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The Myths Surrounding Pre-Exercise Carbohydrate Feeding

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Carbohydrate ingested 30-60 min before exercise may result in hypoglycaemia during exercise, a phenomenon often called rebound or reactive hypoglycaemia. There is considerable confusion regarding pre-exercise carbohydrate feeding with advice that ranges from 'consume carbohydrate in the hour before exercise' to 'avoid carbohydrate in the 60 min prior to exercise'. We analysed the studies available in the literature to draw conclusions about the use of carbohydrate in the pre-exercise period. Without performing a meta-analysis, it is clear that the risk of reduced performance is minimal as almost all studies point towards unaltered or even improved performance. This is despite the rather large metabolic changes that occur in response to pre-exercise carbohydrate feeding. It can be concluded that advice to avoid carbohydrate feeding in the hour before exercise is unfounded. Nevertheless athletes may develop symptoms similar to those of hypoglycaemia, even though they are rarely linked to actual low glucose concentrations. An individual approach may therefore be necessary to minimize these symptoms even though they do not appear to be related to exercise performance.
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Ann Nutr Metab 2010;57(suppl2):18–25
DOI: 10.1159/000322698
The Myths Surrounding Pre-Exercise
Carbohydrate Feeding
AskerE.Jeukendrup SophieC.Killer
School of Sport and Exercise Sciences, University of Birmingham, Birmingham , UK
ilar to those of hypoglycaemia, even though they are rarely
linked to actual low glucose concentrations. An individual
approach may therefore be necessary to minimize these
symptoms even though they do not appear to be related to
exercise performance. Copyr ight © 2011 S. K arger AG, Basel
Introduction
Even textbooks can be confusing when it comes to pre-
ex ercise meal s. Adv ic e ran ges fr om ‘ inge st ca rbohy dr ate in
the hour before’ to ‘avoid carbohydrate in the hour before
exercise’. Early studies have outlined the metabolic effects
of pre- exe rc ise c ar bohyd ra te feed in g. Th ese i nclud e hyp er-
insulinaemia and hyperglycaemia before exercise followed
by a rapid development of hypoglycaemia, high rates of
glycogenolysis and a reduction of lipolysis and fat oxida-
tion during exercise. Although these metabolic perturba-
tions can in theory reduce exercise performance, the re-
sults of studies have been varied. This review will discuss
the development of knowledge in this area from a histori-
cal perspective; it will discuss the evidence with regard to
Key Words
Glucose Exercise performance Timing of carbohydrate
intake Endurance
Abstract
Background/Aims: Carbohydrate ingested 30–60 min be-
fore exercise may result in hypoglycaemia during exercise, a
phenomenon often called rebound or reactive hypoglycae-
mia. There is considerable confusion regarding pre-exercise
carbohydrate feeding with advice that ranges fromcon-
sume carbohydrate in the hour before exercise’ to ‘avoid car-
bohydrate in the 60 min prior to exercise’. Methods: We ana-
lysed the studies available in the literature to draw conclu-
sions about the use of carbohydrate in the pre-exercise
period. Results: Without performing a meta-analysis, it is
clear that the risk of reduced performance is minimal as al-
most all studies point towards unaltered or even improved
performance. This is despite the rather large metabolic
changes that occur in response to pre-exercise carbohydrate
feeding. Conclusion: It can be concluded that advice to
avoid carbohydrate feeding in the hour before exercise is un-
founded. Nevertheless athletes may develop symptoms sim-
Publish ed online: February 22, 2011
Prof. Asker Jeukendrup
School of Sport and Exercise Sciences
University of Birmingham
Edgbaston, Birmingham B15 2TT (UK)
Tel. +44 121 414 4124, Fax + 44 121 414 4121, E-Mail A.E.Jeukendr up
@ bham.ac.u k
© 2011 S. Karger AG, Basel
Accessible online at:
www.karger.com/anm
The Myths Surrounding Pre-Exercise
Carbohydrate Feeding
Ann Nutr Metab 2010;57(suppl2):18–25
19
the metabolic effects as well as performance effects of pre-
exercise glucose ingestion. It will also discuss the potential
need for individualization and it will end with recommen-
dations based on the currently available evidence.
H i s t o r i c a l V i e w
The first paper to describe the effects of pre-exercise
glucose ingestion was probably a paper by Ove Boje in
1940
[1] . In this paper, it was observed that when glucose
was ingested before exercise, blood glucose concentra-
tions dropped during exercise. It was not until the 1970s
that this observation was followed up. At that time the
importance of muscle and liver glycogen during pro-
longed exercise was generally recognized. It was thought
that providing additional substrate either before or dur-
ing exercise should improve exercise performance. Ahl-
borg and Felig
[2] used arteriovenous balance techniques
during low-intensity exercise to study the effects of pre-
exercise glucose feeding. Their subjects received a large
a mou nt of ca rb oh yd rate (20 0 g) 50 mi n b ef ore em ba rk ing
on 4 h of exercise at 30% VO
2 max. Several important ob-
servations were made. First, arterial glucose and insulin
concentrations were high at the start of exercise. Second-
ly glucose uptake by the exercising legs was 40100%
greater when glucose had been ingested before exercise.
