Performance outcomes and unwanted side effects associated with energy drinks

Abstract

Energy drinks are increasingly popular among athletes and others. Advertising for these products typically features images conjuring great muscle power and endurance; however, the scientific literature provides sparse evidence for an ergogenic role of energy drinks. Although the composition of energy drinks varies, most contain caffeine; carbohydrates, amino acids, herbs, and vitamins are other typical ingredients. This report analyzes the effects of energy drink ingredients on prolonged submaximal (endurance) exercise as well as on short-term strength and power (neuromuscular performance). It also analyzes the effects of energy drink ingredients on the fluid and electrolyte deficit during prolonged exercise. In several studies, energy drinks have been found to improve endurance performance, although the effects could be attributable to the caffeine and/or carbohydrate content. In contrast, fewer studies find an ergogenic effect of energy drinks on muscle strength and power. The existing data suggest that the caffeine dose given in studies of energy drinks is insufficient to enhance neuromuscular performance. Finally, it is unclear if energy drinks are the optimal vehicle to deliver caffeine when high doses are needed to improve neuromuscular performance.

Full text

Performance outcomes and unwanted side effects associated
with energy drinks
Ricardo Mora-Rodriguez and Jesús G Pallarés
Energy drinks are increasingly popular among athletes and others. Advertising for
these products typically features images conjuring great muscle power and
endurance; however, the scientific literature provides sparse evidence for an
ergogenic role of energy drinks. Although the composition of energy drinks varies,
most contain caffeine; carbohydrates, amino acids, herbs, and vitamins are other
typical ingredients. This report analyzes the effects of energy drink ingredients on
prolonged submaximal (endurance) exercise as well as on short-term strength and
power (neuromuscular performance). It also analyzes the effects of energy drink
ingredients on the fluid and electrolyte deficit during prolonged exercise. In several
studies, energy drinks have been found to improve endurance performance,
although the effects could be attributable to the caffeine and/or carbohydrate
content. In contrast, fewer studies find an ergogenic effect of energy drinks on muscle
strength and power. The existing data suggest that the caffeine dose given in studies
of energy drinks is insufficient to enhance neuromuscular performance. Finally, it is
unclear if energy drinks are the optimal vehicle to deliver caffeine when high doses
are needed to improve neuromuscular performance.
© 2014 International Life Sciences Institute
INTRODUCTION
Carbonated drinks, sports drinks, and energy drinks are
different beverage categories that consumers can find in
any convenience or large store. While most people can
distinguish between a soda and the other two beverage
categories, sport and energy drinks could easily be con-
fused. Sports drinks originated in the early 1960s with the
formulation of Gatorade by Dr. Robert Cade to aid the
summer performance of the collegiate football team at
the University of Florida.1Several countries have since
regulated the ingredients and labeling of beverages that
are marketed as sports drinks. For instance, the European
Food Safety Authority2advises a narrow range of
osmolalities (200–330 mOsm·kg/H2O) as well as sodium
(20–50 mmol/L), and carbohydrate (2–8% w/v) concen-
trations when defining a sports drink’s composition.Most
sports drinks manufacturers formulate their products fol-
lowing the advice of scientific panels of experts in sports
nutrition (e.g., the European Food Safety Authority and
the US Food and Drug Administration); thus,the compo-
sition of these beverages is quite similar given that the
common aim is to aid performance during prolonged
exercise, especially in a hot environment.
Energy drinks appeared on the Western market 20
years later when the company Red Bull GmbH started
selling their energy beverage products in Austria. In con-
trast with the relatively similar formulations of sports
drinks, the composition of energy drinks is highly vari-
able. Nevertheless, all energy drinks include one or several
stimulants, with caffeine being the most common. The
lack of uniformity in the formulations of energy drinks
most probably originates from the fact that these drinks
lack a unified purpose. Generally, manufacturers claim
that energy drinks will benefit consumers by enhancing
their physical capacity and cognitive performance.
Affiliations: R Mora-Rodriguez and JG Pallarés are with the Exercise Physiology Laboratory at Toledo, University of Castilla-La Mancha,
Toledo, Spain.
Correspondence: R Mora-Rodríguez, Exercise Physiology Laboratory at Toledo, Universidad de Castilla-La Mancha, Avda, Carlos III, s/n,
45071 Toledo, Spain. E-mail: Ricardo.Mora@uclm.es. Phone: +34-925-26-88-00 (Ext. 5510). Fax: +34-925-26-88-46.
Key words: caffeine, endurance performance, energy drinks, neuromuscular performance, Red Bull, side effects, taurine
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Supplement Article
doi:10.1111/nure.12132
Nutrition Reviews® Vol. 72(S1):108–120108

