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220
International Journal of Sport Nutrition and Exercise Metabolism, 2013, 23, 220 -229
© 2013 Human Kinetics, Inc.
Logan-Sprenger and Spriet are with the Dept. of Human Health
and Nutritional Sciences, University of Guelph, Guelph, ON,
Canada. Heigenhauser and Jones are with the Dept of Medicine,
McMaster University, Hamilton, ON, Canada.
Increase in Skeletal-Muscle Glycogenolysis
and Perceived Exertion With Progressive Dehydration
During Cycling in Hydrated Men
Heather M. Logan-Sprenger, George J.F. Heigenhauser,
Graham L. Jones, and Lawrence L. Spriet
This study investigated the effects of progressive mild dehydration during cycling on whole-body substrate
oxidation and skeletal-muscle metabolism in recreationally active men. Subjects (N = 9) cycled for 120 min at
~65% peak oxygen uptake (VO2peak 22.7 °C, 32% relative humidity) with water to replace sweat losses (HYD)
or without uid (DEH). Blood samples were taken at rest and every 20 min, and muscle biopsies were taken at
rest and at 40, 80, and 120 min of exercise. Subjects lost 0.8%, 1.8%, and 2.7% body mass (BM) after 40, 80,
and 120 min of cycling in the DEH trial while sweat loss was not signicantly different between trials. Heart
rate was greater in the DEH trial from 60 to 120 min, and core temperature was greater from 75 to 120 min.
Rating of perceived exertion was higher in the DEH trial from 30 to 120 min. There were no differences in
VO2, respiratory-exchange ratio, total carbohydrate (CHO) oxidation (HYD 312 ± 9 vs. DEH 307 ± 10 g), or
sweat rate between trials. Blood lactate was signicantly greater in the DEH trial from 20 to 120 min with no
difference in plasma free fatty acids or epinephrine. Glycogenolysis was signicantly greater (24%) over the
entire DEH vs. HYD trial (433 ± 44 vs. 349 ± 27 mmol · kg–1 · dm–1). In conclusion, dehydration of <2% BM
elevated physiological parameters and perceived exertion, as well as muscle glycogenolysis, during exercise
without affecting whole-body CHO oxidation.
Keywords: hydration, exercise, uid intake, body-mass loss, substrate oxidation, sweat rate
It has been widely demonstrated that athletes replace
less than 50% of sweat losses during exercise, resulting
in signicant uid decits (Burke 1997). Consequently, a
1–2% body-mass loss is commonplace among athletes in
training and competition (Godek & Godek, 2005; Godek,
Godek, McCrossin, & Bartolozzi, 2009; Logan-Sprenger,
Palmer, & Spriet, 2011; Maughan, Merson, Broad, &
Shirreffs, 2004; Palmer, Logan, & Spriet, 2010; Palmer &
Spriet, 2008). The cardiovascular, thermoregulatory, and
cognitive penalties associated with exercising dehydrated
are well documented and consistent in demonstrating
that the exercise-induced increases in heart rate (HR),
core temperature (Tc), and rating of perceived exertion
(RPE) are exacerbated with dehydration and directly
proportional to the degree of dehydration (Armstrong et
al., 1997; Gonzalez-Alonso, Mora-Rodriguez, Below, &
Coyle, 1995; Montain & Coyle 1992b; Nadel, Fortney,
& Wenger, 1980; Sawka, Young, Francesconi, Muza, &
Pandolf, 1985).
Despite the large number of studies documenting
the cardiovascular and thermoregulatory changes with
dehydration, only one study has investigated the effects
of dehydration in a thermoneutral environment (~20 °C)
on whole-body substrate oxidation and skeletal-muscle
metabolism. Hargreaves, Dillo, Angus, and Febbraio (1996)
investigated the effects of dehydration on muscle metabo-
lism in men and reported that a 2.9% body-mass loss over
2 hr of cycling resulted in 16% more glycogen use than
maintaining body mass with uid ingestion. The respira-
tory-exchange ratio (RER) was signicantly higher in the
uid-restricted trial after 60 and 120 min of exercise, with
the difference between trials being greater in the second
hour of cycling. This was accompanied by signicantly
higher rectal and muscle temperatures at the end of exer-
cise only. No studies have examined how the time course
of progressive dehydration affects muscle glycogenolysis
and metabolism during prolonged exercise.