Thirdly glycerol concentrations were lower suggesting a
suppression of lipolysis and fat metabolism. Similar ob-
servations were made by Costill et al.
[3] . They observed
that ingesting glucose in the 30–45 min before exercise
accelerated glycogen breakdown and induced hypogly-
caemia during exercise
[3] .
The performance effects of pre-exercise carbohydrate
feeding were first studied by Foster et al.
[4] . They re-
ported reduced endurance capacity (cycling at 80%
VO
2 max to exhaustion) during the glucose trial com-
pared with water. This paper became the basis of a rec-
ommendation that would stand for many years, namely:
‘a voi d car bohyd ra te in th e ho ur befor e e xer ci se’. A coupl e
of years later, Koivisto et al.
[5] published a paper that
would become much cited. This paper reinforced the
message that carbohydrate was to be avoided in the hour
before exercise. In this publication, it was concluded that
glucose ingested 45 min before exercise resulted in hy-
perglycaemia and hyperinsulinaemia at the onset of ex-
ercise, followed by a rapid development of hypoglycae-
mia during exercise. It was suggested that these effects
could be prevented by using a low glycaemic index (GI)
carbohydrate (fructose). In the years to follow, much at-
t en tio n w a s p a id t o t h is p he no men on t ha t be c am e k no wn
as reactive hypoglycaemia or rebound hypoglycaemia. It
was generally assumed that carbohydrate ingestion prior
to exercise would cause hypoglycaemia, suppress fat me-
tabolism, accelerate glycogen breakdown and thus re-
duce exercise performance.
Research in the Last 30 Years
Since these early studies in the 1970s and early 1980s,
large numbers of studies have investigated the effects of
pre-exercise carbohydrate feeding on metabolism and
performance
[6–16] . Despite this wealth of information,
it is not easy to draw firm conclusions about the effects
on metabolism and in particular, the development of hy-
poglycaemia. The results of all these studies are rather
mixed, most likely the results of studies using different
types of carbohydrates, different modes of exercise, dif-
ferent intensities of exercise, different subjects (some
trained, some untrained) and different timing of carbo-
hydrate intake. Since all of these factors may affect the
outcome, this makes it very difficult to compare the re-
sults and determine the exact causality of the different
effects.
To clarify these results, we performed a systematic se-
ries of studies to investigate the effects of pre-exercise
carbohydrate feeding
[10 , 12 , 17–19] . All studies had a
similar design and only 1 variable was changed at a time
(e. g. ti mi ng o f intak e or ty pe of ca rbo hyd rate). Each st udy
had a control condition where 75 g of glucose was ingest-
ed 45 min prior to exercise. The exercise consisted of 20
min of steady-state exercise at 70% VO
2 max followed by
a performance test (a time trial lasting approximately 40
min). All subjects were trained and hence representative
of the main population that the advice would be aimed
at. The overall conclusion of these studies was that there
is no effect of pre-exercise carbohydrate feeding on per-
formance, even though in some cases hypoglycaemia did
develop. Furthermore, there was no relationship between
low blood glucose concentrations and performance.
Below we will briefly discuss the metabolic events
that are responsible for rebound hypoglycaemia, fol-
lowed by a discussion of the effects of different amounts
of carbohyd rate as well as t he effects of timing of intake,
the type of carbohydrate and the form in which it is in-
gested.
Jeukendrup/Killer
Ann Nutr Metab 2010;57(suppl2):18–25
20
Rates of Glucose Appearance versus Glucose
Disappearance
When carbohydrate is ingested, an insulin response
is triggered almost instantly. It has been shown that even
an artificial sweetener can result in an insulin response
indicating that the carbohydrate ingested is sensed well
before the arterial blood glucose concentration rises.
Once the carbohydrate is absorbed, insulin is released at
an increased rate from the pancreas. Interestingly, the
ingested carbohydrate will blunt the hepatic glucose
production
[20, 21] . Insulin will recruit GLUT-4 trans-
porters to the muscle membrane and facilitate glucose
uptake. Depending on the type of carbohydrate, the rate
of ingestion and individual differences, plasma glucose
and insulin concentrations peak 20–40 min after inges-
tion of a single bolus, whereas glucose uptake is in-
creased as soon as insulin concentrations rise. When ex-
ercise is in iti ate d in the pre sence of high insu lin conc en-
trations, muscle glucose uptake will be further increased
through an independent calcium-dependent pool of
GLUT-4 transporters
[22] . The rapid decrease in blood
glucose concentration seen in the first 15 min of exercise
is the result of this rapid increase in glucose uptake that
is not compensated by an increased rate of appearance
of glucose. In some cases, hypoglycaemia will develop
(blood glucose ! 3.5 mmol/l). However, in other cases,
the glucose concentration will stay above this arbitrary
threshold. The symptoms of hypoglycaemia are most
likely related to a reduced delivery of glucose to the
brain.
