kind of exercise, caffeine and alcohol consumption for at least 24 h prior to all trials.
The participants completed a random crossover
design of three trials: in a double blind fashion
participants ingested either placebo capsules with
distillated water (PLAc ) or they ingested carbohydrate capsules with distilled water (CHOc ). In
the third trial, they ingested carbohydrate capsules
with distilled water whilst both researchers and
participants knew that carbohydrates were being
ingested (CHOk ). For the double blind trials, an individual that was not involved with the collection of
data was the only individual that knew the codes
for the capsules.
Fig. 1 depicts a timeline of the experiment trials and procedures. Each participant performed the
trials at the same time of day and the experiments
were separated by a period of at least 3 days.
There was no overnight fast; participants consumed
a normal breakfast as prescribed by the dietitian
C. Nassif et al.
on the day of the trials. They then reported to the
laboratory approximately 2 h prior to the experimental trial at which time they voided. Following
this procedure, a blood sample was taken from the
middle finger for the determination of blood lactate and glucose concentrations. The participants
then mounted the cycle ergometer and remained in
a seated position for 5 min until exercise started.
Participants exercised until voluntary exhaustion
at 60% PPO in an environmental chamber in a
warm environment (28 ◦ C and 79% relative humidity). Only naturally circulating air was provided in
the chamber during exercise. They were instructed
to achieve a workload of 60% PPO (approximately
70% VO2peak ) at 90 rpm and maintain this throughout
the trial. When this cadence could not be maintained exercise was terminated. The decision to
terminate the endurance protocol was established
by the following steps: First, when participants
could not maintain the cadence, they received a
warning; they were then advised to maintain the
cadence with a tolerance of up to 2 min given. If the
cadence could not be maintained after this period,
exercise was terminated. The test was terminated
when one or a combination of the following criteria
was achieved: rectal temperature reached 39.5 ◦ C,
an RPE of 20, the participant voluntarily terminating the exercise or any sign of illness or discomfort
by the participant. Exercise duration was measured
using a Timex® Triathlon chronometer. Participants
were not given any feedback about how long they
had been exercising and at no time were blood
results and heart rate measurements available to
the participants.
Each participant received 252 ± 4 mL of distilled
water (4 mL kg−1 of body mass) every 15 min in addition to 127—149 g of CHO depending on the trial
(described subsequently) in the three trials with all
hydration procedures undertaken according to the
Figure 1 Timeline of experimental trials of PLAc (double blind placebo with capsules) vs. CHOc (double blind carbohydrate ingestion with capsules) and CHOk (unblinded carbohydrate with capsules).
Carbohydrate ingestion and exercise duration
ACSM19 recommendations. The amount of powder
was calculated to match the volume of water so
that the mixture of the water and capsules would
correspond to a 6% CHO solution. For each trial
the mean number of capsules ingested was 252 for
PLA, 281 for CHOc and 330 for CHOk . Each capsule
contained 452 mg of carbohydrates whereas each
capsule of PLA contained 500 mg of placebo powder
(gelatin).
The placebo capsule containing gelatin was used
under advice of a qualified physician/pharmacist,
so that the amount of powder that was ingested
during the experiments had no laxative effect. The
gelatin is composed mainly of amino acids such
as glycine, proline, hydroxyproline, glutamic acid,
arginine and alanine. The carbohydrate capsules
contained a commercial isotonic drinking powder
in a proportion of 6% carbohydrates with glucose,
fructose and malt dextrin and the following minerals per 100 g: sodium (625 mg), potassium (300 mg),
chlorine (285 mg), calcium (584 mg), phosphate
(296 mg), magnesium (30 m) and chrome (60 mg).
Vitamins and minerals were administrated as they
formed part of the composition of the commercial
powder; therefore, it was not considered worthwhile to evaluate the effects of these components
as they comprised a negligible amount. No distinguishable taste of the drinks could be readily
recognized by participants since powder was in capsules. In the CHOk trials, the powder was also given
in capsules to remove any possible difference in
absorption between trials.
