Journal of Strength and Conditioning Research, 2004, 18(3), 463–465
᭧ 2004 National Strength & Conditioning Association
PHYSIOLOGIC EFFECTS OF CAFFEINE ON CROSS-
LARRY J. BIRNBAUM AND JACOB D. HERBST
Department of Exercise Physiology, College of St. Scholastica, Duluth, Minnesota 55811.
Birnbaum, L.J., and J.D. Herbst. Physiologic effects
capsules), and nutritional and genetic differences among
of caffeine on cross-country runners. J. Strength Cond. Res.
the subjects. Some studies controlled for previous caffeine
—This study determined the physiological
use by asking subjects to abstain for a period of time—12
effects of caffeine on cross-country runners during submaximal
hours (2), 6 hours (8), 2 weeks (17), or 15 hours (19)—but
exercise. Ten college-age subjects (5 women; 5 men) volunteered
the effectiveness is questionable in light of how ubiqui-
to participate in this study. After completing a V
subject completed 2 30-minute runs at 70% V
tous caffeine is in the American diet. It is unlikely that
treadmill, 1 after ingesting caffeine and the other after ingesting
the physiologic adaptations to long-term caffeine use are
a placebo. A caffeine dosage of 7 mg·kgϪ1 of body weight was
reversed by abstaining for a few hours, although Powers
administered. The same dosage of vitamin C was used as a pla-
et al. (17) asked subjects to abstain for 2 weeks. It is also
cebo. The order of treatments was randomly assigned, and the
difficult to control the diet of voluntary subjects. The in-
trials followed a double-blind format. The physiological data
gestion of caffeine as coffee versus as powder in gelatin
were analyzed using a repeated measures analysis of variance
capsules may contribute to equivocal results. Graham
(SPSS). Tidal volume (T ), alveolar ventilation (V ), and rating
and Spriet (10) suggest that components in coffee may
of perceived exertion (RPE) were significantly different (p
negate the ergogenic actions of caffeine.
0.05) between treatment and control groups. The results suggest
Given the discrepant results of past caffeine studies,
that the ingestion of caffeine at 7 mg·kgϪ1 of body weight priorto submaximal running might provide a modest ergogenic effect
the present study sought to determine if caffeine affected
via improved respiratory efficiency and a psychological lift.
any 1 of several physiological parameters during sub-maximal exercise. Specifically, the responses measured or
KEY WORDS. ergogenic aid, oxygen consumption, RPE, tidal vol-
calculated included relative oxygen consumption (V
heart rate (HR), systolic and diastolic blood pressure(SBP, DBP), double product (DP), pulmonary ventilation
(V ), frequency of breaths (F ), tidal volume (T ), alveolar
ventilation (V ), and rating of perceived exertion (RPE).
affeine has been studied as an ergogenic aid by
effect for caffeine has been reported as either an
Experimental Approach to the Problem
an improvement in performance times (2, 6, 18). Other
Ten college students, all of whom were cross-country run-
studies have reported no significant effect for caffeine on
ners, volunteered to participate in this study. Their de-
mographic data are provided in Table 1. As cross-country
ical variables (4, 19, 20). These divergent results may be
runners, they were in postseason training, which involved
partly explained by differences in caffeine dosage, exer-
moderate-intensity training. All subjects were instructed
cise protocols, or unknown factors. For example, in Mc-
to refrain from any exercise 24 hours prior to testing.
Naughton’s study (15), 15 mg·kgϪ1 of caffeine produced a
Both men and women were included, as no gender differ-
significant improvement in performance time, but a 10-
ences in caffeine sensitivity were reported by investiga-
mg·kgϪ1 dose had no effect. This result would appear to
tors who included both men and women in their studies
be a function of the dosage. However, some studies that
(4, 6, 7), and all subjects were tested at 70% V
used a relatively low dosage of caffeine (approximately 5
negate differences in absolute oxygen consumption. Each
mg·kgϪ1) did not observe any significant effects (4, 11),
subject signed an informed consent and completed a Par-
while others that used the same or very similar dosage
Q health screening questionnaire. The study was ap-
reported significant changes in performance time and/or
proved by the College’s Institutional Review Board.
