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The Effects of Caffeine
on the Athletes Performance
By Sheila G. Dean MS, RD, LD
In the quest for achieving
maximal performance, athletes will often rely on products to give
them that extra “boost” or edge
These types of products
are called ergogenic aids. If you break down the word ergogenic,
“erg” refers to a unit of work while “genic”
refers to the “generation or producing of.” Put it together
and these “work-producing” products may have the properties
needed to enhance physical performance. It is not uncommon to see
the professional as well as the recreational athlete ingest caffeine
to aid exercise performance. Although the physiological effects
may not have been understood until more recently, the
stimulatory
effects of caffeine have been known for thousands of years.
It is important to
note that caffeine is metabolized by the liver into three other
compounds known as theobromine, theophylline, and paraxanthine,
collectively referred to as methylxanthines (1). It does not appear
to be entirely clear which one or group of these substances is actually
responsible for the “coveted caffeine high” that is
felt after, say, a cup of coffee. And although coffee is known for
containing significant amounts of caffeine, the two should not be
equated since coffee also contains hundreds of different chemicals.
How Does It Work?
Caffeine is theorized
to have several mechanisms for its ergogenic effect during exercise.
Central Nervous System
In a review article
by Graham and Spriet (2), one th
eory suggests that caffeine affects
certain areas of the central nervous system that are responsible
for the neural stimulation of muscle contraction and decreasing
the perception of fatigue. Another theory described by Haas (3)
related to the central nervous system suggests that caffeine interferes
with adenosine, a chemical that plays a role in the brain by inducing
sleep. Since caffeine has a similar molecular structure to adenosine,
it can fit into adenosine receptors in the brain, which in turn
does not cause the sedating effect of adenosine. In this sense,
caffeine does not truly act as a stimulant, but rather a blocker
of the calming effects of adenosine. Haas also suggested that caffeine
may enhance the main excitatory amino acid neurochemicals in the
brain such as glutamate and aspartate (3).
Skeletal Muscle
A second theory of
how caffeine acts as an ergogenic aid involves the skeletal muscle
directly. Under normal conditions, the concentration of calcium
ions in relaxed muscle is very low. Contraction is initiated by
release of calcium into the myoplasm (muscle cell) and relaxation
follows calcium removal. Caffeine appears to influence ions such
as potassium and calcium out of the plasma and into the muscle cell.
Because of the lowered potassium in the plasma, the excitability
of the cell membranes in the contracting muscles increases. In essence,
the muscle contractions have a greater force due this ion transport
of calcium and potassium into the cell, which ultimately translates
into improved endurance (4).
Metabolic Changes
A third and popular
theory is that caffeine spares muscles glycogen by using up stored
and muscle triglycerides (fat) for fuel. Specifically, at caffeine
doses of at least 5 mg/kg, free fatty acids are increased at the
onset of exercise, glycogen is spared at the initial 15 minutes
and intramuscular triglyceride use is increased during the first
30 minutes of exercise (4). This suggests that caffeine affects
fat mobilization early in exercise in contrast to an older study
(1968) by Belect (8) which showed that the fat-burning response
to caffeine did not begin until 3-4 hours after ingestion. Although
caffeine stimulates the release of the hormone, epinephrine (aka
adrenalin), wheth
er this release mobilizes free-fatty acids from
triglyceride stores in fat or muscle tissue or the caffeine itself
does is unclear due to conflicting research results.
Caffeine Caveats
Although from the
research presented, it would be appear that caffeine has a definite
ergogenic effect, a few points should be considered.
Diuretic. Firstly,
caffeine is a well-established diuretic (a substance that makes
you lose water). Depending on the amount of caffeine one ingests,
dehydration can result before and during exercise, which can seriously
impair athletic performance.
Overheating.
Second, caffeine can potentially raise metabolic rate and thus body
temperature. Dehydration and overheating can obviously be a devastating
combination that may not only impair performance but damage one’s
health.
High-carbohydrate
diet. Conflicting results have been reported for
athletes who
load on carbohydrates. Apparently, in one study (5) the subjects
didn’t seem to get the same “boosting” effect
from the caffeine. According to Weir et al (5), the typical elevated
blood fat content is negated. However, according to studies conducted
by Spriet (6), a high-carbohydrate diet did not interfere with the
noted ergogenic effects of caffeine.
