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Carbohydrate Hydrogel Products Do Not Improve Performance
or Gastrointestinal Distress During Moderate-Intensity
Endurance Exercise
Andy J. King
Australian Catholic University
Joshua T. Rowe
University of Leeds
Louise M. Burke
Australian Catholic University and
Australian Institute of Sport
The benefits of ingesting exogenous carbohydrate (CHO) during prolonged exercise performance are well established. A recent
food technology innovation has seen sodium alginate and pectin included in solutions of multiple transportable CHO, to
encapsulate them at pH levels found in the stomach. Marketing claims include enhanced gastric emptying and delivery of CHO to
the muscle with less gastrointestinal distress, leading to better sports performance. Emerging literature around such claims was
identified by searching electronic databases; inclusion criteria were randomized controlled trials investigating metabolic and/or
exercise performance parameters during endurance exercise >1 hr, with CHO hydrogels versus traditional CHO fluids and/or
noncaloric hydrogels. Limitations associated with the heterogeneity of exercise protocols and control comparisons are noted. To
date, improvements in exercise performance/capacity have not been clearly demonstrated with ingestion of CHO hydrogels
above traditional CHO fluids. Studies utilizing isotopic tracers demonstrate similar rates of exogenous CHO oxidation, and
subjective ratings of gastrointestinal distress do not appear to be different. Overall, data do not support any metabolic or
performance advantages to exogenous CHO delivery in hydrogel form over traditional CHO preparations; although, one study
demonstrates a possible glycogen sparing effect. The authors note that the current literature has largely failed to investigate the
conditions under which maximal CHO availability is needed; high-performance athletes undertaking prolonged events at high
relative and absolute exercise intensities. Although investigations are needed to better target the testimonials provided about
CHO hydrogels, current evidence suggests that they are similar in outcome and a benefit to traditional CHO sources.
Keywords:encapsulated carbohydrate, glycogen, gut, sports nutrition, oxidation, sports drink
Recent interest in the 2-hr marathon (Caesar, 2019)has
focused attention on an important sports nutrition strategy; con-
sumption of carbohydrate (CHO) during exercise to contribute to
the substantial fuel costs of some endurance events. Events of
sufficient intensity and duration to be limited by CHO availability
benefit from an exogenous CHO supply (Stellingwerff & Cox,
2014), with mechanisms including fuel provision once muscle
glycogen is depleted (Coyle et al., 1986), spared liver (Gonzalez
et al., 2015;Wallis et al., 2006)andmuscle(King et al., 2018;
Tsintzas et al., 1995,1996) glycogen use, and central nervous
system benefits (Burke & Maughan, 2015). A sliding scale of
intake is recommended, according to event fuel needs and specific
mechanisms underpinning performance benefits (Thomas et al.,
2016). Upper targets for fuel-demanding events (80–90+ g·hr
–1
CHO), which aim to maximize the contribution of exogenous
CHO to substrate use, are often challenged by the ability to
consume, tolerate, and absorb large amounts of CHO (de
Oliveira & Burini, 2014). Factors include the availability of
foods/drinks to meet CHO targets in practical amounts/volumes,
the effect of the mode and intensity of exercise on gastrointestinal
(GI) comfort and function (de Oliveira & Burini, 2009), the role of
specific“gut training”(Cox et al., 2010), and characteristics of the
CHO source. Here it has been shown that the use of CHO blends
(“multiple transportable CHO”such as glucose [G] and fructose
[F]) can maximize gut uptake via the use of different intestinal
transport mechanisms, assisting with substrate delivery and the
management of gut comfort (Jeukendrup, 2010).
Recently, specialized sports foods claiming to address such
factors via the use of “hydrogel technology”have become com-
mercially available (Sutehall et al., 2018). These supplements,
combining typical CHO sources with pectin (a soluble fiber)
© 2020 The Authors. Published by Human Kinetics, Inc. This is an Open Access article
distributed under the terms of the Creative Commons Attribution-NonCommercial 4.0
International License, CC BY-NC 4.0, which permits the copy and redistribution in any
medium or format, provided it is not used for commercial purposes, the original work is
properly cited, the new use includes a link to the license, and any changes are indicated.
See http://creativecommons.org/licenses/by-nc/4.0. This license does not cover any
third-party material that may appear with permission in the article. For commercial use,
permission should be requested from Human Kinetics, Inc., through the Copyright
Clearance Center (http://www.copyright.com).
