Effect of Glycemic Index of a Pre-exercise Meal on Endurance Exercise Performance A Systematic Review and Meta-analysis

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SYSTEMATIC REVIEW
Effect of Glycemic Index of a Pre-exercise Meal on Endurance
Exercise Performance: A Systematic Review and Meta-analysis
Catriona A. Burdon1,2 • Inge Spronk3 • Hoi Lun Cheng1,4,5 • Helen T. O’Connor1,6
� Springer International Publishing Switzerland 2016
Abstract
Background Low glycemic index (GI) pre-exercise meals
may enhance endurance performance by maintaining eug-
lycemia and altering fuel utilization. However, evidence
for performance benefits is equivocal.
Objective To evaluate the effect of a low GI (LGI) versus a
high GI (HGI) pre-exercise meal on endurance perfor-
mance using meta-analyses.
Methods Data sources included MEDLINE, SPORTDis-
cus, AUSPORT, AusportMed, Web of Science, and Sco-
pus. Eligibility criteria were randomized, crossover trials
with an endurance exercise (C60 min) component, e.g.,
time trial (TT), time to exhaustion (TTE) test, or sub-
maximal bout followed by TT or TTE. Participants were
healthy, active individuals aged C16 years. Interventions
included a LGI (B55) and HGI (C70) meal ingested
30–240 min before exercise. Study quality was assessed
using an adapted version of the validated Downs and Black
tool. Effect size (ES) and 95 % confidence interval were
calculated for each study and pooled according to perfor-
mance test type and whether exogenous carbohydrate
(CHO) was given during exercise. Potential effect modi-
fiers including exercise duration, pre-exercise meal timing,
glycemic load (GL), and fitness were assessed using meta-
regression.
Results The search netted 3431 citations with 19 studies
eligible for inclusion (totaling 188 participants; 91 %
male; VO2max: [50 ml/kg/min). Meals with 0.18–2 g
CHO/kg body mass, and a mean GI and glycemic load of
82 (GL: 72) and 35 (GL: 32) for HGI and LGI, respec-
tively, were given between 30 and 210 min before
exercise. All test types without CHO ingestion during
exercise showed slightly improved performance with
LGI, but no significant pooled effects were observed
(ES: -0.17 to -0.36; p[ 0.05). Studies where exoge-
nous CHO was ingested during exercise showed con-
flicting results (ES: -0.67 to 0.11; p = 0.04 to 0.94). No
significant relationship was observed with any of the
effect modifiers (p[ 0.05). No consistent metabolic
responses (glucose, insulin, lactate, respiratory exchange
ratio) during exercise were observed with either meal
type.
Limitations There were small numbers of studies within
each exercise testing protocol and limited statistical power
within studies. Pre-exercise meal timing, GL, meal com-
position and participant fitness varied across studies, lim-
iting the capacity to assess the influence of these factors on
study outcomes.
Conclusion There was no clear benefit of consuming a LGI
pre-exercise meal for endurance performance regardless of
carbohydrate ingestion during exercise.
& Helen T. O’Connor
helen.oconnor@sydney.edu.au
1 Discipline of Exercise and Sport Science, Faculty of Health
Sciences, The University of Sydney, 75 East Street,
Lidcombe, Sydney, NSW 2141, Australia
2 School of Medicine, University of Wollongong, Wollongong,
NSW, Australia
3 Wageningen University, Wageningen, The Netherlands
4 Academic Department of Adolescent Medicine, The
Children’s Hospital at Westmead, Westmead, Sydney, NSW,
Australia
5 Discipline of Child and Adolescent Health, Sydney Medical
School, The University of Sydney, Westmead, Sydney, NSW,
Australia
6 Charles Perkins Centre, The University of Sydney,
Camperdown, Sydney, NSW, Australia
123
Sports Med
DOI 10.1007/s40279-016-0632-8
Key Points
Meta-analysis did not find significantly improved
endurance exercise performance with a low glycemic
index (LGI) pre-exercise meal, regardless of
exogenous carbohydrate (CHO) ingestion during
exercise. However, a small, non-significant
performance benefit across all exercise test types was
observed after a LGI meal when no CHO was
ingested during exercise.
Maintenance of CHO availability (glucose or
glycogen) is one of the main reasons a LGI pre-
exercise meal is proposed to enhance endurance
performance. As most, if not all, athletes have
regular access to exogenous CHO during competitive
events, it would seem that exogenous CHO ingestion
would replace the need for a LGI pre-exercise meal.
In prolonged exercise situations where access to
exogenous CHO is restricted, limited (e.g., military
rescue situations or in endurance events for athletes
with a disability where access to exogenous CHO
may be logisitically difficult) or not well tolerated
(gastrointestinal upset), a LGI pre-exercise meal may
be theoretically useful. Further research evidence
addressing this question is warranted.
1 Introduction
Consumption of carbohydrate (CHO) or a CHO-based meal
before exercise is a widely accepted practice for endurance
athletes. It is used not only to satisfy hunger and help meet
daily nutrient requirements, but it is also important for
supporting the energy needs of exercise and optimizing
performance [1]. Pre-exercise CHO ingestion can increase
glycogen stores and/or blood glucose levels, and is known
to increase CHO utilization during exercise [2]. The
majority of studies examining pre-exercise CHO con-
sumption have demonstrated null to beneficial effects on
endurance performance when compared to exercising in
the fasted state [1–3].
One of the more controversial aspects of the pre-exer-
cise meal has been the glycemic index (GI). The GI is
determined by comparing the post-prandial blood glucose
response of a food/meal to that of glucose (which has a
reference GI value of 100). A low GI (LGI) meal is
absorbed slowly from the gut, such that the rise in glucose
and insulin are blunted in comparison to a high GI (HGI)
meal [4, 5]. Various techniques can be used to manipulate
the GI of a meal. These include altering the macronutrient
composition, form or particle size (e.g., liquid or solid),
cooking method, and, more commonly, the type of CHO
within the meal [4, 6, 7]. Interest in manipulating the GI of
pre-exercise meals was initially driven by concerns that
pre-exercise CHO consumption can induce a nadir in blood
glucose levels, and potentially hypoglycemia, during the
initial stages (*30 min) of endurance exercise [8]. Early
studies in the area (predominantly from the 1980s)
explored simple versus complex CHO meals, as GI was not
yet established as a scientific concept [9–11]. Outcomes
from these studies were equivocal, although practical
advice to athletes before and during the early 1990s often
included avoidance of simple CHO or sugars in the pre-
exercise period [12]. Simple sugars were considered most
likely to induce a nadir in blood glucose levels, as they
were believed to be linked with higher insulin secretion
[13].
Over the past two decades, the classification of CHO
based on GI became more widely accepted, and research
on LGI foods confirmed its effects on attenuating post-
prandial insulin levels [14, 15]. This was recognized as
potentially beneficial for maintaining euglycemia during
exercise, and the sustained release of slowly-digested LGI
CHO was seen as attractive for maintaining CHO avail-
ability, particularly in the later stages of prolonged exercise
[16]. Attenuation of post-prandial insulinemia was also
believed to facilitate free fatty acid release and fat oxida-
tion, which may reduce muscle glycogen depletion [4].
Despite these purported benefits, studies comparing HGI
versus LGI pre-exercise meals on exercise performance
have yielded mixed results and several critical reviews
report that the evidence is inconclusive [1, 17, 18]. Smallsample size is often identified as a limitation in these
reviews, and a meta-analysis in this instance would be
advantageous for providing greater power to detect ergo-
genic effects of LGI pre-exercise meals.