Exercise protocols have also varied in the published
studies. Some studies had subjects run or cycle to ex-
after ingesting caffeine and the other after ingesting a
haustion, but the exercise intensity varied (3, 4, 6, 15, 17,
˙ O max test and each 30-minute trial were
18). Other investigators exercised subjects for a set period
spaced 1 week apart. One week was deemed sufficient to
of time at a fixed intensity (7, 8, 11, 18, 19, 20). The re-
fully recover from exercise testing and to eliminate caf-
sults of these studies were mixed, and a clear pattern
feine from the body. According to Wadler and Hainline
regarding exercise protocols and significant effects of caf-
(22), caffeine has a half-life of 2–10 hours. A prepared
caffeine dosage of 7 mg·kgϪ1 of body weight was admin-
Other factors that may contribute to the varied results
istered as a powder in gelatin capsules. This dosage was
include previous caffeine use by subjects, the manner in
selected since smaller dosages did not achieve an effect
which caffeine was administered (e.g., coffee vs. caffeine
in several studies (3, 4, 17, 19), and higher doses might
Subject characteristics (mean Ϯ SD
sphygmomanometer on the left arm. The treadmill was a
A repeated measures analysis of variance (SPSS) was
used to analyze the data. Results were considered statis-
tically significant at p
Յ 0.05. Exercise values were de-
rived by averaging the 10th-, 20th-, and 30th-minute val-
produce adverse symptoms in caffeine-naive subjects. Vi-
No significant differences were observed in the cardio-
tamin C served as the placebo and was also administered
vascular responses or respiratory responses with the ex-
as a powder in the same gelatin capsules and in the same
ception of T and V (Table 2). Tidal volume (p
dosage. Vitamin C was selected as a placebo because it
and V (p
ϭ 0.039) increased significantly in the caffeine
has no known exercise benefits (14, 22); it is also harm-
group. A statistically significant decrease in RPE (p
less in low to moderate doses and cannot be distinguished
0.037) was also found in the caffeine group.
from caffeine in powder form. The vitamin C and caffeinecapsules were indistinguishable. Neither the subjects nor
the assistants collecting the data knew which capsulescontained the placebo or the caffeine. The primary inves-
The results of this study indicate that caffeine does not
tigator randomly selected which subjects received caffeine
alter physiological parameters, with the exception of T ,
or vitamin C on their first trial by flipping a coin (heads
V , and RPE, during submaximal exercise (70% V
ϭ caffeine capsules; tails ϭ vitamin C capsules). The
30 minutes duration). Consequently, it is unlikely that
treatment and control were switched for the second trial.
caffeine provides an improvement in energy expenditure
Subjects were asked to abstain from caffeine for 4 days
prior to testing to decrease the effect of caffeine tolerance.
sponses were unchanged. These results agree with sev-
Only 1 subject admitted to being a regular caffeine user.
eral other studies. In particular, Toner et al. (21) and
In a study by Fisher et al. (8), when habitual caffeine
Gastin et al. (9) did not produce statistically significant
cantly after ingesting 5 mg·kgϪ1 of caffeine. A list of com-
feine did not produce significant changes in V
mon beverages and foods that contain caffeine was given
marathoners who ran for 45 minutes at 75% of their
to each subject. Subjects were also asked to fast for 3
˙ O max but did significantly decrease RPE (5). Bond et
hours before arriving at the test site. On arrival, each
al. (3) reported that caffeine did not alter V
subject was given the caffeine or placebo with approxi-
during conditions ranging from rest to maximum exer-
mately 12 oz of water and then tested 1 hour later since
cise. Tarnopolsky et al. (19) also failed to find significant
caffeine levels peak 60 minutes after ingestion (22). Ini-
˙ O , HR, or RPE in varsity male runners who
tially, resting data were collected, which included SBP,
ran on a treadmill for 90 minutes at 70% V
et al. (20) employed 60 minutes of treadmill running at
collected every 10 minutes during exercise along with
60% of HR max and reported no significant changes in
RPE using the Borg scale. Calculated data included DP,
Conversely, Fisher et al. (8) reported that, after ab-
The Medical Graphics Cardio O metabolic analyzer
staining from caffeine for 4 days, 5 mg·kgϪ1 of caffeine
produced a significant increase in submaximal V
lyzer was calibrated with standard gases (21% O /0% CO
max) in subjects habituated to caffeine. Engels et al.
and 12% O /5% CO ) prior to testing each subject. Both a
(7) also reported a significant increase in V
3-lead ECG and a Polar heart rate monitor were used to
measure HR. Blood pressure was determined by the same
responses were also increased, but HR was not changed.
investigator for all subjects using a stethoscope and
Physiologic responses during moderate exercise.†
ϭ standard deviation; HR ϭ heart rate; SBP ϭ systolic blood pressure; DBP ϭ diastolic blood pressure; DP ϭ double product;
V ϭ pulmonary ventilation; F ϭ frequency of breaths; T ϭ tidal volume; V ϭ alveolar ventilation.