There are additional effects of carbohydrate inges-
tion. As more carbohydrate becomes available to the
muscle, glycolysis will be stimulated
[23] . This stimula-
tion, in combination with an insulin-induced inhibition
of lipolysis in both adipose tissue and muscle, results
in a reduction in fat oxidation. The mechanisms of this
increased carbohydrate oxidation and concomitant re-
duction in fat oxidation can be found in more detail in
other literature
[24, 25] . Initially it was thought that the
increased glycogenolysis would result in premature gly-
cogen depletion and early onset of fatigue
[3] . How-
ever, the effect is transient, approximately lasting only
for the first 20 min of exercise. Thus, it appears that
these relatively small differences in glycogen breakdown
have no significant effect on exercise performance.
Amount of Carbohydrate
A study by Short et al. [26] show ed t hat a higher insuli n
concentration at the start of exercise, resulting from 75g
carbohydrate ingestion, did not further decrease blood
glucose concentrations compared with the ingestion of
22 g of carbohydrate. In agreement, Sherman et al.
[16]
did not find significantly different blood glucose respons-
es during exercise when subjects ingested either 78g or
156 g of a maltodextrin and glucose mixture 60 min be-
fore exercise. Despite higher insulin concentrations at the
onset of exercise following the ingestion of 156 g com-
pared with 78 g of carbohydrate, blood glucose concentra-
tions in both trials decreased. Similar findings were also
obtained by Jentjens et al.
[11] when 25, 75 or 125 g of car-
bohydrate was ingested 45 min prior to exercise.
Taken together, these studies suggest that the fall in
blood glucose concentration during submaximal exercise
(62–72% VO
2 max), following the consumption of a mod-
erate amount of carbohydrate (75 g of carbohydrate)
within the hour before exercise, cannot be prevented ei-
ther by ingesting a smaller (about 22 g) or a larger (more
than 155 g) amount of carbohydrate. Furthermore, there
appear to be no performance differences when a smaller
or a larger amount of carbohydrate is ingested prior to
exercise.
Timing of Carbohydrate Intake
Very few studies have directly investigated the effect
of timing of carbohydrate intake across a range of times
in the immediate pre-exercise period. Moseley et al.
[12]
investigated the metabolic response to 75 g of glucose in-
gested 15, 45 or 75 min before exercise. Plasma glucose
concentrations were significantly higher immediately be-
fore exercise in the 15 min pre-exercise feeding group
compared with the 45min and 75min pre-exercise
groups. Furthermore, insulin concentrations immediate-
ly before exerci se wer e als o si gnif icantl y higher when ca r-
bohydrate was consumed 15 min before exercise com-
pared with 45 min before exercise. The lowest insulin
co ncent ra tions were obser ved when c arbo hyd rate wa s in-
gested 75 min before exercise. Interestingly, differences in
plasma glucose concentration disappeared within 10 min
of exercise and no significant differences in performance
were found. In addition to these findings, Pritchett et al.
[27] studied the effects of a nutrient bar (20 g carbohy-
drate, 12 g protein and 4.5 g fat) consumed at either 15 or
60 min before exercise. Unlike Moseley et al.
[12] , no sig-
The Myths Surrounding Pre-Exercise
Carbohydrate Feeding
Ann Nutr Metab 2010;57(suppl2):18–25
21
nificant differences were seen in glucose concentrations
between groups. It is possible that this was the result of
the volume of carbohydrate consumed and its co-inges-
tion with other nutrients. In accordance with Moseley et
al.
[12] , the timing of the ingestion of pre-exercise nutri-
tion had no significant effect on exercise performance.
Although research has consistently found clear meta-
bolic differences in response to the timing of pre-exercise
carbohydrate ingestion within the hour before exercise,
the performance effects have been somewhat equivocal.
With the exception of 1 study
[4] , research has either
found no performance effects
[6–13] or a performance
improvement
[14 –16] . Based on current research, it would
appear that there is little evidence to suggest avoiding
carbohydrate intake in the hour before exercise. Further-
more, the ingestion of carbohydrate during this period
may lead to enhanced performance. Therefore, individu-
al experimentation is required to find one’s optimal pre-
exercise nutrition routine.