The provision of exogenous CHO during exercise
(which alters CHO availability and fuel utilization) in some
studies further complicates interpretation of research find-
ings [17]. Additionally, research has demonstrated that
glycemic response to a CHO food is not only dependent on
the GI value itself, but also the total amount of CHO
ingested [19, 20]. This combined effect, known as the
glycemic load (GL), has been shown to explain*90 % of
the variability in the blood glucose response [21]. Pre-ex-
ercise GL has only been assessed in one endurance exercise
study which showed a null effect on performance [22].
However, as most studies examining meal GI typically
report CHO content, it is possible to undertake a more
comprehensive evaluation of GL using existing literature.
The primary aim of this study was to conduct a sys-
tematic review and meta-analysis of randomized crossover
trials in trained/highly-trained participants to examine how
C. A. Burdon et al.
123
the GI of a pre-exercise meal affects endurance perfor-
mance. A secondary aim was to investigate the influence of
potential effect modifiers including the GL, time between
meal ingestion, and exercise onset (pre-exercise meal
timing), CHO ingestion during exercise and participant
fitness. Clarification of the potential benefit of LGI meals
prior to exercise will enable athlete recommendations to be
more confidently determined.
2 Methods
2.1 Search Strategy
This review was conducted in accordance with the Pre-
ferred Reporting Items for Systematic Reviews and Meta-
analysis (PRISMA) Statement [23]. A systematic search
was conducted from the earliest record using the databases
MEDLINE (via OvidSP), SPORTDiscus (via EBSCOhost),
AUSPORT and AusportMed (via Informit), Web of Sci-
ence, and Scopus. The search strategy (Fig. 1) combined
terms covering the areas of glycemic index (glycemic
index; glycaemic index; GI) and sport performance (per-
formance; exercise; physical activity; sport; competition;
event). Reference lists of included studies, review articles,
and publications from known authors in the research area
were hand-searched. The search was performed by one
reviewer (IS). After removal of duplicates and exclusion by
title and abstract, two reviewers (IS and CB) independently
evaluated studies for inclusion with disagreements resolved
via discussions with a third reviewer (HOC). Reviewers
were not masked to study titles or authors. Papers in lan-
guages other than English were translated by a native
speaker, and only excluded if a translation was not possi-
ble. The initial search was conducted in February 2014 and
updated in January 2015.
2.2 Eligibility Criteria
To be included, studies were required to have a crossover
design where participants were randomized to receive a
HGI and LGI meal 30 to 240 min prior to exercise. This
time frame was informed by current guidelines for pre-
event CHO consumption of 1–4 h prior to exercise [24]. As
we were aware a priori that meals fed as little as 30 min
prior to exercise were used to explore the risk of hypo-
glycemia (which may compromise endurance perfor-
mance), we set the lower limit for consumption at 30 min
prior to exercise. This was also practically a feasible
minimum time for a meal to be consumed. Definitions of
C70 for HGI and B55 for LGI were used as exclusion cut
points, in accordance with the international tables of GI
and GL values [14]. Studies using meals that were\5 GI
units outside these cut points were included if the GI values
of the meals were C15 units apart, emulating the above-
mentioned international cut-offs for HGI and LGI. Original
and complete research only was included, therefore review
and opinion articles and abstracts were excluded, and non-
randomized trials were excluded to maximize study
quality.
Study participants needed to be physically active, heal-
thy, able-bodied, and C16 years of age. Where age range
was not provided, studies were accepted if the mean age
minus one standard deviation (SD) was C16 years. Studies
were excluded if participants had a disability (e.g., spinal
cord injury), or any condition that affected performance
(e.g., chronic obstructive pulmonary disease) or CHO
metabolism (e.g., diabetes). Studies needed to include
some form of endurance exercise (C60 min) as part of or in
combination with a performance test, for example time trial
(TT), submaximal bout followed by time trial (sub-
max ? TT), time to exhaustion (TTE), submaximal bout
followed by TTE (submax ? TTE), or incremental TTE
protocols.
2.3 Data Extraction
Participant characteristics (age; sex; source population;
fitness/aerobic capacity), test conditions (environmental
conditions; food/drink consumed during exercise; exercise
protocol and duration; performance test type), intervention
characteristics (meal GI and CHO content; meal compo-
sition; pre-exercise meal timing) and performance
Search terms: performance, 
exercise, physical activity, 
sport, competition or event
n = 1,184,900
Search terms: GI, 
glycemic index, 
glycaemic index
n = 158,958
Articles screened
(after duplicate removal)
n = 3,431
Full-text articles assessed for 
eligibility
n = 112
Articles excluded
(after title and abstract review)
n = 3,319
Hand searched 
articles
n = 8
Studies included in qualitative 
synthesis
n = 19
Full-text articles excluded
n = 101
Irrelevant: n = 25
Not RCT or crossover design: n = 12
Under 16 years: n = 2
Sedentary participants: n = 1
Performance not measured: n = 20
Meal consumption <30 min or >240 min 
before exercise: n = 9
or 
GI difference <15: n = 5
No GI reported for meals: n = 1
Not a single pre-exercise meal: n = 2
Not an endurance exercise protocol: n = 2
Review, abstract or thesis: n = 22
Studies included in quantitative 
synthesis (meta-analyses)
n = 19
Fig. 1 Flowchart of the study selection process. HGI high glycemic
index, LGI low glycemic index, RCT randomized controlled trial
Effect of Glycemic Index of a Pre-exercise Meal on Endurance Performance
123
outcomes were extracted by one reviewer (IS), and checked
independently by another (HLC). For studies that tested
more than two pre-exercise meals, the two with the highest
and lowest GI values were selected. GL was calculated
(GI 9 CHO content in grams) from data reported in each
manuscript. As CHO content of meals needed to be rea-
sonably well matched for the study to be included, dis-
crepancies in GL were primarily attributed to differences in
GI. A range of GLs were observed across the included
studies, enabling its impact to be qualitatively assessed.
Descriptive summary data on participants’ metabolic
response to the meal were also extracted. Where necessary,
SD was calculated from standard errors. To facilitate
comparison between studies, exercise duration and aerobic
capacity were calculated from available data. For studies
that only presented data graphically, mean and SD were
estimated in duplicate using a graduated measure.
2.4 Assessment of Methodological Quality
Study quality was assessed by two reviewers (CB and IS)
using a modified Downs and Black checklist, with a
maximum of 21 points [25]. Disagreements were resolved
by discussion with a third reviewer (HOC).
Quality items not relevant to a two-visit, crossover,
randomized controlled trial design (i.e., patients lost to
follow-up stated; representative facilities; data dredging;different follow-up lengths; consistent population recruited
and time frame; randomization concealed; accounting for
subjects lost to follow-up; and adjusting for confounders)
were removed. For the item assessing whether study sam-
ples were representative of trained populations, maximal
oxygen consumption ( _VO2max) thresholds of [55 and
[40 ml/kg/min were used for men and women, respec-
tively [26]. Populations were described as well trained if
_VO2max was[65 ml/kg/min for men and[55 ml/kg/min
for women [26]. Assessment of participant blinding was
expanded to two quality items, in order to assess blinding
to both meal GI and performance tests. The statistical
power of each study was evaluated via power calculations
using the effect size (G*Power version 3.1, Franz Faul,
Universita¨t Kiel, Germany) [27].