However, it is not clear if the aim is to enhance short,
high-intensity bursts of exercise or to fuel and stimulate
the body during prolonged aerobic exercise. Energy
drinks seem to be marketed to improve performance in
extreme sports requiring peak neuromuscular power and
a high degree of athletic ability and coordination.
However, energy drink advertisements may also feature
long-duration sports like car and mountain bike racing or
freestyle windsurfing that require a high degree of whole-
body and local muscle endurance.
This review updates the current scientific informa-
tion on the positive effects that energy drinks may have
on exercise performance, and balances that with informa-
tion on the possible negative side effects derived from
their consumption. Due to the wide variability in the
composition of energy drinks, this review analyzes the
most common ingredients from the original energy
drink, Red Bull, which are as follows: caffeine, taurine,
glucuronolactone, glucose, and B vitamins. The review’s
focus is restricted to the claimed benefits that energy
drinks provide for physical performance, since other
reviews in this issue of the journal and elsewhere provide
information on claims related to cognitive improve-
ments3and weight loss.4Studies in humans are preferen-
tially presented. Only laboratory studies in which
variables were tightly controlled were included.
EVIDENCE FOR PROLONGED EXERCISE ENDURANCE
Energy drinks and prolonged muscle contraction
Complex carbohydrates and water are nutrients that
have been repeatedly shown to delay fatigue during pro-
longed5dehydrating6exercise. However, energy drinks
do not seem to be formulated to maximize the incorpo-
ration of glucose or water into the blood during exercise.
One liter of Red Bull (i.e., 4 cans containing 250 mL
each) contains 4 g of taurine (an amino acid), 2.4 g of
glucuronolactone, 0.32 g of caffeine, 108 g of carbohy-
drates, and 0.14 g of B vitamins. The carbohydrate con-
centration is 11% and osmolality is 601 mOsm·kg/H2O.7
In contrast, a sports drink (e.g., Gatorade Orange) has a
lower carbohydrate concentration (6%) and osmolality
(297 mOsm·kg/H2O). Carbohydrates comprise the main
macronutrient in both drinks and determine the
beverage’s caloric content.
It has been reported that the caloric content of a
beverage influences the rate of gastric emptying at rest8
and when ingested during exercise.9High gastric empty-
ing rates are important to ensure bioavailability of the
ingested drink. Thus, ingestion of a drink with a carbo-
hydrate concentration of 8% or higher could result in
delayed incorporation into the bloodstream and reduced
availability of the ingredients to the contracting muscu-
lature. Furthermore, increasing the osmolality of a 6%
carbohydrate solution to 414 mOsm·kg/H2O, reduced the
absorption of the ingested fluid from 82% to 68% in com-
parison to water placebo.10 The combination of high
osmolality and carbohydrate concentration of Red Bull
(601 mOsm kg/H2O and 11%, respectively) probably
reduces its absorption in comparison to a commercial
sports drink.
In addition, energy drinks do not include salts in
their formulation. Salt (sodium and chloride) is lost in
sweat during exercise in amounts proportional to the
exercise intensity.11 Thus, the salt ingested in a beverage
prior to or during prolonged exercise has an import-
ant role in maintaining cardiovascular stability, fluid
balance, and even exercise performance.12 In their favor,
energy drinks contain caffeine, which has an ergogenic
effect during prolonged exercise.13,14 Caffeine ingestion
increases endurance performance by delaying central
nervous system fatigue,15 and could increase neuro-
muscular performance through a direct effect on the
muscle,16 resulting, in both situations, in increased energy
expenditure during exercise.
Recently, companies have introduced sugar-free ver-
sions of energy drinks. These beverages have zero calories
and low osmolality (140 mOsmol·kg/H2O for Red Bull
Sugarfree), which solves the reduced absorption prob-
lems of the regular sugar-containing version. On the
other hand, the caffeine in these drinks leads to extra
energy expenditure, energy that is not provided by the
sugar-free drink. Thus,the sugar-free drink may result in
a faster drainage of endogenous energy stores (muscle
glycogen, phosphocreatine, and ATP) counteracting its
ergogenic actions. Hence, energy drinks containing
stimulants and fluid but no carbohydrate (e.g.,sugar-free
versions) may improve endurance performance due to
more extensive use of endogenous energy stores.
Energy drinks and rehydration
During prolonged exercise in warm environments, bev-
erages are consumed in an attempt to maintain fluid
balance by replacing the fluid lost via sweating. Fluid
deficit (dehydration) increases the cardiovascular and
thermal strain of exercise.17 Even dehydration levels of
less than 2% (e.g., 1.4 L fluid loss for a 70-kg man)
increase core temperature and could decrease cycling
performance.18 The kidneys determine a body’s fluid
balance over the long term. Kidney function may be
affected by the amount and composition of the
rehydration fluid used during prolonged exercise. In
addition, the kidneys help regulate blood pressure and
acid-base balance, which are also important for perfor-
mance during endurance exercise.
Nutrition Reviews® Vol. 72(S1):108–120 109

It is estimated that all our extracellular fluid passes
through our kidneys 16 times per day. However, most of
the fluid that is filtered at the glomerulus is reabsorbed and
less than 1% ends up in the bladder (1 mL/min of urine
formation rate).Renal blood flow, pressure, and hormones
(mainly vasopressin and aldosterone) dictate the rates of
glomerulus filtration and renal tubule reabsorption,
respectively.Any component of a drink that induces vaso-
constriction or alters the actions of the fluid-regulating
hormones to reduce renal tubule reabsorption (i.e.,
diuretic effect) will negatively affect fluid balance and
could, thus, impair endurance performance. Potentially, a
beverage that contains a diuretic substance could increase
body water loss through urine, reduce plasma volume, and
negatively affect thermoregulation and cardiovascular
function. Caffeine-containing drinks have been shown to
increase urine production when ingested prior to19 and
after20 exercise.
In addition to being diuretic, caffeine enhances
urine sodium losses (i.e., natriuretic). Sodium has been
deemed to be a key element in the maintenance of
plasma volume during prolonged dehydrating exercise.
This has recently been illustrated in a study comparing
control subjects with patients that excrete a lot of
sodium (cystic fibrosis) during prolonged dehydrating
exercise.21 The authors observed larger plasma volume
reductions in the cystic fibrosis patients for the same
level of dehydration. Likewise, the loss of sodium
induced by caffeine ingestion could potentially alter car-
diovascular performance during exercise. In addition, a
negative sodium balance during prolonged exercise
could weaken leg isometric strength.22 Therefore, despite
the fact that energy beverages are popularly associated
with prolonged exercise, it is unclear if they can be rec-
ommended to rehydrate during long-duration physical
activity due to their relatively high caffeine content and
lack of added sodium.
DIURETIC EFFECT OF THE MOST COMMON ENERGY
DRINK INGREDIENTS
B vitamins and glucose
B vitamins are water soluble and are, thus, distributed in
the ample pool of body water. One liter of Red Bull con-
tains 150 mg of B vitamins, any excess of which could be
readily excreted by a normally functioning renal system.
Upon ingestion, glucose is either utilized as an energy
substrate or stored in the liver and muscles. The ingestion
of 108 g of carbohydrate (4 cans of Red Bull) should not
represent a problem for the kidneys. The exception is
with the diabetic population for whom this amount of
glucose could cause glycosuria (presence of glucose in
urine) and the accompanying excessive water loss into the
urine with resultant dehydration (i.e., osmotic diuresis).
Glucuronolactone
This naturally occurring metabolite of glucose is formed
in the liver. Glucuronolactone is rapidly absorbed,
metabolized, and excreted in urine as glucaric acid,
xylitol, and L-xylulose. Glucuronic acid is an important
constituent of fibrous and connective tissues. In 2003,the
European Food Safety Authority raised concerns about
the safety of its inclusion in energy drinks.23 Their con-
cerns were based on the finding of unspecified kidney
lesions (inflammation in the papilla of the kidney) after
13 weeks of supplementation in rats. However, rats
differ from humans in the way they metabolize glucur-
onolactone. In a follow-up study, which included a larger
sample of rats, no effects on kidneys were reported,
leading researchers to conclude that a dose of
1 g·day·kg/BW was safe.24 It is estimated that the popula-
tion with the highest energy drink exposure (i.e., 95%
percentile) could be ingesting 1.5 cans per day of a Red
Bull–like product, which will amount to 840 mg/day of
glucuronolactone. Although this amount of glucuro-
nolactone is much higher than the typical exposure in
omnivore diets (1–2 mg/day), it is still well below the level
that would trigger food safety concerns.23 With regard to
humans, no studies were found describing the effects
of glucuronolactone on fluid regulation or in exercise
performance.
Taurine
Taurine is present in high concentrations in skeletal
muscle, the heart, and the central nervous system. It
has been proposed that taurine participates in
osmoregulation, stabilizes membrane potential in skeletal
muscle, affects calcium ion kinetics, has an antioxidant
and anti-inflammatory effect, and acts as a neurotrans-
mitter.25 One clinical study even suggests that oral treat-
ment with taurine improves cardiac performance in
humans with congestive heart failure.26 However, in one
study of healthy individuals, chronic supplementation
with taurine (5 g/day for 7 days) had no effect on exercise
heart rate or oxygen consumption during prolonged
submaximal exercise.27 Furthermore, muscle energy
stores (e.g., glycogen, ATP, creatine, phosphocreatine)
were not affected by a week of supplementation. Repeated
taurine ingestion over 7 days had no effect on muscle
metabolic responses to 120 min of exercise at moderate
intensity.27
In one study, taurine infusion in cirrhotic patients
resulted in transient diuresis and natriuresis, apparently
through the inhibition of the renin-aldosterone axis.28
Nutrition Reviews® Vol. 72(S1):108–120110