To minimize the risks of hypoglycaemia, carbohy-
drate can be ingested just prior to exercise (in the last
5 min) or during the warm-up. Brouns et al.
[28] gave a
carbohydrate-containing beverage (sucrose, fructose,
maltodextrin, or glucose) or a placebo to subjects during
a warm-up. This warm-up was followed by a short break
and an exercise bout. Results showed that the warm-up
and final exercise led to increased catecholamine concen-
trations and a blunted insulin response. Data also showed
that the intake of carbohydrate-containing beverages
during a warm-up followed by a small break does not lead
to rebound hypoglycaemia, independent of the amount
of carbohydrate ingested, but instead increases blood glu-
cose. When carbohydrate is ingested just before exercise
( ! 10 min), exercise will start before the insulin concen-
tration has increased and therefore this timing strategy
would provide the carbohydrate but minimize the risk of
reactive hypoglycaemia.
G l y c a e m i c I n d e x
The GI is a functional tool used to categorize carbohy-
drates based on their blood glucose and insulin response
to a known food. GI is calculated by the glucose area under
the curve during approximately 2 h following ingestion of
50 g carbohydrate compared to a reference food such as
glucose or white bread (GI: 100). Carbohydrates are gener-
ally categorized i nto either low ( ! 55), moderate (56–70) or
high (70–100) GI carbohydrates
[29] . A low GI carbohy-
drate results in a slow and gradual rise in plasma glucose
and insulin whereas a high GI carbohydrate results in a
rapid rise in glucose and insulin concentration to peak val-
ues, before returning to baseline relatively quickly.
T he G I has be en w id ely us ed by exerc is e scient is ts si nce
its introduction in 1984
[30, 31] to understand the effects
of pre-exercise carbohydrate on both metabolism and ex-
ercise performance. Studies confirmed that the ingestion
of high GI carbohydrates in the hour before exercise leads
to elevated plasma glucose and/or insulin concentrations
compared to low GI
[15 , 32] or moderate GI [33, 34] car-
bohydrates. At the onset of exercise however, these high
concentrations of plasma glucose have been shown to rap-
id ly decrea se t o hy pog lycaem ic leve ls ( ! 3.5 mmol/l) with-
in just 10–20 min. Although it is often assumed that the
reduced glycaemic response of low GI foods is due to a
lower rate of appearance of glucose in the circulation, it
has also been suggested that this may be due to an earlier
postprandial hyperinsulinaemia and an earlier increase
in the rate of disappearance of glucose, which attenuated
the increase in the plasma glucose concentration
[35] .
Hypoglycaemia during exercise has not been observed
in all individuals following the intake of high GI carbo-
hydrates
[3, 11, 19, 34, 36] . It is therefore thought that
some individuals may be more susceptible to hypoglycae-
mia. It was originally proposed that insulin sensitivity
may be a determinant of rebound hypoglycaemia
[37] ;
ho we ve r, J ent je ns et a l.
[11] found no relationship between
one’s sensitivity to insulin and the prevalence of rebound
hypoglycaemia. It is therefore still to be determined
which factors contribute to an individuals susceptibility
to rebound hypoglycaemia.
In addition to the more pronounced glycaemic and in-
sulinaemic responses caused by high GI carbohydrates,
there is also a trend for increased carbohydrate oxidation
during exercise
[9, 15, 38, 39] . This occurrence is due to
the increased glucose uptake and decreased plasma free
fatty acids
[9] . Furthermore, data have also shown that
the ingestion of low GI carbohydrates 45 min before ex-
ercise increases fat oxidation during exercise compared
with high GI carbohydrates
[11] .
Although it is clear that the GI of a carbohydrate has
significant effects on metabolism when consumed before
exercise, there is little evidence to suggest that there are
any performance effects. Original findings by Thomas et
al.
[15] suggested that ingestion of low GI carbohydrates
1 h before exercise could increase time to exhaustion
(TTE) by 20 min compared to high GI carbohydrates.
Since these initial findings, a multitude of studies have
taken place but few have been able to reproduce these re-
sults
[11, 13, 32–34, 39, 40] . Very few studies [41] have
Jeukendrup/Killer
Ann Nutr Metab 2010;57(suppl2):18–25
22
shown low GI carbohydrates to improve exercise perfor-
mance above high GI carbohydrates when consumed ap-
proximately 1 h before exercise. Studies which have used
cycling time trial performance
[11, 32, 34, 40] or TTE [13 ,
33, 39] have found no significant differences in perfor-
mance outcomes.
It is worth noting that when consumed 3 h before ex-
ercise, low GI carbohydrates do have the potential to en-
hance performance. Recent findings have shown signifi-
cant improvements in both running TTE
[42] and time
trial
[43] following ingestion of low GI carbohydrates
compared with high GI carbohydrates consumed 3 h be-
fore exercise. In these studies, however, no further carbo-
hydrate was ingested in the hour before exercise or during
exercise. Most athletes will ingest additional carbohy-
drate approximately 1 h before exercise begins or may use
carbohydrate during exercise and thus the practical rel-
evance of these findings is unknown.