No studies were eliminated and no additional sub-group
analyses were undertaken on the basis of methodological
quality. Assessment of confounding was improved via
three additional items. These assessed whether studies
controlled for: (1) participant glucose tolerance; as well as
(2) diet; and (3) exercise in the 24 h prior to the experi-
mental days. Correspondingly, the item relating to
bias/compliance was attributed an additional point to
evaluate participant adherence to diet and exercise control
guidelines. This additional point was only awarded if a
study reported evidence of compliance (via food and
exercise diaries or other tool to quantify diet and exercise
in the 24 h prior to experimental days).
2.5 Data Analyses
Comprehensive Meta-Analysis Version 3 (Englewood, NJ,
USA) was used to calculate the standardized mean differ-
ence or effect size (ES) and 95 % confidence interval
(95 % CI) for each study, and to combine data using the
random effects model for meta-analysis. Separate meta-
analyses were conducted for each exercise performance
test category, and for studies that did not provide CHO
during exercise. Results were summarized using forest
plots, with a negative ES representing an ergogenic benefit
of a LGI meal and vice versa. Heterogeneity between
studies was assessed using I2 [28]. The weighted mean
difference (WMD) for relative change in performance was
calculated as the mean of the percent change between the
HGI and LGI trials, with each study weighted according to
sample size. No assessment of publication bias was
undertaken as there were too few studies within each per-
formance test category [29]. Studies within the forest plots
were arranged from smallest to largest GL differences
between the HGI and LGI pre-exercise meals. Meta-re-
gression was undertaken to examine whether variations in
GL, pre-exercise meal timing, and participant fitness
between studies influenced the strength of individual study
outcomes.
3 Results
3.1 Identification and Selection of Studies
Upon completion of the initial search and removal of
duplicates, 3431 citations were available for review. After
elimination of papers based on the eligibility criteria, 19
articles were included (Fig. 1).
3.2 Study Characteristics
Participant and meal characteristics for each study are
presented in Table 1. Across the 19 included studies,
sample size ranged from six to 34. Participant age ranged
between 21 and 33 years. Seventeen studies recruited only
males, and two included participants of both sexes. This
equated to a combined total of 188 participants, with a
91 % male representation. Trained and well trained par-
ticipants were used in 15 and two studies, respectively,
with _VO2max not reported in two studies [30, 31]. All of the
highly trained participants were male.
C. A. Burdon et al.
123
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Effect of Glycemic Index of a Pre-exercise Meal on Endurance Performance
123
The HGI and LGI pre-exercise meals provided similar
amounts of energy (HGI: 2228 ± 808; LGI: 2321 ±
793 kJ), CHO (HGI: 90.6 ± 31.7; LGI: 91.3 ± 31.6 g),
protein (HGI: 20.1 ± 8.8; LGI: 24.6 ± 10.5 g), and fat
(HGI: 9.1 ± 7.0; LGI: 9.0 ± 6.9 g). The CHO content of
pre-exercise meals varied from 0.18 to 2 g/kg body mass.
Mean GI was 82 (range 69–100) and 35 (range 26–52) for
the HGI and LGI meals, respectively. This corresponded to
a mean GL of 72 (range 14–124) for the HGI meals and 32
(range 5–59) for the LGI meals. Mean time lapse between
the pre-exercise meal and onset of exercise was 95 min
(range 30–210 min). Timing of pre-exercise meals was: 30
(n = 3); 45 (n = 4); 60 (n = 2); 120 (n = 7); 180 (n = 2);
and 210 min (n = 1). Additional CHO was given during
exercise in five studies; two with a LGI or a HGI CHO as
per the pre-exercise meal [31, 32], one with a radio-labeled
glucose solution [33], and two with a commercial CHO-
electrolyte drink [34, 35]. GI values for the commercial
drinks were unavailable, but were treated as high due to
their glucose content [36].
A summary of the exercise protocols used for each study
is presented in Table 2. Four different protocols were used
to measure performance across the 19 studies: TT (n = 5);
submax ? TT (n = 9); TTE (n = 4); and submax ? TTE
(n = 1). Total exercise duration ranged from 65 to
150 min, and was conducted on either a cycle ergometer
(n = 9) or treadmill (n = 10). Meta-regression was carried
out on each of these different exercise protocols except
submax ? TTE as only one study had this type of design
[37].
3.3 Meta-analysis: Time-Trial Performance Tests
Four studies used a TT protocol without any additional
CHO consumption during exercise (Fig. 2) [38–41]. Sig-
nificantly better performance was reported for the LGI trial
in two of the four papers [40, 41]. Upon pooling of these
studies, there was no significant effect of a LGI meal on TT
performance (ES: -0.18; 95 % CI -0.58 to 0.22;
p = 0.37; I2 = 0 %). The WMD represented a 0.7 %
improvement with a LGI meal, with percent change rang-
ing from -1.8 to 3.2 %. One study provided CHO during
exercise, showing an ES of -0.04 (95 % CI -1.02 to 0.95;
p = 0.94) and a 1.4 % WMD favoring a LGI meal [32].
3.4 Meta-analysis: Submaximal Plus Time-Trial
Performance Tests
A total of six studies used a submax ? TT protocol with-
out providing CHO during exercise (Fig. 3)
[22, 30, 42–45]. Of these, five favored a LGI meal for
superior performance [22, 30, 43–45], with only one
reporting a significant effect of a LGI meal [45]. TheT
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C. A. Burdon et al.
123
pooled effect across the six studies was not significant (ES:
-0.17; 95 % CI -0.55 to 0.22; p = 0.40; I2 = 0 %). The
WMD was 1.3 % favoring the LGI trial, with percent
change ranging from -6.9 to 6.4 %. The opposite pattern
was observed in the three studies that provided CHO dur-
ing the submax ? TT protocol [34, 46, 47]. All three
showed slightly better performance with a HGI meal, but
the pooled effect was non-significant (ES: 0.11; 95 % CI -
0.44 to 0.65; p = 0.70; I2 = 0 %). This equated to a 0.9 %
WMD favoring the HGI trial, and percent change ranging
from -1.0 to -0.8 %.