Based on this study, it could be hypothesized that taurine
ingestion may also have a diuretic effect on healthy indi-
viduals. Furthermore, the combination of taurine and caf-
feine in energy drinks could result in a summation of
their diuretic effects. Riesenhuber et al.29 investigated the
additive diuretic effects of caffeine and taurine in a cross-
over design using 12 healthy male volunteers. Participants
received 750 mL of four different test drinks in a blinded
fashion after a 12 h overnight fluid restriction. One drink
was regular Red Bull containing caffeine and taurine and
the other drinks lacked caffeine, taurine, or both,but were
otherwise identical. Urine volume and urine sodium con-
centration were measured for 6 h after drink ingestion.
Caffeine treatment elevated urine output and urine
sodium concentration, while taurine did not add to the
effect of caffeine on urine production. On the contrary,
there was a tendency for taurine to reduce diuresis and
natriuresis.Thus, the currently available information does
not support a diuretic role of taurine at the dose typically
contained in energy drinks.
Caffeine
Caffeine is the most widely consumed drug in the world.
This tri-methylxanthine antagonizes adenosine receptors
and inhibits phosphodiesterase actions. Of all the methy-
lxantines, caffeine has been found to increase urine
output with a diuretic potency that is exceeded only
by that of theophylline.30 Administration of 400 mg of
caffeine to healthy humans reduces kidney sodium
reabsorption and increases the fractional excretion of
water.31 These effects do not seem to be mediated either
by reduction in renal plasma blood flow32 or by increases
in plasma renin and vasopressin,33 both of which remain
unaltered by caffeine ingestion. Studies in mice show that
the antagonism of A1 adenosine receptors is responsible
for the diuretic and natriuretic actions of caffeine.34
Given this diuretic effect of caffeine, some water
balance studies have proposed the ingestion of 1.2 mL of
fluid per mg of caffeine to compensate its diuretic
actions.35 In the case of Red Bull, a 250 mL can has 80 mg
of caffeine and thus surpasses that ratio by 2.5-fold. In
addition, the effects of caffeine on increasing water excre-
tion are dose-dependent and blunted when studying sub-
jects that are in negative water balance (e.g., after an
overnight fast). A review of the diuretic effects of caffeine
in humans at rest proposes that there is a threshold of
around 250–300 mg of caffeine, below which caffeine
ingestion has no noticeable diuretic effect. This threshold
could be even higher than 250 mg in habitual caffeine
users.36
Ragsdale et al.37 found that the ingestion of 250 mL
of Red Bull (i.e., 1 can) had no measurable diuretic effect
in comparison to the same volume of a glucose placebo
solution. However, subjects were exposed to only 80 mg
of caffeine and they started the trial mildly dehydrated
(urine specific gravity ≥1.020). Ingestion of high volumes
of energy drinks (i.e., 4 cans of Red Bull) or of energy
drinks with higher caffeine concentrations (e.g., Monster
Energy Drink, and Rockstar 2x Energy Drink) could have
a diuretic effect and result in fluid deficit. Low levels of
fluid deficit while at rest are rarely problematic, but
during prolonged exercise, dehydration at a level of 1.5%
raises core temperature and heart rate, and increases the
perceived rate of exertion.17 Thus, further study of the
diuretic effects of caffeine during exercise is of interest.
EFFECTS OF CAFFEINE INGESTION DURING EXERCISE IN
HOT ENVIRONMENTS
During exercise, blood flow is redistributed to the
muscles, and the sympathetic nervous system lowers
blood flow to the kidneys to 1% of cardiac output
(∼250 mL/min). The extent of the reduction in renal
blood flow depends on the exercise intensity and dura-
tion. In fact,there is an inverse relationship between renal
flow and heart rate, with progressive reductions in renal
flow as the heart rate increases.38 Renal blood flow paral-
lels the glomerular filtration rate, explaining the reduc-
tion in urine formation during exercise.
Although exercise reduces urine formation, if fluid
ingestion during prolonged exercise in the heat is enough
to prevent dehydration, urine flow during exercise could
reach the same levels as when at rest.39 Some investigators
have wondered whether the inclusion of caffeine in a
rehydration drink could negatively affect fluid balance
and,hence, thermoregulation during exercise.During pro-
longed exercise in a controlled hot environment (i.e.,
33°C), male cyclists were invited to replace fluid losses
(3.6 L) by consuming one of the following drinks: 1) water,
2) water +caffeine, 3) sports drink, 4) or sports drink +
caffeine (Figure 1).39 Trials were compared to no fluid
replacement (NF) with or without caffeine pills. When
dehydration was not prevented (NF trials), urine produc-
tion was found to be very low and adding caffeine had no
diuretic effect.When subjects drank the sports drink, urine
production increased, but caffeine added to the sports
drink had no diuretic effect either. It is possible that the salt
included in the sports drink counteracted the diuretic
effects of the caffeine. However, when caffeine was added
to water, urine production increased by 37% (Figure 1).39
This increase in fluid losses via urine did not affect
whole-body fluid balance, since urination represented a
small percentage of total fluid loss (mostly sweat). Nev-
ertheless, caffeine tended to increase core temperature
when it was combined with the sports drink. This study is
not alone in showing that caffeine has a mild thermogenic
effect during40 and even prior to exercise.41 Finally, sweat
Nutrition Reviews® Vol. 72(S1):108–120 111

