Exercise and Caffeine
Metabolic, catecholamine, and exercise performance responses
to various doses of caffeine.
J Appl Physiol 78(3):867, 1995.
Summary: In an effort to study the mechanism by which
caffeine might exert its ergogenic effect, these authors studied the
physiological and biological effects of different doses of caffeine.
Eight well-trained male distance runners underwent four separate treadmill
tests to exhaustion. An hour before each test either placebo, 210mg,
420 mg, or 630 mg of caffeine was administered. Run time to exhaustion
was measured and blood samples were taken before and at the end of the
run.
Exercise time to exhaustion was improved at all three
caffeine doses compared to placebo. However, the improvement at the
highest dose was not statistically signficant. Blood levels of epinephrine
were highest with the highest doses of caffeine. Yet, blood epinephrine
levels were not clearly caffeine dose dependent. Increases in blood
lactate and blood glycerol did appear to be caffeine dose-dependent.
The authors conclude that the most ergogenically effective
doses of caffeine were low to moderate (210-420mg). It's probable that
the highest dose of caffeine had a negative effect on the central nervous
system. Some of the runners complained of "mental confusion".
Commentary: The results of this study clearly demonstrate
that the mechanism by which caffeine exerts its ergogenic effect is
not necessarily through increases in epinpehrine. Numerous researchers
have speculated that caffeine's ability to increase physical endurance
is due to its ability to increase the hormone epinephrine (adrenaline).
Epinephrine could then go on to stimulate the release of fat from storage.
The more fat released from storage the more fat used by working skletetal
muscles for energy. If fat can be used for energy, then glucose (carbohydrates)
can be spared. The sparing of glucose is thought to be cruical to prevent
aerobic exercise fatigue and allow the individual to exercise longer.
The effect of caffeine on blood glycerol suggests a
more direct fat moblizing effect, independent of epinephrine. However,
this effect may not totally explain caffeine's ergogenic actions.
It is interesting that most of the studies on caffeine
are looking for a metabolic reason for increased endurance. Perhaps
caffeine's central nervous system or stimulant effects are most important.
It may be that the athlete's perception of fatigue is altered by the
caffeine. This could contribute to the increased time till exhaustion
typically noted after caffeine ingestion. If this stimulant effect is
too great, as in large caffeine doses, then caffeine dosing may not
have clear ergogenic effects.
Metabolic and exercise endurance effects of coffee and
caffeine ingestion
J Appl Physiol 85(3):883, 1998.
Summary: These authors wanted to investigate whether
ingesting caffeine as part of a coffee beverage was an effective ergogenic
aid. Nine well-trained adults were recruited for the study. Each participant
underwent five exercise trials. Forty-eight hours before each trial,
subjects were asked to not ingest any caffeine containing beverage.
One hour before each exercise test the subjects ingested one of five
different caffeine or placebo containing supplements or drinks. Each
test substance contained the same amount of caffeine (4.45 mg/kg body
weight). Plasma levels of caffeine were similiar after the regular coffee,
decaf + caffeine drinks, and caffeine capsules.
Time to exhaustion on the treadmill test was increased,
compared to the placebo group, only after the pure caffeine capsule
challenge. Blood lactate, glycerol and glucose values increased from
the beginning of the exercise bout till exhaustion. There were no differences
in these blood levels between caffeine capsules, placebo and caffeinated
beverage drinks. All caffeine trials resulted in higher before exercise
and higher blood epinephrine levels at exhaustion when compared with
placebo and decaf coffee.
Commentary: The average time to exhaustion on the treadmill
test was 32 minutes after the placebo compared to 42 minutes after ingesting
pure caffeine. Thirty-three minutes was the average time to exhaustion
after caffeine consumed as part of a coffee drink. Not surprisingly
some of the many constituents in coffee seem to nullify the ergogenic
effect of the caffeine drug. Since coffee drinks led to similiar blood
levels of caffeine as ingesting the caffeine in capsule form, the different
endurance effects cannot be explained by different blood levels of caffeine.
The results of this study are an excellent example of an herbal extract,
coffee drink, having a different physiological effect than a single
constituent or drug. It seems unwise and misleading to call whole herbs
or herbal extracts, drugs.
As mentioned previously, several authors have suggested
that the ergogenic effect of caffeine is due to its ability to increase
blood epinephrine levels. Like the previous study, the results of this
study do not support that hypothesis.
Effect of caffeine on metabolism, exercise endurance,
and catecholamine responses after withdrawal.
J Appl Physiol 85(4):1493, 1998.
Summary: These authors wanted to investigate whether
caffeine's ergogenic and metabolic effects are altered by an individual's
history of caffeine consumption. Six recreational athletes, all habitual
caffeine users, were recruited for the study. Their average daily caffeine
intake was 761mg/day primarily from coffee. Each participant completed
seven different exercise trials to exhaustion. A "typical"
cup of coffee has about 150mg of caffeine.
The first two trials were conducted with normal caffeine
intakes and either placebo or 420 mg of caffeine prior to the exercise
bout. Subsequent tests were performed after 2- or 4- days of caffeine
withdrawal. Placebo or caffeine was given prior to these withdrawal
tests.
Time to exhaustion was increased in all caffeine trials.