Timing. Timing
appears to play a role in the efficacy of caffeine and its ergogenicity.
A study by Costill et al (7) indicate that caffeine intake one hour
before exercise improved endurance performance and fat utilization,
while sparing muscle glycogen. However, as previously mentioned,
a study by Belect (8) showed that the fat-burning response to caffeine
did not begin until 3-4 hours after ingestion.
Dosages. Overall,
caffeine dosages in studies that were less than 3 mg/kg did not
show significant ergogenic effects. However, dosages ranging between
3- 6mg/kg may produce an ergogenic effect while maintaining urinary
caffeine levels below the International Olympic Committee (IOC)
acceptable urinary level threshold of no greater that 12 mcg/ml
(2). Furthermore, although dosages as high as 9-13 mg/kg show significant
enhanced performance, urinary caffeine levels rise above the IOC’s
acceptable limit.
Habituation.
Habituation to caffeine also affects whether or not it will enhance
the athle
tes performance. The most significant ergogenic effects
appear to be in those subjects who do not ingest caffeine regularly.
In some studies, to produce an ergogenic effect, as much as 9 mg/kg
of caffeine had to be administered. The problem though was that
9mg/kg of caffeine produced urinary caffeine levels that are over
the IOC limit (9).
Athletes vs. non-athletes.
Finally, whether the effect that caffeine has on the athlete v.s
non-athlete population is uncertain. In a review by Bucci (10),
sedentary subjects showed no effects of caffeine on performance
or on physiological responses, while trained athletes show significant
benefits in both. In contrast, Chesley et al (11) and Spriet et
al (12), showed that the muscle glycogen sparing following caffeine
ingestion was greater in untrained males than in trained males.
References
Rosenbloom, C. Sports
Nutrition 3rd ed. (2000). American Dietetic Association.
pg 116.
Graham, T., and Spriet,
L.(1996). Caffeine and Exercise Performance. Sports Sci. Exchange
60 (9) Number 1.
Haas, R. Eat to Win
for Permanent Fat Loss. (2000). Three Rivers Press pg. 103-110.
Lindinger, M. et al.
(1993). Caffeine attenuates the exercise-induced increase in plasma
potassium in humans. J. Appl Physiol. 74:1149-1155.
Weir, J. et al. (1987).
A high carbohydrate diet negates the metabolic effect of caffeine
during exercise. Med. Sci Sports Exerc. 19:100-105.
Spriet, L (1995).
Caffeine and performance. Int. J. Sports Nutr. 5:S84-S99.
Costill, D. L, et
al. (1978). Effects of caffeine ingestion on metabolism and exercise
performance. Med. Sci. Sports 10:155-158.
Belect, S. et al.
(1968) Responses of free fatty acids to coffee and caffeine. Metabolism.
17:702-707.
Graham T. E. (1994).
Caffeine and exercise: metabolism and performance. Can.
J. Appl. Physiol. 2: 111-138.
Bucci, L. Nutritional
Ergogenic Aids. Nutrition in Exercise and Sport. CRC Press, 107-185.
Chesley, A. et al.
(1994). Variable effects of caffeine on muscle glycogenolysis in
recreationally active subjects during intense aerobic exercise.
Can.
J. Appl. Physiol. 19:10P, 1994 abstract.
Spriet, L. et al.
(1992). Caffeine ingestion and muscle metabolism during prolonged
exercise in humans. Am. J. Physio. 262 (Endocrinol. Metab.): E891-E898.
Editor’s Note:
SHEILA G. DEAN,
MS,
RD, LD is a registered and licensed dietitian and exercise physiologist.
An educator at heart, Sheila teaches Human Nutrition for nurses
at St.
Petersburg Junior
College. She lectures for groups
as young as preschool age to the elderly and retired, authors book
reviews and newspaper ar
ticles regularly for The St. Petersburg
Times and frequently appears on WTSP – CBS – News Channel
10 for interviews. She is also the consulting sports nutritionist
and media spokesperson for the Ironman Institute. Sheila is a certified
health and fitness instructor with the American
College
of Sports Medicine.
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