3.5 Meta-analysis: Time-to-Exhaustion
Performance Tests
Three studies used a TTE design without providing CHO
during exercise (Fig. 4) [16, 48, 49], two of which reported
a significant performance benefit after the LGI meal
[16, 49]. However, when these studies were pooled, the
performance improvement with LGI meals was not sig-
nificant (ES: -0.36; 95 % CI -0.93 to 0.22; p = 0.22;
I2 = 0 %) (Fig. 4). This equated to a WMD of 5.0 % in
favor of LGI meals, and a percentage mean change of -4.4
Table 2 Exercise protocols of included studies
Study Food/drink during exercise Exercise
modality
Environmenta Protocol Performance
test
Exercise
duration
(min)Content (volume) Timing
(min into
exercise)
Bennet et al. [38] N/A N/A Running Not reported 5 9 15 min intermittent
varied intensity
5 9 1 min
sprints
90
Burke et al. [33] Rest: 10 % CHO
1 g/kg (4 ml/kg)
During: 3.3 ml/kg
Rest and
20 min
Cycling Not reported 70 % VO2max 15 min
(300 kJ)
TT
135
Chen et al. [22] N/A N/A Running 20–21 ± 1 �C;
61–62 ± 2 % RH
60 min at 70 % VO2max 10 km TT 112
Chen et al. [34] 6.6 % CHO (2 ml/kg) 2.5 km Running 21.3 ± 0.6 �C;
62.0 ± 2.2 % RH
5 km at 70 % VO2max 16 km TT 92
DeMarco et al. [37] Water (150 ml) 20 min Cycling 21 ± 1 �C 120 min at 70 % VO2max TTE 100 %
VO2max
[120
Febbraio and Stewart
1996 [43]
Water (250 ml) 15 min Cycling 20–22 �C 120 min at 70 % VO2max 15 min TT 135
Febbraio et al. [42] Water (250 ml) 15 min Cycling 20–22 �C 120 min at 70 % VO2max 30 min TT 150
Hulton et al. [30] Water (5 ml/kg) 45 min
(15 min
rest)
Running Not reported 4 9 22.5 min varied
intensity, 15 min rest
1 km TT *94
Little et al. [32] HGI or LGI snack
(0.25 g/kg CHO)
45 min Running Not reported 5 9 15 min intermittent
varied intensity
5 9 1 min
sprints
90
Little et al. [39] Water ad libitum
(matched)
15 min Running Not reported 5 9 15 min intermittent
varied intensity
5 9 1 min
sprints
90
Moore et al. [40] Water ad libitum
(equal both trials)
N/A Cycling 22 ± 2 �C;
48 ± 5 % RH
N/A 40 km TT 95
Moore et al. [41] Not reported N/A Cycling 20–22 �C;
48 ± 5 % RH
N/A 40 km TT 96
Reljic et al. [31] HGI or LGI drink
(250 ml 9 2)
30 and
60 min
Running 20–22 �C; 50–60 %
RH
90 % lactate threshold TTE 80
Sparks et al. [44] Water (400 ml) 15 min Cycling 21 �C 50 min at 67 % VO2max 15 min TT 65
Thomas et al. [16] Water (120 ml) 15 min Cycling 23 ± 1 �C;
60–65 %RH
67 % VO2max TTE 117
Thomas et al. [48] Water (120 ml) 15 min Cycling 23 ± 1 �C;
60–65 %RH
67 % VO2max TTE 101
Wong et al. [45] Water (125 ml) 2.5 km Running 22.1 ± 0.4 �C;
64 ± 2 % RH
5 km at 70 % VO2max 16 km TT 99
Wong et al. [47] 6.6 % CHO
2 ml kg-1 at rest
Rest and
2.5 km
Running Not reported 5 km 70 % VO2max 16 km TT 92
Wu and Williams [49] Water (2 ml kg-1) 15 min Running Not reported 70 % VO2max TTE 109
CHO carbohydrate, HGI high glycemic index, LGI low glycemic index, N/A not applicable, RH relative humidity, TT time trial, TTE time to exhaustion,
VO2max maximum oxygen uptake
a Values are mean ± standard deviation or range
Effect of Glycemic Index of a Pre-exercise Meal on Endurance Performance
123
to 17.1 %. One study [31] provided a CHO drink during
exercise (consistent with the GI value of the pre-test meal),
and found a significant 7.7 % performance benefit after the
LGI meal (ES: -0.49; 95 % CI -0.97 to -0.02;
p = 0.04). Only one study used a submax ? TTE protocol
[37], which reported significantly better performance after
consumption of a LGI meal. This contrasted with the cal-
culated effect size which showed a non-significant 36.1 %
improvement in performance after the LGI meal (ES:
-0.67, 95 % CI -1.57 to 0.23; p = 0.13).
3.6 Potential Effect Modifiers: Glycemic Load,
Pre-exercise Meal Timing, and Fitness
Qualitative evaluation of forest plots (Figs. 2, 3, 4), where
studies were arranged from lowest to highest GL differences
between the HGI and LGI meals, showed no consistent
performance patterns with varying GL. This is consistent
with results from the meta-regression (Fig. 5) where no
association was observed between performance effect and
meal GL differences across the studies (coefficient: 0.01;
standard error: 0.01; p = 0.31). Similarly, no significant
relationships were found for pre-exercise meal timing (co-
efficient: 0.00; standard error: 0.00; p = 0.38) and partici-
pant fitness (coefficient: -0.01; standard error: 0.03;
p = 0.74), even when studies without CHO provision during
exercise were analyzed independently (data not shown).
3.7 Metabolic Responses During Exercise
The response of the metabolic variables during exercise is
summarized in Table 3. All studies measured blood glu-
cose and observed normal values at rest prior to meal
consumption.
Study n ES (95% CI) p value 
TT (no CHO during exercise) 
] 8 29 - - ) 
] 10 29 - - ) 
] 14 31 (- ) 
] 13 56 - - ) 
Pooled effect (no CHO) - - ) 
TT (CHO during exercise) 
] 40 - - ) 
- -
 Favors HGI 
Fig. 2 Forest plot showing HGI vs. LGI pre-exercise meal effects on
performance in time trials (TT) without (n = 4; white squares) and
with (n = 1; gray square) CHO ingestion during exercise. Studies are
arranged according to GL differences between the HGI and LGI
meals and size varies with subject number. Diamond represents the
pooled effect from studies without CHO ingestion during exercise.
CHO carbohydrate, CI confidence interval, Diff, difference, ES effect
size, GL glycemic load, HGI high glycemic index, LGI low glycemic
index
 ES (95% CI) p value 
Submax+TT (no CHO during exercise) 
Febbraio ] 8 20 (- ) 
] 8 36 - -
Febbraio ] 6 - - ) 
] 8 38 - - ) 
] 8 41 - - ) 
Hulton ] 8 52 - - ) 
Pooled effect (no CHO) - - ) 
Submax+TT (CHO during exercise) 
] 9 43 (- ) 
] 8 (- ) 
] 6 (- ) 
Pooled effect (with CHO) (- ) 
- -
 Favors HGI 
Study n
Fig. 3 Forest plot showing HGI vs. LGI pre-exercise meal effects on
performance in submaximal exercise plus time trial (submax ? TT)
studies without (n = 6; white squares) and with (n = 3; gray
squares) CHO ingestion during exercise. Studies are arranged
according to GL differences between the HGI and LGI meals and
size varies with subject number. Diamonds represent the pooled
effects from studies without and with CHO ingestion during exercise.
CHO carbohydrate, CI confidence interval, Diff, difference, ES effect
size, GL glycemic load, HGI high glycemic index, LGI low glycemic
index
C. A. Burdon et al.
123
3.7.1 No Carbohydrate (CHO) Consumed During Exercise
Of the 14 studies that did not provide CHO during exercise,
seven reported no difference in blood glucose between the
HGI and LGI trials, six found lower blood glucose with a HGI
meal at some point during exercise, and one reported superior
blood glucose maintenance with a LGI meal. Circulating
insulin was measured in 13 studies, with 11 reporting no dif-
ference between the trials, and two(both with a short pre-
exercise meal timing of 30 and 45 min) showing higher
insulin in the first 20 min of exercise after the HGI meal
[37, 44]. A total of 12 studies measured blood lactate. Ten of
these showed no difference between the HGI and LGI trials,
one reported higher levels throughout exercise after a HGI
meal [22], and one found significantly higher lactate at the
beginning andmid-way through exercise on theHGI trial [16].
Free fatty acid (FFA) concentration was measured in nine
studies, with three finding no difference between trials; the
remaining six observed lower FFA with a HGI meal
throughout or at some point during exercise.