There was no differences between coffee habiuation and withdrawal test
days. Blood levels of epinephrine were higher at exhaustion after all
caffeine compared to placebo tests. Near the end of the exercise bout
blood lactate levels were higher after acute caffeine supplementation
only with the "withdrawal trials.
Comments: Despite the subjects reporting typical caffeine
withdrawal symptoms (headaches, fatigue, lethary) the impact of the
ergogenic effect of caffeine capsules was not altered. The fact that
blood lactate levels were different between withdrawal versus habituation
trials suggest that regular caffeine consumption may lead to an alteration
in the metabolic response to acute caffeine ingestion. The authors use
this data to suggest that caffeine may directly affect tissues (muscle,
fat, etc.) which contain adenosine receptors. These type of tissue receptors
are known to be stimulated by caffeine in laboratory and animal studies.
Caffeine, performance, and metabolism during repeated
Wingate exercise tests.
J Appl Physiol 85(4): 1502, 1998.
Summary: Although the ergogenic effect of caffeine has
been well-established for endurance exercise bouts, the effects on power
exercise bouts are controversial. These authors recruited nine recreationally
active men. The men underwent 30-s maximal sprint bouts on two separate
occasions. One hour before the exercise test the men were given placebo
or 420mg of caffeine.
There was no difference in power output between placebo
and caffeine challenge tests. No metabolic differences were found between
placebo and caffeine preceded exercise bouts.
Commentary: Although a few other studies have found
an increase in power output after caffeine ingestion, this study did
not. Laboratory studies would suggest that caffeine has its greatest
effect on red, not white muscle fibers. In active, but not sprint trained
men, they would be using mainly white skeletal muscle fibers in these
Wingate exercises. This may account for the lack of caffeine's effect.
The other reason caffeine has no ergogenic effect for
anaerobic or power exercise bouts may be its previously described metabolic
(fat mobilizing) and central nervous system stimulant effects. These
actions of caffeine would only be beneficial in an exercise bout that
lasted for minutes, rather than just seconds. Especially, the fat mobilizing
effects may no be evident until the individual is at least 10 minutes
into an exercise bout.
Acute Exercise reduces caffeine-induced anxiogenesis
Med Sci Sports Exerc 30(5): 740, 1998.
Summary: A high anxiety state was induced in a group
of healthy, aerobically fit men by giving them high doses of caffeine.
The authors wanted to test whether an acute exercise bout could reduce
the anxiety produced by caffeine. Although it is generally agreed that
exercise can decrease anxiety, its effectiveness and mechanism of action
are unclear. Caffeine-induced anxiety is used by these authors as a
model to shed light on these issues.
The nine men in the study underwent a 60 minute bicycle
ergometer exercise two times during the first and second weeks of the
study. The subjects came into the lab two other days each of the weeks
and rested for 60 minutes. After the 60 minute-test the subjects took
the State-Trait Anxiety Inventory as an assessment of their anxiety
level. During week one testing the subjects were given 1200mg of lactose
(a placebo), while during week two testing the subjects were given 1200mg
of caffeine (the amount of caffeine in about 10 cups of coffee).
On the days of caffeine consumption anxiety scores were
significantly elevated. The measurements of anxiety following 60 minutes
of exercise or rest suggested that exercise was better at reducing caffeine-induced
anxiety compared to 1 hour of quiet rest.
Commentary: It has been thought that the primary central
nervous system effect of caffeine is explained by its ability to block
adenosine (A1) receptors. These receptors are mainly involved in transmission
of "inhibitory" signals. Therefore caffeine can have stimulant
effects on the nervous system. Anxiety might be considered a symptom
of "overstimulation" of the central nervous system. It would
appear that the anxiety reducing effects of exercise does not work through
adenosine receptors. However, the methods used in the present study
do not provide any information as to how exercise might be anxiolytic.
The results of this study confirm the anxiety reducing
effects of exercise. It would appear that regular exercise could be
used to keep anxiety to a minimum. However, undertaking regular exercise
should not be used to rationalize the consumption of high amounts of
caffeine. Remember that caffeine or caffeine-like drugs are found in
black and green teas, cola beverages, chocolate, as well as coffee.
Column summary:
Clearly caffeine, but not coffee has the ability to
increase physical endurance. It appears that a dose of 200-400mg caffeine
may have the most effective ergogenic effects. All these studies were
conducted in well-trained individuals. Therefore it is not clear that
caffeine would also boost endurance in untrained individuals.
Endurance is increased, but not power or speed by using
caffeine capsules. This suggests that endurance athletes could benefit,
but sprint or power athletes would not find any use for caffeine.
The caffeine effects may be nullified by other constituents
in coffee. So athletes must obtain the pure "drug" if they
expect to have any measureable help. Also the effectiveness of acute
caffeine ingestion does not seem to dependent on the coffee habits of
the athlete.
Besides the probable ban that will be placed on the
use of caffeine in high level endurance competition, the other downside
of regular caffeine use is "overstimulation" of the adrenal
glands. High level endurance training already appears to place a strain
on the adrenal glands (those glands in the body that respond to all
types of stresses, including physical exercise), regularly ingesting
caffeine may simply add to the burden and possibly hasten an athlete's
movement towards overtraining.