Heart rate was continuously monitored (Polar®
Vantage, Kempele, Finland) and recorded at 5 min
intervals. Ratings of perceived exertion (RPE) were
evaluated every 5 min of exercise using the Borg
scale.20
Blood samples were collected on a reagent
strip for lactate at 30 min and for glucose every
15 min and at the termination of exercise. Blood
was collected in a 20 ␮L tube for the determination of lactate using an Accusport BM—–Lactate
device (Roche Diagnostics, Boehringer-Manheim® ,
Germany) and blood glucose levels were measured
by a Accu-check Advantage® (Roche Diagnostics
Corporation, USA).
Rectal temperature was measured using rectal probes (Yellow Springs Incorporated® 4400
series—–4491-E type) inserted 12 cm beyond the
anal sphincter whilst skin temperature was monitored with skin thermistors attached at four sites
(chest, arm, thigh, leg). All thermistors were
connected to a telethermometer (Yellow Springs
Incorporated® 4400-A) and recorded every 5 min.
Mean skin temperature was calculated as described
by Ramanathan.21
75
All data are reported as mean ± S.D. Target
responses were normally distributed according
to Shapiro—Wilk’s test. The Latin Square design
was used to control all sources of variation
that could have influenced the target responses
other than treatments, such as individual training
effects.22 Treatment sequences (PLAc —CHOc —CHOk
or CHOk —PLAc —CHOc or CHOc —CHOk —PLAc ) were
randomized using nine individuals in a 3 × 3 Latin
Square. All target variables were measured at predetermined times during exercise, thus, the final
design was a Split Plot testing the factorial (three
treatments in the plots and times in the subplots)
with nine replications, built upon a basic 3 × 3 Latin
Square design repeated three times. Time effect
was studied up to 75 min since that was the maximal value achieved by all the participants. Post
hoc comparisons were made by least significant difference test. The level of significance was set at
p < 0.05.
Results
Fig. 2 shows the completion times for each
trial for individual participants. The duration of exercise was 125.25 ± 37.13 min for
PLAc , 138.85 ± 47.04 min for CHOc and 155.
08 ± 54.02 min for CHOk which was significantly
different to PLAc (p < 0.05). When comparing the
groups the exercise duration was similar between
PLAc and CHOc and between CHOc and CHOk . However, exercise time for CHOk was approximately
24% longer compared to PLAc .
Fig. 3 shows the heart rate response during exercise. At the commencement of exercise heart rate
Figure 2 Individual participant times for completion of
exercise in warm humid conditions. PLAc (double blind
placebo with capsules) vs. CHOc (double blind carbohydrate ingestion with capsules) and CHOk (unblinded
carbohydrate with capsules).
76
Figure 3 Heart rate responses during prolonged exercise
in the three conditions: PLAc (double blind placebo with
capsules) vs. CHOc (double blind carbohydrate ingestion
with capsules) and CHOk (unblinded carbohydrate with
capsules). * PLAc significantly different to CHOk and CHOc
(p < 0.05).
was 64 ± 12, 62 ± 5 and 64 ± 10 beats min−1 whilst
at the end of exercise heart rate was 158 ± 9,
163 ± 6 and 165 ± 8 beats min−1 for PLAc , CHOc and
CHOk , respectively. These values were higher for
both CHOc and CHOk compared to PLAc . The mean
heart rate during exercise in each trial taking into
consideration the total time of each participant was
152 ± 5, 156 ± 4 and 156 ± 7 beats min−1 for PLAc ,
CHOc and CHOk , respectively. Heart rate during
exercise was similar amongst trials except at the
end of exercise, when it was higher for CHOc and
CHOk compared with PLAc . The median RPE at the
end of exercise was 19 (exhaustive) in all trials,
indicating a similar conscious exercise effort by all
participants across all trials.
The urine specific gravity before exercise was
1.011 ± 0.008, 1.012 ± 0.008 and 1.014 ± 0.009
whilst after exercise it was 1.012 ± 0.008,
1.013 ± 0.006 and 1.012 ± 0.006 for PLAc , CHOc
and CHOk , respectively. As these values were
similar between trials indicates that participants
commenced and terminated exercise in a hydrated
state.