Solid versus Liquid
The form of carbohydrates ingested before exercise
has also received attention for its potential effects on both
metabolism and performance. As well as measuring ex-
ercise capacity following the consumption of liquid,
semi-solid (gel) or solid forms of carbohydrate, studies
have also investigated the effects of form on oxidation
rates, glycogen synthesis and gastrointestinal tolerance.
The ingestion of solid food significantly slows gastric
emptying, digestion and absorption rates compared with
a liquid food
[44] . This impacts blood glucose concentra-
tion and hence it has therefore been proposed that the in-
take of solid foods before exercise may be beneficial in
providing a slower, more sustained release of glucose into
the blood
[45] . Interest ingly, data from recent studies com-
paring ingestion of solid versus liquid carbohydrate
[46]
and solid versus gel carbohydrate
[34] found no signif icant
differences in blood glucose concentrations between
groups. Furthermore, additional research has found no
differences in either carbohydrate oxidation rates between
solid versus liquid
[47] or liquid versus gel [48] carbohy-
drate consumed during exercise. Studies that have inves-
tigated performance effects have found no significant dif-
ferences following pre-exercise ingestion of solid versus
liquid
[49] or solid versus gel [34, 46] carbohydrates.
To summarize, there appear to be no data to suggest
that one particular form of carbohydrate can enhance or
reduce exercise performance over and above any other
form. In addition to this, it has been shown that there is
no difference in glycogen synthesis following liquid or
solid carbohydrates
[50, 51] . It would therefore be advis-
able to ingest whichever form of carbohydrate best suits
the individual athlete, based on the practical issues of
consumption and cost-effectiveness of the product. It
must be noted that the studies discussed above use solid
foods that are high in carbohydrate and low in fat, protein
and fibre. It is likely that if the macronutrient composi-
tion of the food is very different from the drink that the
metabolic effects will also be different.
E x e r c i s e I n t e n s i t y
Exercise intensity affects both glucose uptake by the
muscle and endogenous glucose production. In order to
maintain plasma glucose concentration during exercise,
hepatic glucose output is elevated
[20] to match the in-
creased muscle glucose uptake
[52] . During higher-inten-
sity exercise ( 1 80% VO 2 max), hepatic glucose output
may exceed glucose uptake and elevated plasma glucose
concentrations are often observed. This in turn could re-
duce the chances of developing hypoglycaemia. Most pre-
exercise carbohydrate feeding studies have been per-
formed at intensities eliciting 70% VO
2 max. While in
some of these studies hypoglycaemia was reported
[5, 53] ,
in other studies glucose concentration did not decrease
markedly
[13, 38, 54] . The results of studies where sub-
jects were asked to exercise at higher intensities ( 1 80%
VO
2 max) are also inconclusive. In some studies, glucose
concentration did not change or increased
[49, 55] , while
others have reported a decrease in glucose concentration
during the first 15 min of exercise
[4, 8, 56] . These studies
that investigated the glucose response during low-inten-
sity exercise did not describe the changes in glucose con-
centration during the first hours of exercise
[2, 57] . Based
on these data, it seems fair to summarize that effects of
exercise intensity on the development of hypoglycaemia
are inconclusive. It is likely that there are large individu-
al differences in the response to increasing exercise inten-
sities and studies in the literature are difficult to compare
because of the methodological issues mentioned above.
Prevalence of Hypoglycaemia and Factors
Affecting It
From several studies it is clear that hypoglycaemia is
highly individual, with some individuals very prone to
development and others much more resistant. As dis-
The Myths Surrounding Pre-Exercise
Carbohydrate Feeding
Ann Nutr Metab 2010;57(suppl2):18–25
23
cu ssed above, factors such as the amou nt of carbohydrate
ingested can decrease (or increase) the risk of developing
hypoglycaemia. Observations from a number of studies
in our lab revealed some interesting observations that
have not received a lot of attention in the published lit-
erature. From these studies it appeared that some indi-
viduals are clearly more prone to develop hypoglycaemia
than others. For example, in a study by Moseley et al.
[12] ,
it was observed that when carbohydrate was ingested 75
min before exercise, 5 individuals developed hypoglycae-
mia, when ingested 45 min before only 3, and when in-
gested 15 min before only 2 ( fig. 1 ). The 2 subjects who
developed hypoglycaemia in the condition that resulted
in the lowest prevalence also demonstrated hypoglycae-
mia in the other conditions. The subjects who developed
hypoglycaemia 45 min before exercise also developed hy-
poglycaemia 75 min before exercise. This seems to indi-
cate that some individuals are more prone to develop hy-
poglycaemia than others. As mentioned above, we also
found that these individual differences could not be ex-
plained by differences in glucose tolerance as measured
by an oral glucose tolerance test
[11] . It is therefore still to
be determined which factors contribute to an individual’s
susceptibility to develop hypoglycaemia.