composition was measured and increased sweat sodium
excretion was observed when trials with caffeine con-
sumption were pooled together.39 This suggests that caf-
feine may alter fluid and mineral balance during
prolonged exercise in the heat.
It is unknown if these adverse effects of beverages
containing caffeine on urine output, thermoregulation,
and mineral balance could be replicated when using
energy drinks as a source of caffeine. The amount of
caffeine ingested in this study (6 mg/kg body weight) was
equivalent to drinking 5–6 regular cans of Red Bull (1.25–
1.5 liters); however, if that amount of caffeine were to
replicated using the energy drink, only 50% of sweat losses
would have been replaced while providing no salt. That
level of rehydration (50%) is insufficient during exercise in
a hot environment and results in core temperature eleva-
tions and cardiovascular strain17 that may limit endurance
performance. Conversely, full replacement of fluid losses
(2.5 liters) with an energy drink (i.e., Red Bull) would have
resulted in a caffeine dose of 11 mg/kg of body weight.
That dose of caffeine is triple the amount that has been
shown to be ergogenic for endurance performance (i.e.,
3–4 mg/kg).13 Thus, ingestion of energy drinks at high
volumes with the aim of rehydration during prolonged
exercise in the heat could potentially result in all the
adverse effects reported for caffeine ingestion.
EFFECTS OF ENERGY DRINKS DURING PROLONGED
EXERCISE IN A NEUTRAL ENVIRONMENT
Several studies have reported the effects of energy drink
consumption before endurance exercise in a neutral envi-
ronment (18−22°C, ∼64−72°F) in which fluid deficit is not
a concern for performance (Table 1). The earliest report to
support a role for energy drinks in endurance perfor-
mance enhancement is the one by Geiß et al.42 In that
study, 10 endurance-trained young males cycled for
60 min at 70% of VO2max, after which they increased the
workload by 50 watts every 3 min until exhaustion. Using
a double-blind method and three trials, 500 mL of three
different beverages were consumed 30 min into the
60 min submaximal ride. One of the drinks was regular
Red Bull (∼2 mg/kg caffeine and ∼26 mg/kg taurine),
another contained only the carbohydrate content of Red
Bull, and a third drink contained the carbohydrate and
caffeine content of Red Bull (∼2 mg/kg caffeine) only (no
taurine or glucuronolactone). Endurance time was
increased when ingesting regular Red Bull, above the car-
bohydrates only beverage and above the carbohydrates
and caffeine beverage. Since the beverage containing the
caffeine and carbohydrate content of Red Bull resulted in
the poorest performance, the authors suggest the
ergogenic effect of Red Bull may be due to taurine
(Table 1).
Taurine derives from the Latin word taurus, meaning
bull, because it was first isolated in the bile acid of bulls.47
The association between the strength of the bull and a
possible ergogenic effect of taurine is not supported by
data. Galloway et al.27 could not find any cardiovascular
or metabolic difference related to exercising for 2 h after a
week of taurine supplementation in comparison to
placebo ingestion. Furthermore, in a recent study, Pettitt
et al.48 showed that taurine and B vitamins at the levels
present in a can of Red Bull do not affect aerobic metabo-
lism during two bouts of intense exercise. In contrast to
theviewofGeißetal.
42 supporting an ergogenic role for
taurine, a recent meta-analysis of the literature suggests
that caffeine ingestion at the dose administered in this
study (2 mg/kg CAFF) increases endurance perfor-
mance.49 Thus, it is unclear which of the energy drink
components could be behind the improvement in perfor-
mance observed in the study by Geiß et al.,42 although
caffeine seems a likely candidate. In fact, it is intriguing
that the trial investigating the effect of caffeine and
glucose demonstrated the worst performance results of
the three trials.
The performance effects related to the ingestion of
500 mL of Red Bull (i.e., 2 cans) has also been explored by
Ivyetal.
43 In their study, well-trained cyclists simulated a
cycling time trial to complete a load equivalent to a ride
for 60 min at 70% of each participant’s maximal aerobic
Figure 1 Urine flow (UF) prior to and during 120 min of
exercise in the heat at 63% VO2max; without rehydration
(NF), rehydrating 97% of sweat losses with water (WAT),
with a carbohydrate-electrolyte solution (CES), or
combining these treatments with caffeine ingestion
(CAFF +NF, CAFF +WAT, and CAFF +CES). Data for
seven subjects are presented as mean ±SEM. *Different
from NF trial (P≤0.05). †Different from preexercise
(P≤0.05). The right inserts display the main effects of
caffeine ingestion. ‡Different from noncaffeine trials
(P≤0.05).
Reproduced from Del Coso et al.39 with permission.
Nutrition Reviews® Vol. 72(S1):108–120112