Therefore, the aims of this study were to investigate
the time course of changes in skeletal-muscle metabolism
with progressive dehydration during 120 min of cycling
at ~65% peak oxygen uptake (VO2peak) in recreationally
active men and to determine what amount of body-mass
loss is necessary to affect changes in whole-body and
muscle metabolic responses to exercise. We hypothesized
that whole-body carbohydrate (CHO) oxidation and
muscle glycogen use would be greater when dehydrated
starting in the 40- to 80-min period and continuing
www.IJSNEM-Journal.com
ORIGINAL RESEARCH
Effects of Dehydration on Muscle Metabolism 221
between 80 and 120 min of cycling and that the onset
of increased CHO oxidation and muscle glycogenolysis
would correlate with a higher Tc in the dehydrated trial.
Methods
Subject Characteristics
Nine men, M ± SE age 21.6 ± 0.5 years, height 178.1 ±
0.9 cm, body mass 77.3 ± 2.2 kg, and VO2peak 4.4 ± 0.2 L/
min, volunteered to participate in the study. All subjects
engaged in light- to moderate-intensity physical activity
3 or 4 days/week. Subjects were informed both verbally
and in writing of the experimental protocol and potential
risks before giving their written consent to participate.
The research ethics boards of the University of Guelph
and McMaster University approved the study.
Preexperimental Protocol
In preparation for the experiment, subjects visited the
laboratory on three separate occasions. On the first
visit, they performed an incremental cycling test to
exhaustion on an electronically braked cycle ergometer
(LODE Excalibur, Quinton Instrument, Groningen, The
Netherlands) for the determination of VO2peak. Respira-
tory gases were collected and analyzed using a metabolic
cart (MOXUS metabolic system, AEI Technologies,
Pittsburgh, PA). After a 30-min break, subjects cycled for
~20 min at ~65% VO2peak to establish the power output
for the subsequent 120-min trials.
On two subsequent occasions, subjects reported
to the laboratory for practice trials and cycled at ~65%
VO2peak for 120 min without uid (DEH) or with uid
(HYD) to replace sweat losses. DEH trials occurred
rst to ascertain sweat losses over the 120-min trial and
to determine how much uid subjects needed to drink
during the HYD trial to maintain uid balance. All
subjects abstained from strenuous exercise and caffeine
and recorded their diet in the 24 hr before the trials. Two
hours before the practice rides, subjects ingested a meal
provided for them (787 kcal; 144 g carbohydrate, 15 g
fat, 19 g protein) and 250 ml of uid. Subjects also drank
300 ml of water 90 and 45 min before each trial to ensure
that they were hydrated before cycling. On arrival at the
laboratory, subjects provided a small midstream urine
sample to determine urine specic gravity (USG) and
completely voided their bladder. A pretrial body-mass
measurement was made wearing dry shorts only. After 40,
80, and 120 min of exercise, subjects stopped cycling and
dismounted the cycle ergometer, removed their shoes and
shirt, toweled dry, and were weighed wearing only shorts
for the determination of sweat loss during the previous 40
min of exercise. At 40 and 80 min, subjects put on a dry
T-shirt and recommenced cycling. Any urine produced
was collected during each trial to account for total sweat
loss using the equation Sweat loss = [pretrial body mass –
posttrial body mass (kg)] + uid intake (ml) – urine loss
(ml). Three-minute respiratory-gas measurements were
collected every 20 min during exercise to determine the
volume of oxygen consumed (VO2) and the volume of
carbon dioxide produced (VCO2) and to calculate the
RER and whole-body CHO and fat oxidation with use of
the nonprotein RER table and the following equations:
CHO oxidation (g) = 4.585 (VCO2) – 3.226 (VO2)
(Ferrannini, 1988)
Fat oxidation (g) = 1.695 (VO2) – 1.701 (VCO2)
(Peronnet & Massicotte, 1991)
Practice trials were separated by 5–7 days.
Experimental Protocol
Subjects came to the laboratory on two occasions for
the actual experiments. During the experimental trials
subjects cycled at ~65% VO2peak for 120 min with uid
to match sweat losses (HYD) or without uid (DEH).
Subjects replicated the same procedure for the practice
trials. In addition, HR was collected using a Polar RS400
downloadable HR monitor (Polar Electro, Lachine, QC,
Canada), and Tc was determined using an individually
calibrated ingestible thermistor (HQ Inc., Palmetto, FL)
that was ingested 3–5 hr before each trial. Before exercise,
a Teon catheter was inserted into an antecubital vein for
blood sampling and was ushed with 0.9% saline to main-
tain patency. One leg was also prepared for percutaneous
needle-biopsy sampling of the vastus lateralis muscle by
the Bergström technique (Bergström, 1962). Three inci-
sions were made in the skin and deep fascia under local
anesthesia (2% xylocaine without epinephrine) for three
separate biopsies. Immediately before exercise, a venous
blood sample (~5 ml) and a muscle biopsy were obtained
while the subject rested on a bed. All muscle samples
were immediately frozen in the needle in liquid nitrogen
and stored in liquid nitrogen for subsequent analyses.