Symptoms of Hypoglycaemia Do Not Mean
Hypoglycaemia
An interesting finding of the studies in our lab was
that some individuals developed symptoms of hypogly-
caemia in all conditions whereas others did not develop
these symptoms. Moreover, these symptoms were often
reported in the absence of true hypoglycaemia. In con-
trast, some subjects had extremely low plasma glucose
concentration but did not report any symptoms. This
finding was not new. In 1979, Foster et al.
[4] reported
that the symptoms reported did not match the serum glu-
cose concentrations. For example, 3 subjects reported ex-
treme symptoms of hypoglycaemia just before stopping
the ride. The blood glucose concentrations at this point
were 3.7, 4.6 and 3.1 mmol/l. This means that only one of
these values was low enough to be classified as hypogly-
caemia. On the other hand, one subject had a blood glu-
cose value of 2.4 mmol/l at that time point but did not
display any symptoms or unusual fatigue. At present, the
cause of the symptoms is still unknown but it is clearly
not related to a threshold blood glucose concentration. Or
if it is, this threshold may be individually determined and
cannot be captured by an average value of 3.5 mmol/l.
R e c o m m e n d a t i o n s
Based on the currently available evidence there ap-
pears to be no reason not to consume carbohydrate before
exercise as there do not seem to be any detrimental ef fects
on performance. Individuals prone to developing reactive
hypoglycaemia and/or symptoms that are often associ-
ated with it can find solutions to avoid it. These solutions
could include choosing low GI carbohydrates, ingesting
carbohydrate just before exercise or during a warm-up or
alternatively, avoiding carbohydrate in the 90 min before
exercise altogether.
Disclosure Statement
Asker Jeukend rup’s research ha s been funded by the Wellcome
Trust, Nestec, GSK, Cargill, GSSI, and Unilever. Sophie Killer has
nothing to disclose.
75-Pre
45-Pre
15-Pre
Fig. 1. Individuals who developed hypoglycaemia when carbohy-
drate was ingested 15, 45 or 75 min prior to exercise. Data from
Moseley et al.
[12] .
Color versi on available online
Jeukendrup/Killer
Ann Nutr Metab 2010;57(suppl2):18–25
24
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... l -1 ) (9,10,12,15,18,23,24,26). This phenomenon has been termed "rebound", "reactive" or "transient" hypoglycemia (20,26). On the other hand, other studies also in cycling have not reported transient hypoglycemia (1,13,14,25,27,36,38). ...
... Also, the higher muscle mass involved in running did not affect blood glucose response during exercise since, with the exception of one person, the same individuals who developed rebound hypoglycemia in CHO-Run, also developed rebound hypoglycemia in CHO-Cycle. Consistency about the phenomenon of transient hypoglycemia has been reported when more than one CHO trial is performed by the same subject in cycling (20). However, to the best of the authors' knowledge the reproducibility of this metabolic phenomenon has not been studied under identical experimental conditions where the type and amount of CHO intake, feeding time before exercise, exercise intensity and mode are kept the same. ...
... Individuals, who experience transient hypoglycemia as a result of pre-exercise CHO intake, will probably develop this metabolic disturbance to the same extent either during cycling or running. However, this perturbation is not associated with a detrimental effect during subsequent endurance exercise, since many studies have reported an improvement in endurance capacity and performance following pre-exercise CHO intake, despite an observed transient hypoglycemia early in exercise (20,33). ...
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This study examined the phenomenon of transient hypoglycemia and metabolic responses to pre-exercise carbohydrate (CHO) maltodextrin ingestion in cycling and running on the same individuals. Eleven active males cycled or ran for 30 min at 80% maximal heart rate (HRmax) after ingestion of either 1g/kg body mass maltodextrin (CHO-Cycle and CHO-Run respectively) or placebo (PL-Cycle and PL-Run) solutions. Fluids were ingested 30min before exercise in a double-blind and random manner. Blood glucose and serum insulin were higher before exercise in CHO (mean CHO-Cycle+CHO-Run) (Glucose: 7.4 ± 0.3 mmol. l-1 ; Insulin: 59 ± 10 mU. l-1) compared to placebo (mean PL-Cycle+PL-Run) (Glucose: 4.7 ± 0.1 mmol. l-1 ; Insulin: 8 ± 1 mU. l-1) (p<0.01), but no differences were observed during exercise among the 4 conditions. Mean blood glucose did not drop below 4.1 mmol. l-1 in any trial. However, six volunteers in CHO-Cycle and seven in CHO-Run experienced blood glucose concentration < 3.5 mmol. l-1 at 20min of exercise and similar degree of transient hypoglycemia in both exercise modes. No association was found between insulin response to maltodextrin ingestion and drop in blood glucose during exercise. Blood lactate increased with exercise more in cycling compared to running, and plasma free fatty acids (FFA) concentrations were higher in placebo compared to CHO irrespective of exercise mode (p<0.01). The ingestion of maltodextrin 30min before exercise at about 80% HRmax produced similar glucose and insulin responses in cycling and running in active males. Lactate was higher in cycling, whereas maltodextrin reduced FFA concentrations independently of exercise mode.