King and Burke are with the Mary Mackillop Institute for Health Research,
Australian Catholic University, Melbourne, VIC, Australia. Burke is also with
the Australian Institute of Sport, Canberra, ACT, Australia. Rowe is with the
University of Leeds, Leeds, United Kingdom. King (andy.king@acu.edu.au)is
corresponding author.
1
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and alginate (a polymer derived from seaweed) undergo gelation on
contact with low pH solutions, such as stomach acid, to encapsulate
the CHO (Marciani et al., 2019). Enhanced rates of gastric empty-
ing could deliver this “hydrogel”to the small intestine where it
dissolves in the higher pH environment for absorption, leading to
reduced gut discomfort, enhanced muscle CHO delivery, and
performance benefits (Figure 1). Indeed, according to testimonials,
the commercial product has been quickly adopted by elite athletes
(Sutehall et al., 2018) and publicized in sporting successes includ-
ing the 1:59 marathon project, leading to marketing claims that it is
the “world’s fastest sports fuel”(Maurten, 2020). Noting that this has
largely occurred in the absence of scientific validation of these
claims, we undertook a review of newly published investigations of
hydrogel CHO to determine whether they achieve better GI char-
acteristics, substrate delivery, and performance effects under exer-
cise conditions than traditional sports drinks and gels.
Methods
A search of electronic databases for studies published up to May
14, 2020 was independently completed by two authors (A. King/
J. Rowe), with the key methodological process and considerations
involved in including/excluding data summarized in Figure 2.
To be eligible for this review, studies were required to have
investigated a CHO hydrogel compound during prolonged, endur-
ance exercise defined as continuous running, cycling, triathlon,
rowing, swimming, and cross-country skiing greater than 1-hr dura-
tion. Studies with exercise durations lasting 1 hr or less were excluded
because CHO ingestion is unlikely to be beneficial during shorter
duration exercise (Burke et al., 2011;Thomas et al., 2016). CHO
mouth rinse studies were also excluded as the primary mechanism
whereby performance is improved is neurological in origin.
The CHO hydrogel used could be a commercially available
product or a laboratory-manufactured solution, provided that the
active substance included as a gelling agent was known to encap-
sulate ingested CHO in the stomach. A control comparison/
condition was required to be a typical CHO control, matched for
CHO dose and type, or a placebo. Studies were included if they
reported data on one or more of physiological or performance
variables (Table 1). Review articles and case studies were excluded.
From the available studies, between-condition differences for
hydrogel and comparison products were calculated. Standardized
effect sizes (ES) were calculated using Hedge’sgadjustment for
small samples with 95% confidence intervals for the ES computed.
No statistical adjustments for ES were made for carryover effects,
since suitable washout periods were included in these crossover
trials. No assessment of publication bias was undertaken with the
low number of studies and the likely presence of heterogeneity; we
noted that this can affect the robustness of publication bias analysis
(Ioannidis & Trikalinos, 2007). Forest plots were produced to
provide a visual comparison of effects in studies measuring exer-
cise performance.
Results
We located six studies (Table 1) which compared a CHO hydrogel
containing alginate and/or other gelling compounds with a nonca-
loric hydrogel placebo (n= 1) or CHO fluids of matched CHO
(n= 5). Although CHO hydrogels were similar in composition,
containing a mixture of maltodextrin (MD) and F, total CHO
ranged from 68 to 132 g·hr
–1
. Five of the six studies included a
matched condition for dose and type of CHO (i.e., MD + F fluid)
and two studies included a G or MD only fluid matched for CHO
dose. Therefore, four studies met guidelines for upper range CHO
Figure 1 —Mechanisms of CHO hydrogel formation and delivery to the small intestine. Despite benefits to gastric emptying with hydrogel-
encapsulated CHO, the rate-limiting step of exogenous CHO oxidation lies in the intestinal transport of monosaccharides. CHO = carbohydrate.
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intakes: ∼90 g·hr
–1
from multiple transportable CHO sources
(Jeukendrup, 2010).