Table 1 Summary of studies with double-blind, randomized, crossover designs examining the effects of energy drinks on endurance performance.
Reference Subjects Habitual caffeine
consumption
Dose Protocol Findings Improvements
(%; ES)
Geiß et al.,
(1994)42
10 endurance
athletes
Not reported 3 Treatments:
1) A: 500 mL regular Red Bull
2) B: 500 mL Red Bull only CHO
and caffeine without taurine
3) C: 500 mL Red Bull only CHO
without caffeine or taurine
(Caffeine dose in A and B
treatments ∼2.0 mg/kg)
60 min cycling at 70%
VO2max followed by an
incremental test to
exhaustion
↑Endurance time in A
compared to B
treatment
14.9% and 0.74 ES*
↑Endurance time in C
compared to B
treatment
24.5% and 1.03 ES;
8.3% and 0.31,
respectively*
Ivy et al.,
(2009)43
12 young trained
cyclists
Not reported 2 Treatments:
1) 500 mL Red Bull®(∼2.0 mg/kg
caffeine)
2) 500 mL of flavored water
pacebo
Time to complete an
amount of work
equivalent to 1 h
cycling at 70% Wmax
↓Time required to
complete the work
4.7% and ES not
available*
Candow
et al.,
(2009)44
17 moderately
trained subjects
Range, 50–200 mg/day 2 Treatments:
1) Volume of sugar-free Red Bull
to deliver 2.0 mg/kg caffeine
2) Same volume of flavored water
placebo available energy drink
Time to exhaustion at
80% VO2max running
on a treadmill
No significant effect on
performance
6.3% and 0.22 ES
Walsh et al.,
(2010)45
15 recreationally
active subjects
Not reported 2 Treatments:
1) 500 mL of Amino Impact with
2.05 g of a mix of caffeine,
taurine and glucuronolactone
2) 500 mL of flavored water
placebo
Time to exhaustion at
70% VO2max running
on a treadmill
↑Time to exhaustion 12.5% and ES not
available*
Schubert
et al.,
(2013)46
6 endurance-
trained runners
80 mg/day 3 Treatments:
1) 59 mL Red Bull Energy Shot
(∼1.2 mg/kg caffeine)
2) 59 mL Yerba maté shot
(2 mg/kg caffeine)
3) 59 mL placebo
Time to complete 5 km
on a treadmill
No significant effect on
performance
1.8–2.4%
0.24–0.30 ES
* Significant differences (P<0.05).
Abbreviations: CHO, carbohydrates; ES, effect size.
Nutrition Reviews® Vol. 72(S1):108–120 113

power. In this study, the placebo drink contained only
water with artificial color and sweetener. Performance
improved in the Red Bull time trial by 3 min (∼5%) in
most (83%) of the participants. Either the carbohydrate or
the caffeine content in Red Bull could have been respon-
sible for the improved performance. Subjects ingested
2.3 mg of caffeine per kilogram of weight, and similar
doses have been found to be ergogenic when ingested
without the other energy drink ingredients.14 Ingestion of
an energy drink 40 min before exercise elevated blood
glucose, insulin, and blood lactate, reduced plasma free
fatty acids, and maintained higher rates of carbohydrate
oxidation in the latter stages of exercise.43 All these
responses are compatible with an increased glucose
supply to the working muscles, which may allow higher
rates of ATP production. Thus, it is also possible that
ingestion of the 54 g of carbohydrates contained in the 2
cans of Red Bull had some role in the improved endur-
ance performance. Of note, in the study of Ivy et al.,43 the
rate of perceived exertion tended to be lower on the
energy drink trial and β-endorphin levels tended to be
higher than on the control trial.
Not only cycling, but running time to exhaustion at
70% VO2max is also improved after ingestion of an energy
drink containing high levels of caffeine (2 g each of caf-
feine, taurine, and glucuronolactone) when compared to
a placebo without caffeine.45 In contrast, when running
intensity is higher (i.e., 80% VO2max) and running time to
exhaustion is, therefore, reduced, (12–17 min), ingestion
of sugar-free energy drinks that deliver either a low
(1.2 mg/kg) or a moderate (2 mg/kg) caffeine dose has no
effect on performance.46
In summary, the available literature on the effects of
energy drinks on endurance supports an ergogenic effect
when performance is prolonged (∼60 min) and ingestion
of the energy drink provides at least 2 mg/kg body weight
of caffeine. Due to the lack of a proper caffeine placebo
control, there is no information about whether compa-
rable effects could be found when similar amounts of
fluid with only caffeine and carbohydrates are consumed.
Data from Kovacs et al.14 suggest this may be the case;
however, when exercise intensity is increased and exer-
cise duration falls below 20 min, energy drink consump-
tion to provide up to 2 mg/kg of caffeine does not seem to
result in an ergogenic effect.
EFFECTS OF ENERGY DRINK INGESTION ON
SHORT-TERM HIGH-INTENSITY
EXERCISE PERFORMANCE
Compared to the literature on endurance performance, a
relatively larger number of studies are available on the
effects of energy drinks during high-intensity exercises.
Within this category are the following different perfor-
mance durations: 1) trials lasting beyond 1 min to the
point of muscle failure50–55; 2) efforts lasting below 1 min,
such as the Wingate test and 20-meter sprints51,52,56–59;and
3) single maximal isometric, isokinetic,or isoinertial con-
tractions55 lasting only a few seconds.
All the literature is very recent, but Forbes et al.,51 in
2007, appear to be the first research group to have exam-
ined the ergogenic effects of a marketed energy drink on
neuromuscular performance. They found that a 500 mL
serving of Red Bull (2.0 mg/kg caffeine) significantly
increased the total number of bench press (BP) repeti-
tions over 3 sets at 70% of one-repetition maximum (RM)
in 15 young adults (with 1RM being the maximum
amount of weight lifted while completing a full range of
motion). In this study, during the Red Bull trial partici-
pants achieved 34 repetitions versus 32 repetitions when
they received the placebo (5.9% and 0.24 effect size).
However, no differences were reported for peak or
average power during three consecutive 30-second leg
cycling Wingate tests spaced by 2 minutes of recovery.
In two similar studies,50,56 a commercially available
energy drink (unreported brand; 2.1 mg/kg of caffeine)
was provided to recreationally active subjects and
resulted in a significant increase in total leg press lifting
volume (12.3% and 0.24 effect size). However, it had no
effect on bench press weight lifted50 or on 2 ×20-second
Wingate test.56 Duncan et al.,54 using a noncommercial
self-prepared sugar-free energy drink (5 mg/kg of caf-
feine diluted into 250 mL of sugar-free artificially sweet-
ened water), found a significant increase in bench press
repetitions to failure with 60% of 1RM in 13 moderately
trained athletes. Similar to Duncan et al.,54 Woolf et al.52
formulated their own noncommercial sugar-free energy
drink (5 mg/kg caffeine and 0.125 g/kg carbohydrate) to
determine its effect on a single dash, repeated sprint
ability and local muscle endurance tests. They found no
ergogenic effect on 40-yd dash, 20-yd shuttle run, or
bench press repetitions to failure using a fixed load (84 or
102 kg) in 17 college football players.
Gwacham and Wagner58 found no ergogenic effects
of an AdvoCare Spark pouch of 120 mL (1.2 mg/kg caf-
feine) on the Running-based Anaerobic Sprint Test
(6 ×35 meters) in a sample of American college football
players. Similarly, Astorino et al.59 did not find any
ergogenic effects of a can of Red Bull (1.3 mg/kg caffeine)
on repeated sprint performance (ttest – 3 ×8 sprints) in
female soccer players. Hoffman et al.57 also reported that
120 mL of Redline Extreme (2.0 mg/kg caffeine) ingested
10 minutes before exercise had no effect on anaerobic
power measured by 3 repeated 20-second Wingate tests
separated by a 10-minute rest in 12 male strength-trained
athletes. In the only study that has addressed the isolated
effects of both caffeine and taurine on neuromuscular
performance, Eckerson et al.55 found that neither 500 mL
Nutrition Reviews® Vol. 72(S1):108–120114