... Finally, the concentration is an important consideration, with research in rugby currently limited to 6-9% CHO. However, evidence from soccer suggests that a higher concentration (~12%) might have a more pronounced effect when CHO is ingested within close proximity (10-15 minutes) of the activity, may be a more feasible alternative to regular CHO feeding as this can be difficult to implement, and can reduce the risk of hypoglycaemia [30]. ...
... Secondly, due to the data being collected on all participants simultaneously with limited equipment and personnel, we were unable to measure blood glucose concentration within a timeframe that would allow for accurate interpretation. Whilst blood glucose would have supported our interpretation, numerous studies exist reporting the changes in glucose following ingestion have demonstrated that changes in performance do not appear to be directly associated with blood glucose concentration [22,23,30]. Thirdly, we were unable to include a control trial within this study due to the logistics of conducting research during a competitive season. ...
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The purpose of this study was to investigate the effects of a 12% carbohydrate (CHO) beverage on tackling technique and running performance during rugby league activity. Using a double-blind, placebo-controlled, randomised, crossover design, 15 academy rugby league players ingested a 250 ml bolus of a 12% CHO solution (30 g maltodextrin and 30 g sucrose in 500 ml) 15 minutes before two bouts of rugby activity. The rugby league match simulation for interchange players was used to standardise the movement patterns of activity and provide reliable outcome measures, whilst also reflecting the duration of a typical field-based conditioning session. Measures of tackling technique, external responses (e.g., fatigue index from sprint data) and rating of perceived exertion (RPE) were recorded throughout. Gut discomfort was measured before each bout. The interaction effect was largely compatible with the hypothesis for relative distance ( P <0.001, η ² = 0.217) and fairly compatible for tackling technique ( P = 0.068, η ² = 0.0640). The time effect for tackling technique, relative and high-intensity distance, sprint, and sprint to contact velocity, time at high metabolic power, PlayerLoad™, and RPE (all P <0.05; η ² = 0.131–0.701) was compatible with the hypothesis. Data for tackling technique, relative and high-intensity distance, sprint, and sprint to contact velocity, sprint, and sprint to contact fatigue index (all P <0.05; η ² = 0.189–0.612) was compatible with a supplement effect overall despite few differences in the pattern of change (interaction). Minimal gut discomfort was reported for the CHO (bout 1 = 27 ± 17; bout 2 = 23 ± 17 AU) and placebo (bout 1 = 23 ± 18 AU; bout 2 = 24 ± 13) trials. This study shows that a 12% CHO beverage before two bouts of standardised rugby activity is a practical and effective strategy for retaining tackling technique, increasing external responses, and reducing RPE without compromising gut comfort.
... Endogenous carbohydrate stores (e.g., liver and muscle glycogen) are an important, readily available energy source used to fuel physical performance (1). Reductions in glycogen content through prolonged aerobic exercise and suboptimal daily carbohydrate or energy intake are associated with declines in physical performance (2)(3)(4). A recent metaanalysis of 181 studies assessing muscle glycogen content reported that normal/adequate resting glycogen content was ∼450 mmol/kg dry muscle wt, high glycogen content was ∼550 mmol/kg dry muscle wt, and low glycogen content was ∼200 mmol/kg dry muscle wt (5). ...
... Two separate investigations by Weltan et al. (20,21) manipulated glycogen content by Data are presented as mean values. 1 Direct measurement of glycogen was conducted using a colorimetric assay in freeze-dried skeletal muscle. 2 No direct measurement of skeletal muscle glycogen content. 3 Reported as significantly different than low glucose in the primary manuscript. ...