All studies were randomized controlled crossover trials. They
investigated trained male athletes (VO
2
max 55–70 ml·kg
−1
·min
−1
),
with the exception of Pettersson et al. (2019) and (Flood et al.,
2020), who included six female cross-country skiers (VO
2
max:
59.9 ± 2.6 ml·kg
−1
·min
−1
) and seven female cyclists (54.3 ±
12.3 ml·kg
−1
·min
−1
). Exercise modes included cycling (four stud-
ies), running (one), and cross-country skiing (one). A period of
fixed-intensity exercise was included in all protocols (90–180 min)
with five of the six studies then measuring exercise performance/
capacity using a time trial (three studies), time to exhaustion test
(one), or repeated sprint test (one). Each of the studies in which
performance was measured involved a preexercise meal according
to current recommendations (Burke et al., 2011) or a self-selected
“CHO-rich”meal (Flood et al., 2020). The only study undertaken
under fasting conditions (Barber et al., 2020) focused on exoge-
nous CHO oxidation as the primary outcome, which is not affected
by prior muscle glycogen content (Margolis et al., 2019). Between-
conditions comparisons remain valid despite the difference in
breakfast protocol to other studies.
Exercise Performance
Performance was similar with MD + F hydrogel, isocaloric MD + F
fluids (Figure 3;Baur et al., 2019;Flood et al., 2020;McCubbin
et al., 2019;Mears et al., 2020b), or noncaloric hydrogel
(Pettersson et al., 2019). Relative performance changes between
CHO hydrogel and fluids ranged between +1.05% and +3.8% but
were not statistically significant (p<.05). The largest change
reported by Mears et al. (2020b) during a ∼20-min fixed-work
time trial was a moderately higher workload (3.8% improvement,
ES = 0.51) with CHO hydrogel ingestion compared with the dose
matched MD + F control. All other performance effects were very
small (ES <0.10).
Physiological Measures
Exogenous CHO oxidation was measured in two studies, but only
one included a comparative CHO condition (Barber et al., 2020)
with exogenous CHO oxidation peaking at 1.1 ± 0.3 g·min
–1
in both
the MD + F hydrogel and MD + F fluid (Table 1). MD + G ingestion
resulted in a moderately lower oxidation rate (0.92 ± 0.3 g·min
–1
).
Total exogenous CHO oxidation over the final (second) hour of
running was not modified by MD + F hydrogel (48.25 ± 16.5 g vs.
50.25 ± 16.5 g in MD + F solution) but both were higher than
MD + G (41.25 ± 15.0 g). Pettersson et al. (2019) reported exoge-
nous CHO oxidation of 1.22 (0.89–1.66) g·min
–1
with MD + F
hydrogel, indicating that a higher CHO dose (132 g.hr
–1
)with
hydrogel may result in higher exogenous CHO utilization.
Total rates of CHO and fat oxidation during the steady-state
exercise varied between studies, a consequence of different exercise
intensities. Therefore, it is of interest to examine relative contribu-
tions to total fuel use (Figure 4) as well as absolute rates of oxidation
(Table 1). When comparing MD + F in hydrogel versus fluid form,
divergent effects were reported. Mears et al. (2020b) and Baur et al.
(2019) found no differences in relative substrate contribution during
submaximal cycling at 50% W
max
but the nonsignificant increase in
fat oxidation (and decrease in CHO oxidation) reported by Baur et al.
(2019) is notable, and consistent with Barber et al. (2020). Flood
et al. (2020) also reported similar CHO and fat oxidation with CHO
and hydrogel ingestion during low-intensity cycling. However,
Baur et al. (2019) also noted lower absolute total CHO and fat
(to Mears et al. [2020b] in both conditions), despite similar ingestion
rates (68 and 78 g.hr
–1
). McCubbin et al. (2019) did not report
differences in substrate oxidation during steady-state running, and
neither CHO nor fat oxidation were modified by the hydrogel form
during a subsequent incremental test. Barber et al. (2020) however,
reported a reduced contribution of endogenous CHO during steady-
state running with hydrogel CHO. Glycogen contributed ∼60% of
total energy expenditure with MD + G and MD + F fluids, but ∼50%
with MD+ F hydrogel. Differences in glycogen use were explained
by higher exogenous CHO oxidation in comparison with MD + G
fluid, and increased fat oxidation in relation to MD + F fluid
(Figure 4). Comparisons between studies employing different exer-
cise modalities for whole-body substrate utilization are difficult as
noted previously (Achten et al., 2003).