PHYSIOLOGIC EFFECTS OF CAFFEINE ON CROSS-COUNTRY RUNNERS 465
ercise is often viewed as a decrease in running economy
cardiorespiratory endurance in men and women. Res. Q. Exerc.
(1). The increase in BP may be a function of caffeine’s
influence on peripheral resistance (16).
CASAL, D.C., AND A.S. LEON. Failure of caffeine to affect sub-
An explanation for these discrepant results is not
strate utilization during prolonged running. Med. Sci. SportsExerc.
readily apparent. The same caffeine dosage has produced
COSTILL, D.L., G.P. DALKSY, AND W.J. FINK. Effects of caffeine
conflicting results (3, 7, 8, 17), as has similar exercise
ingestion on metabolism and exercise performance. Med. Sci.
protocols (4, 7, 8, 20). The manner in which caffeine is
administered (coffee vs. capsules) does not appear to offer
ENGELS, H.J., J.C. WIRTH, S. CELIK, AND J.L. DORSEY. Influ-
an explanation either. However, this study did find a sig-
ence of caffeine on metabolic and cardiovascular functions dur-
nificant increase in T (11.5%) and V (7.4%) during sub-
ing sustained light intensity cycling and at rest. Int. J. Sport
maximal exercise. This is most likely due to caffeine’s
ability to produce bronchodilation (12, 22). Although the
FISHER, S.M., R.G. MCMURRAY, M. BERRY, M.H. MAR, AND
mechanism by which it changes airway tone is unclear
W.A. FORSYTHE. Influence of caffeine on exercise performance
(13), caffeine may also help individuals who suffer from
in habitual caffeine users. Int. J. Sports Med.
GASTIN, P.B., J.E. MISNER, R. A. BOILEAU, AND M.H. SLAUGH-
exercise-induced asthma. By increasing T and V , the
TER. Failure of caffeine to enhance exercise performance in in-
efficiency of the respiratory apparatus is improved in that
cremental treadmill running. Aust. J. Sci. Med. Sport
the subjects ventilated essentially the same minute vol-
ume (64 vs. 61 L·minϪ1; p
ϭ 0.104) with fewer breaths.
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The decrease in ventilatory effort is further supported by
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9(1):1–10. 1996. (http://
the significant increase in V . Both responses are directly
related to the subjects’ decrease in RPE. As ventilatory
efficiency improves, less exertion is required for respira-
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tion. Thus, the subjects’ perception of effort was de-
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creased during the 30 minutes of submaximal exercise.
This finding is in agreement with Casal and Leon (5), who
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the positive effect on respiratory effort, it is reasonable to
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MCNAUGHTON, L.R. The influence of caffeine ingestion on in-
increase in V , in particular, indicates that the inspired
cremental treadmill running. Brit. J. Sports Med.
air reaching the alveoli and participating in gas exchange
helped to ensure consistency in arterial blood gases,
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which would otherwise compromise the respiratory re-
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in young men. Am. J. Cardiol.
statistically significant, may not be of practical signifi-
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cance since RPE is measured in whole numbers on the
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group and 11.2 for the placebo group would both be 11
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in whole numbers, and the statistical analysis produced
TARNOPOLSKY, M.A., S.A. ATKINSON, J.D. MCDOUGALL, D.B.
a finding of significance indicating that the subjects did
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indeed perceive the effort to be less after ingestion of caf-
during endurance running in habitual caffeine users. Med. Sci.
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then that subject may be able to perform at a higher in-
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to affect metabolism during 60 min submaximal exercise. J.
tensity and/or for a longer duration.
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The authors thank Dr. Tommy Boone, chair of the Exercise
BOND, V. R. ADAMS, B. BALKISSOON, J. MCRAE, E. KNIGHT, S.
Physiology Department, for his assistance throughout this
ROBBINS, AND M. BANKS. Effects of caffeine on cardiorespira-
study. His expertise was invaluable.
tory function and glucose metabolism during rest and gradedexercise. J. Sports Med.
Address correspondence to Dr. Larry J. Birnbaum,
BUTTS, N.K., AND D. CROWELL. Effect of caffeine ingestion on
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