of sugar-free Red Bull (2.0 mg/kg caffeine +24 mg/kg
taurine) nor 500 mL of sugar and taurine-free Red Bull
(2.0 mg/kg caffeine) had an ergogenic effect compared to
a sugar-free caffeine-free placebo when testing 1RM
strength or repetitions to failure at 70% of 1RM for bench
press exercise.
Despite the fact that all these studies have been con-
ducted with appropriate double-blind, randomized, and
crossover designs on well-trained subjects, only 5 of the
11 studies evaluated revealed significant energy drink-
mediated improvements in neuromuscular performance.
Of note, the performance enhancement was always on the
number of repetitions to muscle failure50–54 (5.1–15.5%
and 0.24–0.69 effect size; Table 2). In addition, 2 of the 5
studies that found significant ergogenic effects of energy
drinks on local muscle endurance used a study-specific,
specially prepared energy drink with 5 mg/kg of caffeine.
Given that caffeine has been suggested to be the only
ergogenic ingredient of energy drinks,3this lack of
positive effects on neuromuscular performance could
be related to the low caffeine dose administered in
the 8 studies using commercial energy drinks (range
1.2–2.1 mg/kg).
Recent findings indicate that the minimum caffeine
dose needed to significantly improve muscle strength and
power output in highly trained athletes is dependent on
the resistance that the musculature has to overcome (%
1RM). A dose of 3 mg/kg was enough to improve high-
velocity muscle actions against low loads (i.e., 25–50%
1RM), whereas a higher caffeine dose (9 mg/kg) was nec-
essary against high loads (90% 1RM) (Figure 2).61 The
muscle actions involved in the testing protocols of the
aforementioned studies that used energy drinks as an
ergogenic aid required a different percentage of the ath-
letes’ maximum strength. For instance, the first pedal
strokes on a Wingate test or the first strides of a running
20-meter shuttle test require 90–100% 1RM. However, as
the event proceeds and the body accelerates, muscle
recruitment frequency (cadence) is greatly increased
while the percentage of the maximum force required is
drastically reduced (20–30% 1RM).62 The absence of
ergogenic effects on neuromuscular performance when
testing single actions near 1 RM may be explained by the
insufficient caffeine dose (range, 1.0–2.0 mg/kg) that a
single can of a commercial energy drink normally con-
tains (Table 2).
These findings are consistent with previous studies
in which caffeine doses above 5 mg/kg tended to produce
ergogenic effects on neuromuscular performance,63,64
while doses below 3 mg/kg usually did not promote sig-
nificant improvements.63,65 From a practical point of
view, data suggest that if an athlete wishes to improve
short-term, high-intensity exercise performance via
energy drinks the minimum amount consumed must be
the equivalent of 3–4 cans of Red Bull per 60 kg of body
weight, or 4–5 cans for a subject weighing approximately
80 kg (∼5 mg/kg of caffeine). However, when ingesting
that amount of Red Bull (5 cans) to achieve a caffeine
ergogenic dose for neuromuscular performance, subjects
would also be ingesting 135 g of carbohydrate, 5 g of
taurine, 3 g of glucuronolactone, and 0.175 g of B vita-
mins. The interaction of these components at these high
concentrations is still unknown. In addition, energy
drinks are not formulated to speed up fluid absorption,
and different rates of incorporation of different compo-
nents into the blood could result in unwanted side effects.
Thus, when high doses of caffeine are needed, energy
drinks may not be the optimal choice.
CONFOUNDING FACTORS IN THE PERFORMANCE
EFFECTS OF ENERGY DRINKS
Besides the volume of energy drinks and, thus, caffeine
ingested, other confounding variables may be behind the
absence of significant effects on neuromuscular and
endurance performance among studies that follow
similar protocols and energy drink consumption
volumes.Among these confounding variables, the follow-
ing stand out: 1) the timing of energy drink ingestion
relative to the time of testing, 2) the role of caffeine
habituation,and 3) the time-of-day and circadian rhythm
effect on the ergogenic potential of energy drinks.
Studies assessing the ergogenic effect of energy
drinks report different elapsed times between drink
intake and the beginning of the testing protocols.
For instance, 10–20 min,57 30–40 min,43,48,50,56 and
50–60 min.44,46,51,52,60 In most people, the plasma caffeine
concentration peaks 30–60 min after ingestion,66,67 with
an elimination half-life ranging between 2.5 and
10 hours.68 However, the absorption rate constant of caf-
feine is influenced by the physicochemical properties of
the dose formulation, including pH, volume, and compo-
sition.69 For example, caffeine absorption is faster from
chewing gum than from a capsule,70 and from a capsule
than from coffee.71 Thus, it is not clear what amount of
time should elapse between energy drink ingestion and
performance testing to optimize results.
Since no other ingredient of energy drinks has been
consistently reported to be ergogenic,3it seems reason-
able to standardize the time between energy drink inges-
tion and the beginning of the testing protocols or sports
events to 60 min to allow caffeine to reach peak plasma
concentration. Moreover, although the caffeine half-life
normally exceeds 2.5 hours, the potential ergogenic effect
of caffeine in athletic efforts exceeding 1 hour is presently
being questioned.72 However, caffeine seems to maintain
its ergogenic properties when ingestion is repeated
during long-term, exhaustive endurance exercise.73 In
Nutrition Reviews® Vol. 72(S1):108–120 115