Figure 2 —Methodology and review considerations. GI =
gastrointestinal; CHO = carbohydrate.
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Table 1 Studies Investigating CHO Hydrogel Formulations With Isocaloric CHO or Placebo Solutions
Study Participants Design/protocol
Preexercise
CHO Supplement
Performance
Δ
Exogenous
CHO Ox
(g·min
–1
)
Whole-body
substrate
Ox (g·min
–1
)
GI symptoms
(hydrogel
comparisons
only)
Baur et al.
(2019)
Nine male cyclists
(trained, VO
2
max =
55.5 ± 3.6 ml·kg
−1
·min
−1
)
RCT; variable-
intensity cycling
(55 min SS @
50% W
max
+2×
4×2 min @ 80%
W
max
+ 5 min
@ 50% W
max
+
10 ×max sprint)
2 hr preexercise
(20–25% daily
ER—liquid meal
replacement)
MD + F hydrogel 284 ± 51 W
(↑2.5%)*
n/a CHO: 1.50 ± 1.26
FAT: 0.25 ± 0.26
No significant
Treatment ×Time
interactions
MD + F solution 281 ± 46 W
(↑1.08%)*
CHO: 1.53 ± 1.37
FAT: 0.19 ± 0.27
Nonsignificant ↑
fullness vs. MD
(ES = 0.54)
MD solution 277 ± 48 W CHO: 1.40 ± 1.21
FAT: 0.26 ± 0.29
All 78 g·hr
−1
in
1 L·hr
−1
McCubbin
et al.
(2019)
Nine male runners
(trained, VO
2
max =
59.0 ± 8.0 ml·kg
−1
·min
−1
)
RCT; fixed-inten-
sity running + TTE
(180 min
@ 60% VO
2
max
TTE: SS pace +
2 km·hr
−1
↑/3 min)
2 hr preexercise MD + F hydrogel 744 ± 182 s
(↑1.6%)*
n/a CHO: ∼1.92 ± 0.30
FAT: n/a
11% vs. 22% inci-
dence of severe
symptoms hydrogel
vs. MD; F
24 kJ·kg
−1
(1 g·kg
−1
CHO,
0.15 g·kg
−1
PRO)
MD + F solution
90 g·hr
−1
in
0.57 L·hr
−1
756 ± 187 s CHO: ∼1.95 ± 0.30
FAT: n/a
Breath H
2
not diff
Pettersson
et al.
(2019)
12 elite CX skiers: six
males, six females
(VO
2
max = 69.1 ± 2.9 and
59.9 ±2.6 ml·kg
−1
·min
−1
)
RCT; fixed-inten-
sity CX skiing + TT
(120 min @
70% VO
2
max)
1 hr preexercise MD + F hydrogel
132 g·hr
−1
in
0.6 L·hr
−1
239 ± 16 W
(↓0.4%)*
1.22 ± 0.4 CHO: 2.38 ± 0.25
FAT: 0.70 ± 0.06
No significant
Treatment ×Time
interactions
1 g·kg
−1
CHO PLA (noncaloric)
hydrogel
238 ± 16 W 0 CHO: 2.00 ± 0.26
FAT: 0.83 ± 0.08
Mears et al.
(2020b)
Eight cyclists
(well-trained, VO
2
max =
62.1 ± 6.9 ml·kg
−1
·min
−1
)
RCT; fixed-inten-
sity cycling + TT
(120 min @
50% W
max
)
2 hr preexercise MD + F hydrogel 1219 ± 84 s
(↑3.8%)*
n/a CHO: 2.59 ± 0.60
FAT: 0.42 ± 0.15
Fullness ↑p= .02 in
hydrogel
1.5 g·kg
−1
CHO MD + F solution
68 g·hr
−1
in
0.5 L·hr
−1
1,267 ± 102 s CHO: 2.56 ± 0.44
FAT: 0.42 ± 0.12
No other significant
Treatment ×Time
interactions
Barber
et al.