Respiratory exchange ratio (RER) was measured in 12
studies; four reported no difference between trials. Six
studies observed higher and two observed lower RER
throughout or at some point during exercise after a HGI
meal. Carbohydrate and fat oxidation were measured in 11
and 10 studies respectively, with four reporting no differ-
ence in oxidation rates of either fuel. Higher CHO oxida-
tion in five and lower CHO oxidation in two studies were
observed during the HGI trial. Fat oxidation was lower in
four and higher in two studies after the HGI meal.
3.7.2 CHO Consumed During Exercise
The five studies where CHO was given as a part of the
performance test protocol found no differences between
HGI and LGI trials for circulating insulin, blood lactate,
FFA, RER, or CHO and fat oxidation during exercise
[31–35]. One study by Reljic et al. reported higher blood
glucose levels during exercise after the LGI meal [31].
3.8 Methodological Quality
Methodological quality of the included studies was mixed
(Table 4). Studies scored an average of 14 points out of 21
(range 11–17), with excellent reporting of some items.
Reporting of participant description, adverse events, and
actual probability values was poor. Only four
[16, 22, 40, 48] of the 19 studies blinded participants to the
meal, 12 [16, 22, 30, 32, 33, 37–41, 45, 48] to exercise
performance as an outcome, and six [22, 31, 32, 38, 40, 45]
blinded researchers. Two [33, 40] studies verified compli-
ance with pre-trial instructions; however, another nine
[22, 30, 32, 34, 39, 41, 44, 45, 47] reported checking
compliance but did not report results. Confounding factors
(glucose tolerance, pre-trial diet, and exercise control) were
generally well controlled for, although only four studies
screened for glucose tolerance. Only three studies were
adequately powered [16, 37, 45].
4 Discussion
This is the first systematic review with meta-analysis
evaluating the impact of the GI of a pre-exercise meal on
endurance performance. The review demonstrates no sig-
nificant ergogenic benefit of a LGI over a HGI pre-exercise
meal. This was the case regardless of whether exogenous
CHO was consumed during exercise. Metabolic responses
including blood glucose and insulin secretion, FFA oxi-
dation and RER were inconsistent between studies. Despite
nydutS ES (95% CI) p value 
TTE (no CHO during exercise) 
] 6 43 (- ) 
] - - ) 
] 8 56 - - ) 
Pooled effect (no CHO) - - ) 
Submax+TTE (no CHO during exercise) 
DeMarco ] 10 - - ) 
TTE (CHO during exercise) 
Reljic ] 34 9 - - - ) 
- -
 Favors HGI 
Fig. 4 Forest plot showing HGI vs. LGI pre-exercise meal effects on
performance in time to exhaustion (TTE) trials without (n = 3; white
squares) and with (n = 1; gray square) CHO ingestion during
exercise. Studies are arranged according to GL differences between
the HGI and LGI meals and size varies with subject number. Diamond
represents the pooled effect from studies without CHO ingestion
during exercise. CHO carbohydrate, CI confidence interval, Diff,
difference, ES effect size, GL glycemic load, HGI high glycemic
index, LGI low glycemic index
Effect of Glycemic Index of a Pre-exercise Meal on Endurance Performance
123
the studies all having randomized cross-over designs, there
was wide variability in research methodologies including
the use of four main types of exercise performance tests,
large differences in the timing and CHO content of pre-
exercise meals, and a substantial range in the athletic cal-
iber of participants between studies. Meta-regression did
not identify any significant impact of these potential con-
founding factors on performance, although the above
inconsistencies and the relatively small number of studies
within each exercise type may have limited the power to
assess the impact of pre-exercise meal GI. The quality of
the included articles was moderate and many studies were
underpowered to assess the performance outcomes.
The GI of the pre-exercise meal has been a relatively
controversial aspect of the practical guidance provided to
endurance athletes. Theoretically, a LGI pre-exercise meal
may result in improved endurance performance as inges-
tion is proposed to improve maintenance of blood glucose,
reduce insulin secretion, facilitate the oxidation of free
fatty acids, and spare muscle glycogen [50–52]. Despite
these potential metabolic shifts induced by a LGI pre-ex-
ercise meal, no significant benefit was observed from the
meta-analysis conducted in this study across any of the four
exercise testing protocols used. However, it should be
noted that in none of the studies did the authors report a
significant benefit of a HGI meal. All of the significant
studies (n = 7) used LGI pre-exercise meals with exoge-
nous CHO consumed during exercise in only one of these.
Clearly, when CHO is ingested during exercise, this
exogenous CHO is made available for oxidation and would
support the pool of CHO available from body stores and
decrease the risk of inadequate CHO availability during
exercise [13]. This practice essentially negates a key ben-
efit of a LGI pre-exercise meal. It should also be noted that
there was a wide range of performance benefits reported in
the seven significant studies (2.8–36.1 %) and only two
used a TT design which is a more robust (lower coefficients
of variation [53]) and relevant test of exercise performance.
One of the limiting factors of the current literature on
LGI pre-exercise meals and endurance performance is the
relatively small number of studies within each exercise
protocol used as well as the variability in the pre-exercise
meal time, GL of meals, and training status of the partic-
ipants. Pre-exercise meal time is an important factor,
especially for LGI meals, as slower digestion may result in
a delay in CHO availability for oxidation [54]. There was a
wide range in pre-exercise meal time (30–210 min) across
the studies, with some having as little as 30 min. This may
have limited the capacity for CHO absorption and hence
oxidation during exercise. All of the studies showing a non-
significant benefit of a HGI meal had a pre-exercise meal
timing of B2 h (excluding those where exogenous CHO
was provided during exercise). In these studies, CHO from
the HGI meal would have been available for oxidation
earlier in exercise due to its faster absorption, whereas the
CHO from the LGI meal was most likely less available
earlier in exercise. Despite this, there were a number of
a Coefficient: 
Standard error: 
p 
Ef
fe
ct
 s
iz
e 
(H
ed
ge
s’ 
g)
 
-
-
-
-
-
-
-
b Coefficient: 
Standard error: 
p 
Ef
fe
ct
 s
iz
e 
(H
ed
ge
s’ 
g)
 
-
-
-
-
-
Pre-exercise time (min) 
c Coefficient: -
Standard error: 
p 
Ef
fe
ct
 s
iz
e 
(H
ed
ge
s’ 
g)
 
-
-
-
-
-
-
-
VO2max (ml/kg/min) 
Fig. 5 Relationships between performance effect size and between-
study variationsin: a HGI and LGI glycemic load difference; b pre-
exercise meal timing; and c participant fitness. HGI high glycemic
index, GL glycemic load, LGI low glycemic index, VO2max maximum
oxygen uptake
C. A. Burdon et al.
123
T
a
b
le
3
E
ff
ec
ts
o
f
th
e
H
G
I
an
d
L
G
I
p
re
-e
x
er
ci
se
m
ea
ls
o
n
ex
er
ci
se
p
er
fo
rm
an
ce
an
d
m
et
ab
o
li
c
m
ar
k
er
s
S
tu
d
y
P
er
fo
rm
an
ce
te
st
ty
p
e
T
es
t
m
et
ri
c
P
er
fo
rm
an
ce
o
n
H
G
Ia
P
er
fo
rm
an
ce
o
n
L
G
Ia
E
ff
ec
t
d
ir
ec
ti
o
n
fa
v
o
rs
(b
y
%
d
if
fe
re
n
ce
)b
S
ig
n
ifi
ca
n
t
ef
fe
ct
re
p
o
rt
ed
?
