1. Erin Carlson
HN310 Fall 2013
ErgogenicAidAssignment
The Effect of Caffeine Intake on Metabolism and Endurance Performance
Coffee is one of the most renowned sources of caffeine for people around the
world. Each morning millions of cups are brewed, spilt, and worshipped to get people
through their day-to-day, even nights, activities due to the source of caffeine to power
them through their hectic schedules. Coffee can contain as much as 95mg of caffeine in
1 cup. Caffeine, as defined in Merriam-Webster’s dictionary, is a bitter alkaloid
(C8H10N4O2) found especially in coffee, tea, cacao, and kola nuts and used medicinally
as a stimulant and diuretic. Caffeine is supposed to make you feel more energetic and
awake, and has been studied for its potential use as an ergogenic aid. Studies have
demonstrated an improvement in exercise performance in endurance activities.
According to study #1; many studies have reported an enhancement of prolonged,
submaximal exercise after caffeine ingestion. The mechanism proposed to explain these
benefits includes an increased utilization of plasma free fatty acid and/or intramuscular
triacylglycerol, which acts to reduce the rate of muscle glycogenolysis, as well as
favorable changes to the central nervous system and/or neuromuscular function.
According to study #2, the metabolic consequences of caffeine ingestion followed by
prolonged exercise have been explained by increasing fat mobilization and the
subsequent sparing of muscle glycogen stores. If this ‘‘glycogen-sparing’’ theory is correct
and is the only mechanism by which caffeine influences exercise capacity then caffeine
ingestion should have no impact on short-term intense exercise. Under these conditions,

2. Erin Carlson
HN310 Fall 2013
ErgogenicAidAssignment
the energy is predominantly derived from anaerobic metabolism and the oxidation of
carbohydrates; i.e., fat metabolism is not important.
In the first study they tested twelve highly trained male cyclists or triathletes of age
27. Their first intention was to determine the importance of the timing of intake of
conventional doses of caffeine (6 mg/kg BM), comparing a protocol in which the caffeine
was ingested 60 minutes before an endurance cycling task with a protocol in which this
dose was consumed throughout the exercise task. They hypothesized that the different
ingestion protocols would have diverse effects on substrate metabolism during
submaximal exercise, but that both would enhance the performance of a subsequent time
trial (TT). Their secondary aim of this first study was to compare the effects of Coca-Cola
ingestion late in the exercise protocol against the intake of larger (conventional) caffeine
doses. They chose cycling because it is an exercise task involving a preload of steady-
state (SS) which SS metabolic measurements could be made, followed by a TT of known
and appropriate reliability to measure performance. In both Study A and Study B, each
subject undertook 4 experimental trials, with training and nutritional status being
controlled before each trial. Subjects refrained from consuming caffeine-containing
substances for 48 hours before each experiment. Diet and exercise diaries were used to
standardize food intake and training for each subject for the period lasting 24–48 hours
before each trial. During the 24-hour period immediately before each trial, subjects were
instructed to refrain from all training and were provided with a pre-packed standard diet
with an energy content of 200 kJ/kg BM, composed of 63% CHO (8 g/kg BM), 20% fat,
and 17% protein. Each experimental trial consisted of 120 minutes of SS cycling at 70%

3. Erin Carlson
HN310 Fall 2013
ErgogenicAidAssignment
V˙ O2 peak immediately followed by a 7 kJ/kg TT. Each trial was undertaken under
conditions designed to mimic recommended nutritional practices for endurance sport.
Specifically, exercise commenced 2 hours after the intake of a standardized CHO-rich
meal providing 2 g CHO/kg BM. In addition, subjects were provided with a commercial
sports drink (6.3% CHO, 18 mmol/l sodium) to allow replacement of fluid. Twelve milliliters
of blood were collected at each sampling time, of which 50 microliters were immediately
analyzed for blood glucose and lactate concentrations by a blood and oximetry analyzer.
The side effects are that it may cause a diuretic effect and it may create a tolerance and
cause withdrawal symptoms if consumed in exceeding amounts.
In the second study they tested 14 young adults (3 women and 11 males) who
were active recreational or varsity athletes of endurance activities ages 23-25. The
second study the subjects performed an incremental V ˙ O2max test on a cycle
ergometer. The study tested on a separate day, the subject performed a practice trial
consisting of three 2-minute exercise bouts at a power output hat required the subject’s
V ˙ O2max, with 6 minutes of rest between. Each subject completed two trials, one with
caffeine ingestion and one with placebo. The order was randomized and administered
with a double-blind procedure. The subjects abstained from all caffeine-containing foods
and beverages for 48 hours before the tests and were instructed to prepare for the trials
as they would for an athletic competition. The subjects kept a food and activity diary for
48 hours before each test. Each subject’s tests were conducted at the same time of day,
and the trials were separated by 1 week. Eight of the subjects had blood and muscle
samples taken in order to determine the effects on metabolism. The side effects are that

4. Erin Carlson
HN310 Fall 2013
ErgogenicAidAssignment
it may cause a diuretic effect and it may create a tolerance and cause withdrawal
symptoms if consumed in exceeding amounts.
In conclusion, these two experimental studies have been proven to be credible
sources to detect the effects of caffeine as an ergogenic aid. In both studies they are
testing subjects who are active and in shape because they have experience with
endurance sports. They test the effects of caffeine by testing before the experiment and
after the experiment at different intervals to see the differences of what the caffeine will
do. Each experiment uses the cycle ergometer and has different protocols to view the
effects of different caffeine sources such as a placebo, sports drink, or Coca-Cola. In
study #1 caffeine intake of 6 mg/kg BM enhanced the performance. This performance
enhancement occurred whether the caffeine was consumed before the cycling bout or
throughout exercise. These results are consistent with the reporting enhanced capacity
for submaximal exercise with intakes of caffeine in the hour before exercise. The results
show that the intake of moderate doses of caffeine (6mg/kg) enhanced the performance
of a cycling TT, undertaken with a sports-specific protocol and under the dietary
conditions promoted for optimal sports performance. This ergogenic benefit to
performance was observed whether caffeine was ingested 1 hour before exercise or in a
series of doses throughout the exercise bout and was achieved without contravening the
re- porting limit for caffeine use. While for the metabolism study, it has shown that this
glycogen sparing effect is limited to the first 15–30 minutes of exercise, but is associated
with enhancement of endurance performance. In study #2, caffeine ingestion resulted in
a significant increase in endurance. Their study confirmed the finding of caffeine

5. Erin Carlson
HN310 Fall 2013
ErgogenicAidAssignment
enhancement, but the glycogen and lactate data failed to support the theory that the
cause was metabolic in nature. During these two exercise periods the net decrease in
muscle glycogen was not affected by caffeine administration. While this has not been
previously examined in intense exercise, investigations of prolonged exercise have
demonstrated that caffeine results in reduced muscle glycogenolysis. It appears that with
the high-energy demands of the intense exercise, the metabolic signals negate the
mechanisms by which glycogen sparing is induced in less intense metabolic conditions.
This may be due to factors such as greater changes concentrations of intracellular
modulators such as calcium and hydrogen ions, free ADP and AMP, and different motor
unit recruitment during the intense exercise. Both experiments proved enhancement of
caffeine in endurance performance, but showed little to no effect on metabolism.