(2020)
Nine runners
(well-trained, VO
2
max =
63 ± 3.6 ml·kg
−1
·min
−1
)
RCT; fixed-inten-
sity running
(120 min @ 60%
VO
2
max)
Nil: 8 hr fast MD + F hydrogel n/a 1.1 ± 0.3 CHO: 2.60 ± 0.75
FAT: 0.47 ± 0.22
No significant
Treatment ×Time
interactions
MD + F solution 1.1 ± 0.3 CHO: 3.04 ± 0.69
FAT: 0.31 ± 0.15
MD + G solution
90 g·hr
−1
in
0.57 L·hr
−1
0.9 ± 0.5 CHO: 2.88 ± 0.62
FAT: 0.43 ± 0.09
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Table 1 (continued)
Study Participants Design/protocol
Preexercise
CHO Supplement
Performance
Δ
Exogenous
CHO Ox
(g·min
–1
)
Whole-body
substrate
Ox (g·min
–1
)
GI symptoms
(hydrogel
comparisons
only)
Flood et al.
(2020)
14 cyclists: seven males,
seven females
(recreational/trained,
VO
2
max = 56.4 ±
7.6 ml·kg
−1
·min
−1
; 54.3 ±
12.3 ml·kg
−1
·min
−1
)
RCT; fixed-inten-
sity cycling + TT
(90 min @ 45%
VO
2
max)
3 hr preexercise MD + F hydrogel 192 W
(↑14%)
CHO: 1.73 ± 0.75
FAT: 0.21 ± 0.35
IFABP (end
exercise)
Self-selected
“CHO rich”
MD + F solution 190 W
(↑13%)
CHO: 1.70 ± 0.40
FAT: 0.27 ± 0.12
↑Water (∼500
pg·ml
−1
vs. hydro-
gel [∼150
pg·ml
−1
**]
MD + F [by ∼70
pg·ml
−1
**])
PLA
90 g·h
−1
in 0.78
L·h
−1
168 W n/a CHO: 1.35 ± 0.40
FAT: 0.39 ± 0.15
Lactulose: Rham-
nose ↓in MD + F
and hydrogel**
No significant
treatment effects for
GI symptoms. Full-
ness ↑in MD + F
and hydrogel
Note. RCT = randomized crossover trial; SS = steady state; TTE = time to exhaustion; TT = time trial; CHO = carbohydrate; ER = energy requirement; CX = cross-country (skiing); IFABP = Intestinal fatty acid binding protein;
MD = maltodextrin; F = fructose; G = glucose; GI = gastrointestinal; PLA = noncaloric placebo.
*Performance change not significantly different (p>.05). Performance changes in Baur et al. (2019) are relative to MD condition and in Flood et al. (2020) relative to water. **Gastrointestinal effects significantly different (p<.05) to
placebo.
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Gastrointestinal discomfort was reported in all studies with
similar responses from the hydrogel trial. McCubbin et al. (2019)
reported a slightly, but not significantly, higher overall incidence of
GI symptoms while running with MD + F fluids than MD + F
hydrogel (22% vs. 11%). In cycling, Mears et al. (2020b) and
Baur et al. (2019) reported slightly increased stomach fullness with
CHO hydrogel ingestion but similarly, differences were not sig-
nificant. Fullness in Flood et al. (2020) did not differ between CHO
hydrogel and MD + F. Baur et al (2019) also reported moderately,
but not significantly increased nausea with the hydrogel compared
with MD + F fluids (ES = 0.53, p= .23). Data from Pettersson et al.
(2019) reported low levels of GI issues during skiing in cold
conditions in either trial, but a small, nonsignificant decrease in
stomach rumbling with the MD + F hydrogel in comparison with a
noncaloric hydrogel. In the only study to report GI data through
nonsubjective measures, Flood et al. (2020) found significantly
higher intestinal fatty acid-binding protein and lactulose to rham-
nose ratio with placebo compared with both CHO hydrogel and
matched MD + F ingestion.
Discussion
This is a timely investigation of the evidence that intake of hydrogel
encapsulations of multiple transportable CHO during prolonged
endurance exercise provides benefits over traditional sports drinks,
in response to the recent interest in newly available commercial
hydrogel products. We summarized studies where hydrogels,
formed by combining MD + F with gelling agents such as alginate
and pectin, were compared with typical CHO fluids containing
single or multiple CHO sources, or a noncaloric hydrogel treat-
ment. Despite marketing claims and lay media discussion about
MD + F products in hydrogel format, currently available studies
fail to show benefits in terms of muscle oxidation of exogenous
CHO, GI comfort, or performance. The current literature, com-
prised of robust randomized controlled trials, is small and includes
nuances around total substrate oxidation and gut comfort due to
exercise mode and intensity, as well as total CHO intake. Further-
more, the conditions under which it is promoted to achieve its key
benefits (high rates of CHO intake during prolonged exercise at
high absolute and relative intensities) have not been investigated,
potentially due to the challenge of involving elite competitors
within traditional research protocols and the technical challenges
of undertaking measurements of interest (e.g., gastric emptying,
tracer determined substrate oxidation) under these conditions.