Table 2 Summary of studies with double-blind, randomized, crossover designs examining the effects of energy drinks on power-based performance.
Reference Subjects Habitual caffeine consumption Energy drink dose Protocol Findings Improvements
(%; ES)
Forbes et al.,
(2007)51
15 healthy
physically active
subjects
From naive to
>200 mg/day
2 Treatments:
1) 500 mL of Red Bull(∼2.0 mg/kg caffeine)
2) 500 mL of isoenergetic, isovolumetric,
noncaffeinated placebo
Total number of BP repetitions
over 3 sets at 70% 1RM
↑total repetitions 5.9% and 0.24 ES*
Peak and mean power during
repeated Wingate tests
No significant effect on
Wingate peak or mean
power
0.1–1.7% and
0.01–0.11 ES
Wolf et al.,
(2008)60
18 male highly
trained athletes
40.8 ±51.0 mg/day 2 Treatments:
1) Self-prepared beverage with 5 mg/kg
caffeine +125 mg/kg of CHO
2) CHO placebo drink
Total weight lifted in BP and
leg press to muscle failure
↑total weight lifted in BP 15.5% and 0.69 ES*
30 s Wingate test ↑Wingate peak power 5.1% and 0.56 ES*
Wolf et al.,
(2009)52
17 male collegiate
football players
<50 mg/day 2 Treatments:
1) Self-prepared beverage with 5 mg/kg
caffeine +125 mg/kg of CHO
2) CHO placebo drink
40-yd dash test No significant effect on
performance
<0.5% and ES <0.1
20-yd shuttle test
Total number of BP repetitions
with a fixed absolute load
(84 or 102 kg)
Hoffman et al.,
(2009)57
12 male
strength-trained
athletes
Not reported 2 Treatments:
1) 120 mL of Redline Xtreme (∼2.0 mg/kg
caffeine)
2) 120 mL of flavored water placebo
20-s Wingate test No significant effect on
Wingate peak or mean
power
<1.5% and ES <0.2
Campbell et al.,
(2010)56
15 recreationally
active subjects
Not reported 2 Treatments:
1) marketed energy drink (∼2.1 mg/kg
caffeine)
2) CHO placebo drink
2×20-s Wingate tests,
separated by 150 s
No significant effect on
performance
1.0% and 0.03 ES
Campbell et al.,
(2010)50
18 recreationally
active subjects
Not reported 2 Treatments:
1) marketed energy drink (∼2.1 mg/kg
caffeine)
2) CHO placebo drink
Total weight lifted in 4 sets of
BP and leg press repetitions
to failure at an intensity of
80% 1RM
↑total weight lifted in leg
press
12.3% and 0.24 ES*
Duncan and
Oxford,
(2011)53
13 moderately
trained athletes
Range 169–250 mg/
day
2 Treatments:
1) Self-prepared beverage with 5 mg/kg
caffeine diluted into 250 mL of sugar-free
artificially sweetened water
2) 250 mL of flavored water placebo
BP repetitions to failure at 60%
of 1RM
↑total repetitions and total
weight lifted
8.9 to 9.4% 0.44 to
0.62 ES*
Duncan et al,
(2012)54
13 strength-trained
athletes
211 mg/day; range,
120–400 mg/day
2 Treatments:
1) 250 mL of diluted Quick Energy (∼3.0 mg/kg
caffeine)
2) 250 mL of flavored water placebo
BP, deadlift, prone row and
back squat repetitions to
failure at 60% of 1RM
↑total repetitions in all
exercises
Average of 7.5% and
0.25 ES*
Gwacham and
Wagner,
(2012)58
20 football players Significant interaction
effect between
caffeine use and the
beverage treatment
2 Treatments:
1) 237 mL of AdvoCare Spark (∼1.2 mg/kg
caffeine)
2) 237 mL of flavored water placebo
Running-based anaerobic
sprint test (RAST) 6 ×35m/
10 s
No significant effect on
performance
0% and 0.0 ES
Astorino et al.,
(2012)59
15 young healthy
female soccer
players
Habitual consumers 2 Treatments:
1) 255 mL of Red Bull (∼1.3 mg/kg caffeine)
2) 255 mL of Canada Dry Ginger Ale placebo
drink (caffeine and taurine free)
Repeated sprint performance
(test – 3 ×8 sprints)
No significant effect on
performance
<1% and <0.1 ES
Eckerson et al.,
(2013)55
17 resistance-
trained athletes
<50 mg/day 3 Treatments
1) 500 mL of sugar-free Red Bull (∼2.0 mg/kg
caffeine +∼24 mg/kg of taurine)
2) 500 mL of sugar-free Red Bull (∼2.0 mg/kg
caffeine without taurine)
3) 500 mL placebo without placebo nor taurine
BP 1RM strength and
repetitions to failure at 70%
of 1RM
No significant effect on
performance
<0.5% and ES <0.1
* Significant differences (P<0.05).
Abbreviations: 1RM, 1 repetition maximum; CHO, carbohydrates; ES, effect size.
Nutrition Reviews® Vol. 72(S1):108–120116