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Individuals with high physical activity levels, such as athletes and military personnel, are likely to experience periods of low muscle glycogen content. Reductions in glycogen stores are associated with impaired physical performance. Lower glycogen stores in these populations are likely due to sustained aerobic exercise coupled with sub-optimal carbohydrate or energy intake. Consuming exogenous carbohydrate during aerobic exercise may be an effective intervention to sustain physical performance during periods of low glycogen. However, research is limited in the area of carbohydrate recommendations to fuel performance during periods of sub-optimal carbohydrate and energy intake. Additionally, the studies that have investigated the effects of low glycogen stores on exogenous carbohydrate oxidation have yielded conflicting results. Discrepancies between studies may be the result of glycogen stores being lowered by restricting carbohydrate or restricting energy intake. This narrative review discusses the influence of low glycogen status resulting from carbohydrate restriction versus energy restriction on exogenous carbohydrate oxidation and examines the potential mechanism resulting in divergent responses in exogenous carbohydrate oxidation. Results from this review indicate that rates of exogenous carbohydrate oxidation can be maintained when glycogen content is lower following carbohydrate restrictions, but may be reduced following energy restriction. Reductions in exogenous carbohydrate oxidation following energy restriction appear to result from lower insulin sensitivity and glucose uptake. Exogenous carbohydrate may thus be an effective intervention to sustain performance following short-term energy adequate carbohydrate restriction, but may not be an effective ergogenic aid when glycogen stores are low due to energy restriction.
... Pre-exercise and optimal starting conditions are a prerequisite. A well-dosed intake of carbohydrates prior to exercise can have a performance-enhancing effect [70,72,[74][75][76]. This may involve replenishing glycogen stores in advance and ensuring a (subsequent) replenishment of blood glucose, thus ensuring the availability of sufficient glucose for energy metabolism [74]. ...
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... We assume that hematologic responses at the preexercise state, following ISO and SUC ingestion, did not translate into thermoregulatory responses during exercise. Indeed, it has been suggested that an elevated plasma Glu concentration after carbohydrate ingestion rapidly decreases to hypoglycemic levels within 10-20 min [42]. This response may occur in both high and low glycemic foods, whilst the magnitude of the response is either similar between these foods [43] or greater in high glycemic food [44]. ...
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Ion‐beam‐induced microstructure of the iron‐rich end of the Fe–Ti and Fe–Ti–C systems was studied by using multilayered thin‐film samples with a linearly varying composition of the metallic constituents. The ion bombardment was carried out using Xe<sup>+</sup><sup>+</sup> ions at 600 keV. A mixing of iron and titanium in binary samples was complete after a bombardment with 8×10<sup>1</sup><sup>5</sup> ions/cm<sup>2</sup> whereas Fe–Ti–C samples were only partially mixed after the fluence of 1.8×10<sup>1</sup><sup>6</sup> ions/cm<sup>2</sup>. Transmission electron microscope studies revealed that the microstructure of Fe–Ti samples was amorphous or amorphous and crystalline iron over a wide range of the compositions investigated. In the case of Fe–Ti–C, the microstructure consisted of crystalline iron and titanium together with a very little amorphous phase.
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The ingestion of CHO solutions has been shown to increase CHO oxidation and improve endurance performance. However, most studies have investigated CHO in solution, and sporting practice includes ingestion of CHO in solid (e.g., energy bars) as well as in liquid form. It remains unknown whether CHO in solid form is as effectively oxidized as CHO in solution. To investigate exogenous CHO oxidation from CHO provided in either solid (BAR) or solution (DRINK) form during cycling. Eight well-trained subjects (age = 31 ± 7 yr, mass = 73 ± 5 kg, height = 1.79 ± 0.05 m, VO2max = 69 ± 6 mL·kg−¹·min−¹) cycled at 58% ± 4% VO2max for 180 min while receiving one of the following three treatments in randomized order: BAR plus water, DRINK, or water. The BAR and DRINK was delivered glucose + fructose (GLU + FRC) in a ratio of 2:1 at a rate of 1.55 g·min−¹, and fluid intake was matched between treatments. During the final 2 h of exercise, overall mean exogenous CHO oxidation rate was −0.11 g·min−¹ lower in BAR (95% confidence interval = −0.27 to 0.05 g·min−¹, P = 0.19) relative to DRINK, whereas exogenous CHO oxidation rates were 15% lower in BAR (P < 0.05) at 120, 135, and 150 min of exercise. Peak exogenous CHO oxidation rates were high in both conditions (BAR 1.25 ± 0.15 g·min−¹ and DRINK 1.34 ± 0.27 g·min−¹) but were not significantly different (P = 0.36) between treatments (mean difference = −0.9 g·min−¹, 95% confidence interval = −0.32 to 0.13 g·min−¹). The present study demonstrates that a GLU + FRC mix administered as a solid BAR during cycling can lead to high mean and peak exogenous CHO oxidation rates (91 g·min−¹). The GLU + FRC mix ingested in the form of a solid BAR resulted in similar mean and peak exogenous CHO oxidation rates and showed similar oxidation efficiencies as a DRINK. These findings suggest that CHO from a solid BAR is effectively oxidized during exercise and can be a practical form of supplementation alongside other forms of CHO.