While further studies with relevant protocols are needed to inves-
tigate the putative benefits of these products, the present literature
fails to endorse the marketing claims.
Over the past 5 years, CHO-containing drinks that achieve
hydrogel encapsulation within the gut have become commer-
cially available, with claims that they achieve superior CHO
delivery to the muscle for lower GI distress, leading to perfor-
mance benefits over traditional CHO-containing sports products
(Maurten, 2020;Sutehall et al., 2018). CHO hydrogel products
have attempted to improve on existing nutrition recommenda-
tions (use of CHO with multiple transporters [Jeukendrup, 2010]
and gut training [Jeukendrup, 2017]) to target the need for high-
performance athletes to achieve high CHO availability during
prolonged events conducted at high relative and absolute inten-
sities (Burke et al., 2019). Although there is evidence that these
products achieve gelation within the acidic stomach environ-
ment as claimed (Marciani et al., 2019;McCubbin et al., 2019),
subsequent effects on gastric emptying, intestinal absorption,
anddeliverytothemuscleduring exercise remain largely
untested. Our review summarized the available literature on
the use of these products during prolonged exercise in terms
of GI comfort, substrate utilization, and performance. The
scarcity and heterogeneity of study protocols prevents a meta-
analytical approach from producing meaningful results. How-
ever, in view of community interest and marketing claims, we
felt it was timely to collate the findings of available studies in
narrative form to summarize the overall findings and alert
researchers to the need for particular protocols.
Figure 3 —Forest plot of standardized effects sizes and 95% confidence intervals for exercise performance in studies comparing CHO hydrogel
formulations with isocaloric CHO or noncaloric placebo solutions. H = hydrogel; MD = maltodextrin; MD + F = maltodextrin + fructose; PLA = placebo;
H (MD + F) = hydrogel of MD + F; CHO = carbohydrate.
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The current literature involves studies of subelite athletes
(although one study of elite performers is noted [Pettersson
et al., 2019]), exercising over 2–3 hr at intensities of approximately
45–70% VO
2
peak. Such protocols likely represent pragmatic
choices based on subject availability and the technical requirements
around the steady-state conditions needed to monitor muscle
substrate use during exercise (Robert et al., 1987). According to
our review, although the GI handling of these products has not been
directly measured, these studies have failed to find evidence of
increased oxidation of exogenous CHO when MD + F is provided
in hydrogel form compared with conventional solutions. Indeed,
rates of exogenous CHO oxidation derived from
13
C isotopic tracer
techniques are in line with results of previous studies of traditional
CHO-containing fluids (King et al., 2019;O’Brien et al., 2013), or
in the case of studies in which direct comparison has been made
between CHO-matched fluids and hydrogel products, no differ-
ences in exogenous CHO use has been detected (Barber et al.,
2020). Since the rate-limiting step of exogenous CHO oxidation
is believed to lie in the GI tract and not cellular glucose uptake
(Hawley et al., 1994), this indirectly indicates that CHO hydrogels
have not achieved a net benefit to the gastric emptying/intestinal
CHO transport processes.
The studies published to date confirm previous knowledge that
whole-body CHO utilization is altered by the intake of CHO during
exercise (Pettersson et al., 2019) and by the choice of multiple
transportable CHO sources that increase the capacity for total
intestinal absorption (Barber et al., 2020;Baur et al., 2019). In
general, Figure 4suggests that hydrogel encapsulation per se does
not change total CHO oxidation during exercise (Baur et al., 2019;
Flood et al., 2020;McCubbin et al., 2019;Mears et al., 2020b).
However, Barber et al. (2020) reported a decrease in endogenous
CHO utilization and increase in fat oxidation when high rates of
CHO intake were consumed in hydrogel form versus fluid. Muscle
glycogen sparing effects have been reported in several studies of
CHO intake during endurance exercise, but performance improve-
ments have not been consistently observed (Newell et al., 2014).