Fig. 4 shows the blood glucose response during
exercise up to 75 min in all trials and at the end of
exercise. Initial blood glucose was similar amongst
trials at 5.6 ± 0.4, 5.8 ± 0.9 and 5.9 ± 0.4 mmol L−1
for PLAc , CHOc and CHOk , respectively. Blood glucose was higher for CHOc and CHOk compared to
PLAc after 45 min of exercise and at the end of
exercise these values were also higher for CHOc
(5.8 ± 0.5 mmol L−1 ) and CHOk (5.9 ± 0.6 mmol L−1 )
compared with PLAc (4.9 ± 0.3 mmol L−1 ; p < 0.05).
The CV of the blood glucose measurements ranged
from 1.5 to 0.7%.
C. Nassif et al.
Figure 4 Blood glucose concentration during prolonged
exercise for the three conditions: PLAc (double blind
placebo with capsules) vs. CHOc (double blind carbohydrate ingestion with capsules) and CHOk (unblinded
carbohydrate with capsules). PLAc significantly different
to CHOk and CHOc (p < 0.05).
At the commencement of exercise blood lactate was 2.6 ± 0.7, 2.4 ± 0.3 and 2.6 ± 0.4 mmol L−1
for PLAc , CHOc and CHOk , respectively. Blood lactate levels at the end of exercise were 2.7 ± 0.3;
2.6 ± 0.5 and 3.2 ± 1.1 mmol L−1 for PLAc , CHOc and
CHOk , respectively. At no time were blood lactate
values different amongst conditions.
Fig. 5 shows the rectal temperature response to
the end of exercise. Rectal temperatures at the
start of exercise were 36.9 ± 0.2, 37.1 ± 0.1 and
37.0 ± 0.3 ◦ C for PLAc , CHOc and CHOk , respectively
and were not different. At the end of exercise rectal temperatures were 38.4 ± 0.5, 38.6 ± 0.4 and
Figure 5 Rectal temperature response for three conditions: PLAc (double blind placebo with capsules) vs. CHOc
(double blind carbohydrate ingestion with capsules) and
CHOk (unblinded carbohydrate with capsules). PLAc is significantly different to CHOk (p < 0.05).
Carbohydrate ingestion and exercise duration
38.6 ± 0.5 ◦ C for PLAc , CHOc and CHOk , respectively
but were only different between CHOk and PLAc
(p < 0.05). The mean skin temperature response was
similar between trials throughout and up to the end
of exercise. The end of exercise mean skin temperature was ∼34.7 ◦ C amongst trials.
Discussion
The findings of the present study show that exercise
duration is significantly increased (∼24%) when the
ingestion of CHO is combined with the knowledge of
ingesting the CHO, compared with PLA and double
blind CHO ingestion in a warm, humid environment.
However, the experimental design used in this study
does not allow us to fully explain the differences in
exercise duration solely on the basis of either a psychological or additive effect. Based on some recent
studies; however, exercise duration was expected
to be improved in CHOc compared with PLAc , but
we expected CHOk would be even better because
of the combined effect of the ingestion of carbohydrates and the knowledge of this ingestion by both
participants and researchers.
Although the results of the present study partly
contradict the expected improvement in exercise duration with CHO compared with PLA, it
is likely that a possible psychological effect was
a determining factor in the outcome of exercise
duration. Clark et al.10 suggested that a psychological effect on performance is possible when
carbohydrates were thought to be ingested. In the
present study, CHO was ingested in capsules which
guaranteed a robust evaluation of the physiological effects of this substance. Although others3
used fluid with artificial sweetener as a placebo,
the use of this traditional method may limit the
masking of the placebo and not guarantee that
individuals will not be able to discriminate the
taste difference between the placebo fluid and
the fluid with carbohydrates. It is important to
note that the use of venous infusion of CHO did
not enhance performance14 and it is possible that
oral and pharyngeal receptors could be activated
when glucose is ingested leading to a psychological effect that can influence fatigue mechanisms.15
The present findings lend indirect support to this as
oral and/or pharangeal receptors could not have
been activated by taste due to the ingestion of
capsules.
In the present study blood glucose was higher in
both CHO trials compared to the PLAc trial, which
is in agreement with previous work where carbohydrates were administered as a bolus solution.1,3,4
In the present study, the elevated blood glucose
77
was not followed by any enhancement in exercise performance using the double blind method.