addition, the dose of caffeine required for an ergogenic
effect may differ if the ingestion takes place once the
exercise has begun.66 The effects of these different com-
binations of timing and dose need to be better studied if
we are to reach a better understanding of the ergogenic
effects of energy drinks.
To date, very little information is available regarding
the potential relationship between caffeine habituation
and the magnitude of endurance or neuromuscular per-
formance improvement following energy drink inges-
tion. Although Gwacham and Wagner58 did not detect
significant ergogenic effects of energy drinks
(∼1.2 mg/kg caffeine) on repeated sprint performance,
they found a significant interaction effect between caf-
feine habituation and the beverage treatment (energy
drink versus placebo), suggesting that athletes not
habituated to caffeine were more likely to improve per-
formance when consuming energy drinks than those
who regularly consumed caffeine. In animal models,
chronic caffeine consumption resulted in an increase in
the affinity of adenosine receptors within the central
nervous system and, therefore, an increase in the
amount of caffeine needed to have the same antagonist
activity on the receptors.74,75 In humans, Van Soeren and
Graham76 measured the time-to-exhaustion after sub-
jects abstained from caffeine ingestion for 0, 2, and 4
days. Although nonsignificant, there was a trend towards
greater improvement following 2 and 4 days
of abstinence. Bell and McLellan,77 using a similar
time-to-exhaustion protocol, showed that improve-
ments in performance were greater for caffeine-naive
(<50 mg/day) compared to habitual caffeine consumers
(≥300 mg/day).
In contrast, both Wiles et al.78 and Tarnopolsky and
Cupido79 found no relationships between caffeine
habituation and 1,500-meter running time and muscle
force development, respectively. All these data suggest
that caffeine habituation and its relationship to the
effects of energy drinks needs further study. Meanwhile,
to avoid the possible contaminating effects of this vari-
able, it is recommended that researchers use partici-
pants who report similar regular caffeine consumption.
Furthermore, as suggested by Ganio et al.,75 athletes
should abstain from caffeine ingestion for no fewer than
7 days before the experimental trials or competition
events. In addition, it is strongly recommended that
each manuscript report the mean, standard deviation,
and confidence interval for the athletes’ regular caffeine
ingestion.
Most of the studies that assessed the potential of
energy drinks to enhance endurance or short-term,
high-intensity exercise performance have been con-
ducted in the mornings43,52–54–60 and a minority in the
afternoons,44 while the rest do not detail the time-of-day
of testing.46,48,50,51,55–58 To date, no study has addressed the
possible implications of circadian rhythm for the
ergogenic potential of energy drinks. It was recently
found that the ergogenic effect of a 6 mg/kg caffeine
dose on the neuromuscular performance in the morning
(8:00 a.m.) (5.4–9.4%; 0.75–1.15 effect size; P<0.05) was
completely lost in the afternoon (18:00 p.m.) with the
same caffeine dose administered.80 The mechanisms
behind this time-of-day-related effect of caffeine on
muscle performance are not clear. One plausible expla-
nation is that neural activation is almost complete in the
afternoon, and caffeine may have minimal room for
improvement at that hour.16 These data suggest that the
time of day at which an energy drink or caffeine is
ingested is a confounding variable that should be taken
into consideration when studying the ergogenic effects
of these beverages.
Figure 2 Dose-response effects of caffeine ingestion on
load-velocity relationship for bench press (A) and full
squat (B) exercises. Data are means ±SD. *Significant
differences (P≤0.05) compared to the PLAC trial within
each load.
Reproduced from Pallarés et al.61 with permission.
Nutrition Reviews® Vol. 72(S1):108–120 117

SIDE EFFECTS OF ENERGY DRINKS
Although various studies81,82 and several reports from
international institutions23,83 have described the possible
negative effects that the habitual consumption of energy
drinks may have on health, to date there is little informa-
tion on the side effects that acute ingestion of energy
drinks may have on the physical performance and per-
ceived fatigue of athletes. For instance, when analyzing
endurance performance, several studies have reported
positive effects on the rate of perceived exertion (RPE)
after the ingestion of one or two cans of commercial
energy drinks (2.0–3.0 mg/kg of caffeine),43,54 while
others did not find any effect on this outcome.44,46,48 Thus,
a reduction in the perception of physical fatigue is not a
consistent effect of energy drink ingestion when perform-
ing endurance exercise.
Using questionnaires to evaluate the side effects of
energy drinks, Hoffman et al.57 found that 120 mL of the
marketed Redline Xtreme energy drink (∼2.0 mg/kg of
caffeine) significantly improved the participants’ subjec-
tive feelings of energy and focus, while no differences
were detected for the feelings of fatigue and alertness.
These data are consistent with the findings of Walsh
et al.,45 who found that recreationally active subjects con-
suming a commercial energy drink (caffeine dose not
reported) felt greater focus and energy as well as less
fatigue (compared to a placebo treatment) before and
during a time-to-exhaustion test. These positive effects
disappeared immediately after exercise. Similarly, the
mood state scores for vigor were significantly greater
and fatigue scores significantly lower 60 min after the
ingestion of a noncommercial, self-prepared, sugar-free
energy drink (5 mg/kg of caffeine) compared to a placebo
beverage.53
Astorino et al.59 found that only 2 of 15 female par-
ticipants felt adverse side effects (i.e., stomach ache and
feelings of mild tremor) following the ingestion of a can
of Red Bull (∼1.3 mg/kg caffeine). In a descriptive cross-
sectional study, Desbrow and Leveritt84 found that an
average caffeine dose of 3.8 ±3 mg/kg produced very
minor and infrequent adverse symptoms during the
Ironman triathlon events. In a recent study, the caffeine
dose was systematically raised (0–3–6 and 9 mg/kg) while
monitoring participants’ positive and adverse caffeine
effects through a validated questionnaire. Although the
positive feelings such us increased vigor/activeness and
perception of performance improvement were already
Figure 3 Dose-response on side effects of caffeine ingestion. Data are presented as percent of prevalence.
Reproduced from Pallarés et al.61 with permission.
Nutrition Reviews® Vol. 72(S1):108–120118

clearly present with the 3 and 6 mg/kg doses, the presence
of negative side effects (i.e., gastrointestinal problems,
headaches, and insomnia) increased markedly with the
9 mg/kg caffeine dose (Figure 3).61 Furthermore, when a
moderate dose of caffeine (6 mg/kg) is ingested in the
evening (18:00 p.m.), the negative side effects felt by the
participants drastically increase compared to the same
dose of caffeine ingested in the morning.80 In summary, in
sports events lasting longer than half a day, the negative
side effects generated by the ingestion of a sufficient
amount of energy drink to provide caffeine doses higher
than 6 mg/kg in the mornings, and 3 mg/kg in the after-
noons, could counteract the ergogenic effects of caffeine
and result in reduced physical performance.
CONCLUSION
Energy drinks are difficult to evaluate from the nutri-
tional and ergogenic perspective due to the variety of
ingredients they contain (e.g., water, sugars, caffeine,
other stimulants, amino acids, herbs, and vitamins),
which is further complicated by the introduction of
calorie-free and concentrated (shot) versions to the mar-
ketplace. While the latter versions are formulated for fast
delivery of their main stimulant (typically caffeine), the
regular versions deliver other nutrients that have not
been proven to be ergogenic (e.g., taurine, glucurono-
lactone, vitamins). Due to their high carbohydrate con-
centration and lack of salts, energy drinks are not a good
beverage choice when prolonged exercise in a warm envi-
ronment is likely to require rehydration. Short-term,
high-intensity performance could be improved by energy
drinks, but achieving this improvement requires the
ingestion of high volumes to deliver enough caffeine.
Ingestion of high doses of caffeine, although ergogenic,
could result in negative side effects that could counteract
the caffeine’s ergogenic effect.
Acknowledgments
Funding. The authors received no external funding rel-
evanttothisreview.
Declaration of interest. The authors have no relevant
interests to declare.
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