M
et
ab
o
li
c
re
sp
o
n
se
d
u
ri
n
g
ex
er
ci
se
B
en
n
et
t
et
al
.
[3
8
]
T
T
M
et
er
s
1
3
3
3
±
1
0
0
1
3
2
8
±
1
4
1
H
G
I
(0
.4
)
N
N
o
d
if
fe
re
n
ce
in
g
lu
co
se
,
in
su
li
n
,
la
ct
at
e,
R
E
R
,
o
r
C
H
O
o
r
fa
t
o
x
id
at
io
n
B
u
rk
e
et
al
.
[3
3
]
S
u
b
m
ax
?
T
T
M
in
u
te
s
1
5
.8
±
0
.9
1
5
.9
±
1
.5
H
G
I
(0
.9
)
N
N
o
d
if
fe
re
n
ce
in
g
lu
co
se
,
in
su
li
n
,
F
F
A
,
R
E
R
,
o
r
C
H
O
o
x
id
at
io
n
C
h
en
et
al
.
[2
2
]
S
u
b
m
ax
?
T
T
M
in
u
te
s
5
2
.6
±
5
.7
5
1
.2
±
5
.7
L
G
I
(2
.7
)
N
R
E
R
,
la
ct
at
e
an
d
C
H
O
o
x
id
at
io
n
lo
w
er
,
an
d
fa
t
o
x
id
at
io
n
h
ig
h
er
in
L
G
I.
N
o
d
if
fe
re
n
ce
in
g
lu
co
se
,
in
su
li
n
,
o
r
F
F
A
C
h
en
et
al
.
[3
4
]
S
u
b
m
ax
?
T
T
M
in
u
te
s
9
1
.5
±
6
.2
9
2
.4
±
6
.8
H
G
I
(1
.0
)
N
N
o
d
if
fe
re
n
ce
in
g
lu
co
se
,
in
su
li
n
,
C
H
O
,
o
r
fa
t
o
x
id
at
io
n
D
eM
ar
co
et
al
.
[3
7
]
S
u
b
m
ax
?
T
T
E
M
in
u
te
s
2
.2
±
1
.2
3
.4
±
2
.3
L
G
I
(3
6
.1
)
Y
B
et
te
r
g
lu
co
se
m
ai
n
te
n
an
ce
in
L
G
I.
R
E
R
lo
w
er
in
L
G
I
F
eb
b
ra
io
an
d
S
te
w
ar
t
[4
3
]
S
u
b
m
ax
?
T
T
k
J
2
5
0
±
4
9
2
6
7
.0
±
2
4
.5
L
G
I
(6
.4
)
N
N
o
d
if
fe
re
n
ce
in
g
lu
co
se
,
in
su
li
n
,
la
ct
at
e,
R
E
R
,
o
r
C
H
O
o
x
id
at
io
n
.
F
F
A
lo
w
er
in
H
G
I
F
eb
b
ra
io
et
al
.
[4
2
]
S
u
b
m
ax
?
T
T
k
J
3
5
5
.0
±
7
0
.7
3
3
2
.5
±
1
0
6
.1
H
G
I
(6
.5
)
N
G
lu
co
se
lo
w
er
in
H
G
I
0
–
3
0
m
in
.
F
F
A
lo
w
er
in
H
G
I
H
u
lt
o
n
et
al
.
[3
0
]
S
u
b
m
ax
?
T
T
M
in
u
te
s
3
.6
±
1
.1
3
.5
±
0
.9
L
G
I
(2
.7
)
N
N
o
d
if
fe
re
n
ce
in
g
lu
co
se
,
in
su
li
n
,
C
H
O
o
r
fa
t
o
x
id
at
io
n
L
it
tl
e
et
al
.
[3
2
]
T
T
M
et
er
s
1
6
0
5
.5
±
6
1
6
.2
1
6
2
7
.4
±
5
3
6
.3
L
G
I
(1
.4
)
N
N
o
d
if
fe
re
n
ce
in
g
lu
co
se
,
C
H
O
,
o
r
fa
t
o
x
id
at
io
n
L
it
tl
e
et
al
.
[3
9
]
T
T
M
et
er
s
1
5
0
0
.0
±
1
6
9
.5
1
5
1
0
.0
±
1
7
6
.7
L
G
I
(0
.7
)
N
N
o
d
if
fe
re
n
ce
in
g
lu
co
se
,
in
su
li
n
,
R
E
R
,
la
ct
at
e,
C
H
O
,
o
r
fa
t
o
x
id
at
io
n
M
o
o
re
et
al
.
[4
0
]
T
T
M
in
u
te
s
9
5
.6
±
6
.0
9
2
.5
±
5
.2
L
G
I
(3
.2
)
Y
D
u
ri
n
g
ex
er
ci
se
,
R
E
R
an
d
C
H
O
o
x
id
at
io
n
h
ig
h
er
,
an
d
fa
t
o
x
id
at
io
n
lo
w
er
in
L
G
I.
A
t
en
d
o
f
ex
er
ci
se
,
g
lu
co
se
h
ig
h
er
in
L
G
I.
N
o
d
if
fe
re
n
ce
in
la
ct
at
e
M
o
o
re
et
al
.
[4
1
]
T
T
M
in
u
te
s
9
6
.0
±
7
.0
9
3
.0
±
8
.0
L
G
I
(3
.1
)
Y
R
E
R
an
d
C
H
O
o
x
id
at
io
n
h
ig
h
er
,
an
d
fa
t
o
x
id
at
io
n
lo
w
er
in
L
G
I.
N
o
d
if
fe
re
n
ce
in
g
lu
co
se
o
r
la
ct
at
e
R
el
ji
c
et
al
.
[3
1
]
T
T
E
M
in
u
te
s
7
3
.6
±
1
2
.7
7
9
.7
±
1
1
.7
L
G
I
(7
.7
)
Y
A
ft
er
6
0
m
in
,
g
lu
co
se
h
ig
h
er
in
L
G
I.
N
o
d
if
fe
re
n
ce
in
la
ct
at
e
S
p
ar
k
s
et
al
.
[4
4
]
S
u
b
m
ax
?
T
T
k
J
2
4
9
.0
±
4
2
.4
2
5
3
.0
±
2
8
.3
L
G
I
(1
.6
)
N
R
E
R
an
d
C
H
O
o
x
id
at
io
n
lo
w
er
in
L
G
I.
A
t
1
0
–
2
0
m
in
,
g
lu
co
se
h
ig
h
er
in
L
G
I.
A
t
2
0
an
d
5
0
m
in
,
F
F
A
h
ig
h
er
in
L
G
I
T
h
o
m
as
et
al
.
[1
6
]
T
T
E
M
in
u
te
s
9
7
.0
±
2
9
.1
1
1
7
.0
±
2
9
.1
L
G
I
(1
7
.1
)
Y
N
o
d
if
fe
re
n
ce
in
g
lu
co
se
,
in
su
li
n
,
F
F
A
,
o
r
R
E
R
.
A
t
0
–
1
5
m
in
an
d
4
5
m
in
,
la
ct
at
e
lo
w
er
in
L
G
I.
T
h
o
m
as
et
al
.
[4
8
]
T
T
E
M
in
u
te
s
9
4
.0
±
2
6
.9
9
0
.0
±
1
9
.6
H
G
I
(4
.3
)
N
N
o
d
if
fe
re
n
ce
in
g
lu
co
se
,
in
su
li
n
,
F
F
A
,
la
ct
at
e,
o
r
R
E
R
.