Given that blood glucose levels were within normal range during all three trials, it is unlikely
that hypoglycemia was related to the cessation of
exercise. According to previous work23 endurance
in a hot environment is unlikely to be related
to carbohydrate availability.8 However, reduction
in muscle glycogen stores could induce fatigue
as glycogen can be depleted in exercise lasting
60—120 min.24 But, more recently it has been shown
that the association between low intramuscular
glycogen stores with fatigue is still unclear, and
data do not support the theory that this is the cause
of fatigue.25 Since glycogen usage was not measured in the present study, this hypothesis could
not be verified. Nevertheless, even if glycogen
stores were completely depleted, this cannot fully
explain the reduced exercise time as other glycolytic energy sources would still be available.26
In addition, when muscle glycogen was measured
following exercise in the heat no differences were
found between glycogen at rest and at the point of
fatigue.23
The initial heart rate was similar between all
three trials and no difference could be observed
up to 75 min of exercise, which is similar to previous studies that also had participants exercise in
a warm environment.3 In the present study, the
heart rate at the end of exercise was higher in the
CHOc and CHOk trials when compared to PLAc . A
higher heart rate in CHOk compared to PLAc can
be explained by the longer exercise duration in
the CHOk trial. However, exercise duration cannot
explain the difference found in heart rate between
PLAc and CHOc .
The rating of perceived exertion (RPE) was not
different amongst trials and lends support to the
notion that the decrease in muscle glycogen stores
at the completion of exercise may not be the primary cause of fatigue.24 Noakes et al.26 proposed
the possibility and the importance of RPE in the
understanding of the mechanisms of fatigue, since
others25 found the same RPE at the end of exercise
in participants with both high and low intramuscular glycogen content. In the present study a similar
RPE at the end of exercise was also observed even
though exercise duration was different between
PLAc and CHOk . Participants followed a diet so
that a similar intramuscular glycogen content could
be expected at the start of exercise in all trials.
Although speculative, the present data lend support to the hypothesis that there might be a central
anticipatory mechanism working toward the maximal RPE that the body can tolerate; presumably
the highest RPE will be reached before complete
78
depletion of the energy substrate.26 The blood
lactate concentrations observed in the present
study indicate that exercise in all trials was predominantly non-glycolytic. These findings suggest
that the blood lactate values were not related to
fatigue.
The participants started exercise with similar rectal temperatures but no differences were
observed during exercise amongst trials. The rectal
temperatures at the end of exercise were higher
in the CHOk trial compared with the PLAc trial
most likely due to the longer exercise duration
in the CHOk . However, since rectal temperature
at the end of exercise was below 39 ◦ C across
all trials, it is unlikely that rectal temperature
per se was the factor responsible for terminating
exercise. These data suggest that the ingestion of
carbohydrates does not reduce thermal strain in
a warm environment and augment metabolic heat
production.
In conclusion, the ingestion of carbohydrate capsules in a double blind fashion did not change
exercise duration of athletes cycling at 60% of
PPO in a warm, humid environment. As there
was no difference in exercise duration between
the double blind placebo and the CHO trial and
no apparent physiological effect of CHO, the differences between the known CHO ingestion and
placebo could have been solely the result of
suggestion.
Practical implications
• Coaches and trainers of endurance athletes
should be aware that knowledge of the performance enhancement supplement may have
a significant psychological effect on endurance
performance.
• Coaches and trainers should consider that
knowledge of the ingested ergogenic aid could
act as an additive effect to the ergogenic aid
itself and thereby improve endurance performance.
Disclosure
This study was supported by Coordenacao de Aperfeicoamento de Pessoal di Nivel Superior (CAPES),
Conselho Nacional de Desenvolvimento Cientifico
e Tecnologico (CNPq), Fundacao de Amparo a
Pesquisa do Estado de Minas Gerais (FAPEMIG),
Brazilian Ministry of Sports and GERMINARE Chemistry.
C. Nassif et al.
Acknowledgements
The authors would like to thank all participants for
their whole hearted dedication during the study
and to Ivan Barbosa Machado Sampaio for assistance
with the writing of the manuscript.
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