Previous work from our group (King et al., 2018) and others (Smith
et al., 2010) has shown that the muscle glycogen response to CHO
ingestion is dose dependent. Precise mechanisms to explain this
effect have not been investigated, but likely sit within the cellular
flux through the glycolytic pathway and the interaction of exoge-
nous glucose and glucosyl units liberated from glycogen at glu-
cose-6-phosphate. Exogenous CHO more consistently reduces
liver glycogen use as long as the ingested dose is sufficient to
inhibit hepatic glycogenolysis and glucose output (Gonzalez &
Betts, 2019). However, liver glycogen capacity is much smaller
than muscle (∼100 g vs. ∼400–500 g) and complete liver glycogen
sparing during exercise has only been reported with an extremely
high CHO dose (Jeukendrup et al., 1999). The ingested doses in the
reviewed studies cover a wide range, and only one study estimated
whole-body endogenous glycogen utilization using expired
13
CO
2
tracer methods, which do not account for specific liver and muscle
contributions; conclusions around a dose effect with CHO hydro-
gels are not possible at this stage. However, if an event requires
maximized CHO availability, recommendations to saturate intesti-
nal CHO transporters should remain if athletes tolerate these doses
in terms of GI distress.
As has been the case in previous investigations of CHO
feeding during exercise, methodological differences between stud-
ies do not allow firm interstudy comparisons. Factors such as CHO
Figure 4 —Comparison of relative contributions to energy expenditure from fat (black bars) and carbohydrate (white bars) with maltodextrin and
fructose (MD + F) ingestion in fluid and hydrogel form. Exogenous (dotted bars) and endogenous (hashed bars) contributions shown where data were
available. MD + F = maltodextrin + fructose; CHO = carbohydrate.
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dose, as well as timing, exercise intensity, mode and duration,
prefeeding state, and potentially training status create differences in
outcomes. Although such variability creates difficulty in piecing
together an emerging literature, it is important to recognize that
many of these factors may be fixed or characteristic of specific
sporting events. Further studies, particularly those using multiple
conditions within the same investigation, might help to isolate any
conditions under which hydrogel CHO might provide an advan-
tage. Exercise intensity has important implications for intestinal
absorption due to the diversion of splanchnic blood flow at higher
intensities (ter Steege & Kolkman, 2012). Therefore, potential
mechanistic benefits of CHO hydrogels in terms of GI tolerance
and absorption may only be observed in the higher intensity
domains, presenting an opportunity for future research. To date,
only one study (Pettersson et al., 2019) has investigated what may be
considered higher intensity exercise. Running also utilizes less
localized muscle recruitment compared with cycling, resulting in
increased surface area blood flow and a greater reduction of
splanchnic blood flow (de Oliveira et al., 2014). Similar responses
are likely in cross-country skiing given the whole-body nature of the
sport and may even demand higher CHO utilization (Losnegard
et al., 2014). While body mass is not considered a methodological
factor for exogenous CHO oxidation (Jeukendrup, 2010), the com-
bination of differences in exercise mode and intensity in the
reviewed studies does not allow for firm extrapolations to higher
intensity exercise, which may also benefit further from optimal, that
is, at intestinal saturation, CHO dosing. Due to the increased duration
of training and competition, cyclists may be at an advantage over
runners if habituated to CHO intake. Increased CHO exposure
causes a “gut training”effect (Cox et al., 2010), leading to enhanced
GI tolerance of ingested CHO during exercise. Gut tolerance is
therefore a further methodological factor that differentiates between
exercise modality and may mean potential GI benefits to CHO
hydrogels may be more likely in individuals with lower natural CHO
tolerance or with less habitual CHO gut training. This would serve as
a useful consideration or screening tool in future study design.