A
t
en
d
o
f
ex
er
ci
se
,
g
lu
co
se
h
ig
h
er
in
L
G
I,
an
d
R
E
R
lo
w
er
in
L
G
I
W
o
n
g
et
al
.
[4
5
]
S
u
b
m
ax
?
T
T
M
in
u
te
s
1
0
1
.5
±
5
.9
9
8
.7
±
5
.7
L
G
I
(2
.8
)
Y
A
t
5
–
1
0
k
m
,
R
E
R
an
d
C
H
O
o
x
id
at
io
n
lo
w
er
in
L
G
I.
A
t
5
–
1
5
k
m
,
fa
t
o
x
id
at
io
n
h
ig
h
er
in
L
G
I.
A
t
1
0
k
m
,
g
lu
co
se
h
ig
h
er
in
L
G
I.
A
t
1
0
–
2
1
k
m
,
g
ly
ce
ro
l
h
ig
h
er
in
L
G
I.
A
t
en
d
o
f
ex
er
ci
se
,
F
F
A
h
ig
h
er
in
L
G
I.
N
o
d
if
fe
re
n
ce
in
in
su
li
n
,
la
ct
at
e
W
o
n
g
et
al
.
[4
7
]
S
u
b
m
ax
?
T
T
M
in
u
te
s
9
1
.1
±
6
.0
9
1
.8
±
6
.6
HG
I
(0
.8
)
N
N
o
d
if
fe
re
n
ce
in
g
lu
co
se
,
in
su
li
n
,
la
ct
at
e,
F
F
A
,
g
ly
ce
ro
l,
C
H
O
,
o
r
fa
t
o
x
id
at
io
n
W
u
an
d
W
il
li
am
s
[4
9
]
T
T
E
M
in
u
te
s
1
0
1
.4
±
1
4
.7
1
0
8
.8
±
1
1
.6
L
G
I
(6
.8
)
Y
A
t
0
–
3
0
m
in
,
g
lu
co
se
h
ig
h
er
in
L
G
I.
A
t
4
5
m
in
,
g
ly
ce
ro
l
h
ig
h
er
in
L
G
I.
A
t
0
–
9
0
m
in
,
fa
t
o
x
id
at
io
n
h
ig
h
er
,
an
d
C
H
O
o
x
id
at
io
n
lo
w
er
in
L
G
I.
F
F
A
h
ig
h
er
an
d
R
E
R
lo
w
er
in
L
G
I
th
ro
u
g
h
o
u
t.
N
o
d
if
fe
re
n
ce
in
la
ct
at
e
C
H
O
ca
rb
o
h
y
d
ra
te
,
F
F
A
fr
ee
fa
tt
y
ac
id
,
H
G
I
h
ig
h
g
ly
ce
m
ic
in
d
ex
,
L
G
I
lo
w
g
ly
ce
m
ic
in
d
ex
,
N
n
o
,
R
E
R
R
es
p
ir
at
o
ry
ex
ch
an
g
e
ra
ti
o
,
S
u
b
m
a
x
su
b
m
ax
im
al
ex
er
ci
se
,
T
T
ti
m
e
tr
ia
l,
T
T
E
ti
m
e
to
ex
h
au
st
io
n
,
Y
y
es
a
V
al
u
es
ar
e
m
ea
n
±
S
D
b
P
er
ce
n
t
d
if
fe
re
n
ce
ca
lc
u
la
te
d
as
th
e
im
p
ro
v
em
en
t
o
f
o
n
e
tr
ia
l
o
v
er
th
e
o
th
er
,
e.
g
.,
(L
G
I-
H
G
I)
/L
G
I
9
1
0
0
,
h
o
w
ev
er
fo
r
th
e
p
u
rp
o
se
o
f
th
e
w
ei
g
h
te
d
m
ea
n
d
if
fe
re
n
ce
,
th
e
sa
m
e
eq
u
at
io
n
w
as
u
se
d
Effect of Glycemic Index of a Pre-exercise Meal on Endurance Performance
123
studies (n = 6) where the LGI meal (without exogenous
CHO consumption) was reported to have a significant
effect. In four of these studies, the meal was provided\1 h
before exercise and it is possible that despite the short pre-
exercise meal time, at least some CHO from the LGI meal
was still available at the later stages of exercise to support
CHO oxidation when endogenous stores were low.
Current sports nutrition guidelines recommend con-
sumption of the pre-exercise meal 1–4 h prior [1]. Time-
frames outside of these recommendations were used in
seven of the 19 studies. At the time when most of the
included studies were conducted, there was concern
regarding the potential for a HGI meal to cause a nadir in
blood glucose close to the commencement of exercise and
so studies were often purposely designed to investigate this
effect. This explains why a number (n = 7) of the studies
in this review were designed with a\1-h pre-exercise meal
time despite the risk that for the LGI condition, this may
have compromised CHO availability during exercise.
Despite concern regarding the potential for hypoglycemia
after a HGI pre-exercise meal, none of the studies reported
any participant experiencing symptoms of hypo-
glycemia although some studies did report a few partici-
pants experiencing blood glucose concentrations below
3.5 mmol/L.
Another parameter which varied widely across the
studies was the GL. As the studies all included a similar
dose of CHO in both arms, it was not possible to quanti-
tatively examine the within-study impact of GL. Qualita-
tive evaluation of the forest plots with studies arranged
from lowest to highest within-study GL differences does
not support any effect of meal GL. The dose of CHO in a
pre-exercise meal recommended by current sports nutrition
guidelines is 1–4 g/kg body mass. All but one of the
included studies used this dose, although all of the studies
were in the lower half of this range (B2 g/kg). At a higher
pre-exercise CHO dose, there may be a greater effect of the
GL, although this has not yet been investigated.
Table 4 Methodological quality of included studiesa
Study Reporting EV Bias Confounding Power Study score (max
21)
1 2 3 4 5 6 7 8 9 10 11 12 13 14b 15 16 17 18 19
Bennett et al. [38] Y Y Y Y Y Y N Y Y N Y Y Y U Y Y N Y Y N 15
Burke et al. [33] Y Y N Y Y Y N N Y N Y N Y Y Y Y N Y Y N 14
Chen et al. [22] Y Y N Y Y Y N N Y Y Y Y Y P Y Y N Y Y N 15
Chen et al. [34] Y Y Y Y Y Y N N Y N N N Y P Y Y Y Y N N 13
DeMarco et al. [37] Y Y Y Y Y Y N N Y N Y N Y U Y Y Y Y Y Y 15
Febbraio and Stewart
[43]
Y Y N Y Y Y N N Y N N N Y U Y Y N Y Y N 11
Febbraio et al. [42] Y Y N Y Y Y N N Y N N N Y U Y Y N Y Y N 13
Houlton et al. [30] Y Y N Y Y Y Y N U N Y N Y P Y Y N Y Y N 11
Little et al. [32] Y Y N Y Y Y Y Y Y N Y Y Y P Y Y N Y Y N 16
Little et al. [39] Y Y N Y Y Y N N Y N Y N Y P Y Y N Y Y N 13
Moore et al. [40] Y Y N Y Y Y N Y Y Y Y Y Y Y Y Y N Y Y N 17
Moore et al. [41] Y Y N Y Y Y Y Y Y N Y N Y P Y Y N Y Y N 15
Reljic et al. [31] N Y Y Y Y Y N N U N N Y Y U Y Y N Y Y N 11
Sparks et al. [44] Y Y N Y Y Y N N Y N N N Y P Y Y N Y Y N 12
Thomas et al. [16] Y Y N Y Y Y N N Y Y Y N Y U Y Y N Y Y Y 14
Thomas et al. [48] Y Y N Y Y Y N N Y Y Y N Y U Y Y N Y Y N 13
Wong et al. [45] Y Y Y Y Y Y N N Y N Y Y Y P Y Y Y Y Y Y 17
Wong et al. [47] Y Y Y Y Y Y N N Y N N N Y P Y Y Y Y Y N 14
Wu and Williams
[49]
Y Y N Y Y Y N N Y N N N Y U Y Y N Y Y N 11
Item score (max 19) 18 19 6 19 19 19 3 4 17 4 12 6 19 6.5 19 19 4 19 18 3
1 hypothesis stated, 2 main outcomes, 3 participant description, 4 intervention described, 5 main findings described, 6 variability estimates, 7
adverse events reported, 8 actual p value reported, 9 representative subjects, 10 subject blind to food, 11 subject blind to performance, 12
researcher blinded, 13 statistical tests, 14 compliance, 15 accurate measures, 16 randomized to groups, 17 glucose tolerance screening, 18 24 h
diet control, 19 24 h exercise control, EV external validity, Max maximum, N no, P partial, U unable to determine, Y yes
a For all items except that assessing compliance, ‘‘Y’’ is given a score of one and ‘‘N’’ or ‘‘U’’ are given a score of zero
b Item 14 on compliance is given a maximum score of two, with ‘‘Y’’ scored as two, ‘‘P’’ as one, and ‘‘U’’ as zero