Typically, the intake of solutions with high CHO content
during prolonged exercise delays gastric emptying and is associ-
ated with higher incidence of GI distress (Rehrer et al., 1992). A
positive finding from this review is that MD + F hydrogel formula-
tions were generally well tolerated across the range of doses and
exercise protocols that were examined. However, hydrogel solu-
tions did not improve GI tolerance per se above comparable CHO
sources in traditional fluid form. It remains to be seen if they
systematically reduce GI symptoms at higher doses approaching
and above intestinal saturation, due to specific interaction with the
digestive system. Reports of a slight increase in gastric fullness
associated with the hydrogel are of interest (Baur et al., 2019), since
even if there is a subsequent increase in gastric emptying associated
with the formation of the gel, it may create an initial sensation of
fullness. Quantitative measures of GI barrier function and damage
reported by Flood et al. (2020) evidence that CHO hydrogels do not
provide a further protective effect to the intestinal membrane over
typical CHO ingestion. These data do however, confirm the
preventative role of CHO for enterocyte injury and small intestine
permeability during endurance exercise (Snipe et al., 2017).
Although an enhancement of gastric emptying is the mecha-
nism most heavily marketed in support of the use of the hydrogel
sports drinks, the importance of gastric emptying in the whole
process of delivering CHO from the mouth to the muscle mito-
chondria, and any benefits achieved by hydrogels, are difficult to
ascertain. So far, the available evidence is limited to a report by
Sutehall et al. (2020) that a commercially available MD + F
hydrogel increased gastric emptying compared with G + F and
MD + F solutions when consumed as a bolus at rest, with time to
empty half the drink being nearly twice as fast for MD + F hydrogel
(21 min) than its fluid counterpart (37 min). Despite significantly
less volume remaining in the stomach at 30 min, differences
became increasingly smaller thereafter (Sutehall et al., 2020). Since
none of the currently available studies have attempted to directly
measure gastric emptying during exercise, it is not possible to
comment on what occurs under these circumstances. However,
according to the general literature, gastric emptying is not consid-
ered to be the rate-limiting step in determining the availability of
exogenous fuels consumed during exercise (see review by
Rowlands et al. [2015]) and its measurement includes artifacts
and practical difficulties. Furthermore, without using complex
invasive measures (Shi et al., 1995), intestinal absorption cannot
be directly measured; therefore, the endpoint of muscle CHO
oxidation is used to reflect the contribution of a combination of
gut processes between consumption and delivery to the mitochon-
dria. The evidence collected from the currently available studies
does not show clear evidence of differences in the overall process.
Even if hydrogel-encapsulated CHO can be shown to have
different gut characteristics per se, the effects of the amount,
timing, and pattern of intake of CHO sources on gastric emptying
are among the many interacting factors that should be considered
in the current story. This is of interest since in many sporting
events, the pattern of intake is dictated by the availability of fluids
at breaks or feed stations rather than continuous or spontaneous
access. Recent work by Mears et al. (2020a) reported that mean
and peak exogenous CHO oxidation is altered slightly (but
significantly) by CHO timing, with better outcomes associated
with consuming sources every 20 min compared with 5-min
intervals. Meanwhile, Menzies et al. (2020) reported better endur-
ance when CHO intake commenced early during a running
protocol than later delivery. Therefore, future research should
systematically investigate conditions under which a hydrogel
CHO might be used. Although this will involve a large number
of permutations of characteristics, we note in particular, that
scenarios from which current testimonials about the use of
hydrogels have emanated have not been investigated. These
include use by elite athletes in events such as the marathon
requiring high CHO availability from endogenous and exogenous
sources to fuel prolonged exercise (∼2 hr+) of high relative and
absolute intensities (Maurten, 2020;Sutehall et al., 2018). Here,
the combination of issues such as high fuel requirement, high risk
of gut distress, and a practical requirement for small fluid volumes
intersect, making them a priority for study.
In conclusion, a small number of studies have investigated the
use of commercially available CHO hydrogels to deliver CHO
during exercise. So far, data do not support the claimed benefits of
enhanced CHO delivery to the muscle, reduced GI distress, and
better performance compared with the use of traditional CHO
solutions. Future research should focus on dose and timing of
hydrogel ingestion, higher exercise intensities where GI issues are
more prevalent and CHO absorption more greatly impaired, and
further mechanistic insights around endogenous CHO responses.
Athletes may continue to use CHO hydrogels to meet current
guidelines for endurance nutrition practices if this is their prefer-
ence. While no disadvantages around the use of these specialized
sports products appear to be present, based on current evidence,
they do not confer metabolic or performance advantages over
typical CHO ingestion strategies.
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Acknowledgments
All authors contributed to the preparation of this manuscript. The authors
declare no conflicts of interest in the preparation of this review.
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