C. A. Burdon et al.
123
Participant fitness is another variable that may influence
performance outcomes. Performance coefficients of varia-
tion in participants of higher athletic caliber are often
smaller, providing a greater chance to observe significant
effects [53]. Fitter participants also have a greater capacity
to oxidize fat, and, thus, their reliance on CHO oxidation
during sub-maximal exercise may be lower [46]. The type
of exercise test is also relevant to measuring performance.
In this review, only two studies used well trained partici-
pants, which limited the generalizability of the results to
elite athletes. However, elite endurance athletes are also
likely to use exogenous CHO during exercise so this is
another key reason why there could be limited applicability
of a LGI pre-exercise meal. Lack of consistency of these
factors limits the capacity to draw firm conclusions,
although meta-regression analysis failed to identify any of
these factors as having a significant impact on the overall
outcomes.
Consistent with the above limitations, the metabolic
responses reported across the studies were also conflicting.
Half of the 14 studies where exogenous CHO was not
consumed reported no difference in blood glucose con-
centration between the LGI and the HGI meals. Ten of the
12 studies reporting on blood lactate also showed no dif-ference between meals. Many of the studies (but not all)
reported lower free fatty acid (six of nine), higher CHO
oxidation rate (five out of 11), and a higher RER (six of 12)
with the HGI pre-exercise meal, which would be antici-
pated on theoretical grounds [50–52]. Lack of control of
pre-exercise diet and exercise regimens as well as between-
study variability in participant fitness and GL may have
influenced the metabolic results. It is also notable that all
except one of the pre-exercise meals had mixed macronu-
trient composition rather than being CHO alone. Other
nutrients, particularly protein, are known to influence post-
prandial insulin release [55]. Although the meals were
matched within each study, substantial variation in meal
composition existed across studies, which likely played a
role in the different metabolic responses observed in this
review.
Given that most elite endurance athletes consume
exogenous CHO during exercise and competitions provide
aid stations to support this, the real-world application of a
LGI pre-event meal in competitive sport may be limited.
However, our analysis suggests there may be some situa-
tions in which a LGI meal could be beneficial. These sit-
uations include prolonged exercise where there is limited
opportunity for exogenous CHO ingestion, such as intense
and sustained physical activity during military operations.
There may also be an application for athletes with a dis-
ability in wheelchair or hand-cycling endurance events as
although there are sufficient aid stations to provide
exogenous CHO, it may be logistically difficult for the
athletes to take their hands off the wheel or bike wheel
crank (unless they are going downhill). Storing drinks
containing CHO can also be difficult in the bike/chair set-
up and fluid requirements are substantially lower in those
with spinal cord injury as they do not sweat below the level
of lesion [56].
There may also be some application for individuals who
are unable to tolerate exogenous CHO consumption at the
high levels required for performance optimization. Over
the past 10 years there has been interest in maximal CHO
oxidation rates during endurance exercise and in strategies
to enhance CHO absorption from the gut [57]. Amounts of
up to 90 g of CHO per hour can be successfully fed using
2:1 glucose to fructose ratios, and the subsequent gain in
CHO oxidation rate may be essential to winning perfor-
mance for top athletes during endurance events [58]. It can
be difficult for some athletes to consume CHO at these
rates due to gastrointestinal upset [59], although CHO
tolerance has been shown to improve with regular CHO
ingestion during exercise [60]. LGI pre-exercise meals may
be useful in athletes whose CHO tolerance remains low.
This potential application of a LGI meal warrants further
investigation.
This review has a number of strengths in that it was
systematic and also included meta-analysis with sub-
analysis conducted for studies where CHO was ingested
during exercise, as is typical for endurance events. Meta-
regression was also used to investigate the influence of
potential effect modifiers: the GL, pre-exercise meal
timing, and participant fitness. Weaknesses include the
quality of the included studies, which was mixed due to
inadequate description of the participants and any
adverse events, lack of participant blinding, inadequate
description/reporting of the diet control in the 24 h prior
to the exercise performance tests, and lack of statistical
power in most of the studies. In many of the studies, the
meals were fed\2 h prior to exercise and it was likely
that much of the LGI CHO remained undigested and
therefore unavailable for oxidation. This limited the
capacity to fully evaluate the effectiveness of the LGI
meal.
Future research in this area would benefit from a focus
on situations where it may be more difficult for participants
to consume CHO during exercise such as extended military
operations and endurance events for athletes with a dis-
ability (wheelchair marathons/hand cycling) where com-
petitors may find it logistically difficult to access CHO
during the event. In these situations, slowly digested LGI
CHO may support CHO oxidation later in the event.
Consumption of a LGI pre-event CHO meal may also be
useful for elite athletes with high rates of CHO oxidation
who experience gastrointestinal distress when consuming
higher amounts of exogenous CHO during the event.
Effect of Glycemic Index of a Pre-exercise Meal on Endurance Performance
123
5 Conclusion
In conclusion, this systematic review and meta-analysis
does not support a significant benefit of a LGI over a HGI
meal for endurance exercise performance. The metabolic
responses to these meals were also inconsistent. Given that
exogenous CHO consumption is common practice for
athletes during endurance events, it would seem that LGI
pre-event meals have limited benefit in competitive
endurance sports. The importance of LGI meals may
become apparent in certain instances where supply of
exogenous CHO is limited or compromised such as in
military operations or in individuals who have difficulty
ingesting sufficient CHO during exercise. Further research
is required to assess the potential benefit of LGI meals in
these situations.
Acknowledgments The authors thank Sandeep Das and Susan
Griffee for assistance with data extraction and methodological quality
assessment.
Compliance with Ethical Standards
Funding No sources of funding were used to assist in the preparation
of this article.
Conflict of interest Catriona Burdon, Inge Spronk, Hoi Lun Cheng,
and Helen O’Connor declare that they have no conflicts of interest.
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