Subjects then cycled for 120 min at ~65% VO2peak at a
constant cadence (80–95 rpm). Venous blood samples
were obtained at 20, 40, 60, 80, 100, and 120 min of
exercise. HR, Tc, and RPE were recorded every 15 min
during exercise. RPE was determined using the Borg
scale (rating 6–20; Borg, 1970). During the HYD trial
subjects were given uid every 15 min to match sweat
loss and drank the uid after HR, Tc, and RPE measure-
ments were recorded. At 40, 80, and 120 min subjects
stopped cycling and a muscle biopsy was taken with the
subjects sitting on the cycle ergometer. After the muscle
biopsy was taken, subjects removed their shoes and shirt,
toweled dry, and were weighed for the determination of
body-mass loss over the previous 40 min of exercise. The
same procedure was replicated for the second trial, with
muscle biopsies taken from the opposite leg, and the trials
were randomized and separated by 7 days.
Analyses
Trial Conditions. Laboratory temperature (°C) and
relative humidity (%) were measured using a digital
thermometer (Fisher Scientic, Ottawa, ON). USG was
222 Logan-Sprenger et al.
measured via handheld pocket refractometer (Model
PAL10S, Atago USA Inc., Bellevue, WA) to assess
hydration status from the preexercise urine sample. The
refractometer was calibrated with distilled water before
each measurement. Stover, Zachwieja, Stofan, Murray,
and Horswill (2006) reported that USG measured with
refractometry correlated strongly with urine osmolality
(r = .995), and a USG of 1.020 correlated with an urine
osmolality of ~800 mOsm/kg. In light of this and the
published position stand from the American College of
Sports Medicine, a USG below 1.020 was considered to
indicate a hydrated state (Sawka et al., 2007).
Blood Measurements. Venous blood was collected
in sodium heparin tubes. A portion of whole blood
(200 μl) was added to 1 ml of 0.6-M perchloric acid
and centrifuged. The supernatant was stored at –20 °C
and later analyzed for blood glucose and lactate with
uorometric techniques (Bergmeyer, 1974). A second
portion (1.5 ml) was centrifuged and the supernatant
was analyzed for plasma free fatty acids with an
enzymatic colorimetric technique (NEFA C test kit,
Wako Chemicals, Richmond, VA). A third portion
(1.5 ml) was added to 30 ml of EGTA and reduced
glutathione and centrifuged (10,000 g) for 3 min, and
the supernatant was analyzed for epinephrine with an
enzymatic immunoassay kit (Epinephrine RIA kit, Rocky
Mountain Diagnostics Inc., Colorado Springs, CO). The
remaining venous blood was used for the determination
of whole-blood hemoglobin and hematocrit. Hemoglobin
was measured in duplicate using an automated blood-
analysis machine (OSM3 Hemoximeter, Radiometer,
Copenhagen, Denmark). Hematocrit was measured in
triplicate using capillary tubes and a microhematocrit
centrifuge and reader (microcapillary reader, Damon/IEC
Division, USA). The percent plasma volume change was
calculated using whole-blood hemoglobin and hematocrit
measurements (Dill & Costill, 1974).
Muscle Metabolites. Each muscle biopsy was
freeze-dried, powdered, and dissected free of visible
connective tissue, fat, and blood. One aliquot of freeze-
dried powdered muscle (~10 mg) was extracted in
0.5-M HCLO4–1-mM EDTA and neutralized with
2.2-M KHCO3. The supernatant was used to measure
phosphocreatine, creatine, adenosine triphosphate
(ATP), and lactate (Harris, Hultman, & Nordesjo, 1974).
Muscle metabolites were normalized to the highest total
creatine content measured from all biopsies from each
subject. Muscle glycogen content was determined in
duplicate using two additional aliquots of freeze-dried
muscle (2–4 mg). Glycogen was extracted in 0.1-M
NaOH and neutralized with 0.1-M HCL–0.2-M citric
acid–0.2-M Na2PO4, and amyloglucosidase was added
to degrade glycogen to glucose, which was measured
spectrophotometrically and normalized for total creatine
(Bergmeyer, 1974).
Muscle Calculations. Free ADP (ADPf) and AMP
(AMPf) contents were calculated by assuming equilibrium
of the creatine kinase and adenylate kinase reactions
(Dudley, Tullson, & Terjung, 1987). Specically, ADPf
was calculated using the measured ATP, creatine, and
phosphocreatine values, an estimated H+ concentration,
and the creatine kinase constant of 1.66 × 109 (Sahlin,
Harris, Nylind, & Hultman, 1976; Saltin, 1990). AMPf
was calculated from the estimated ADPf and measured
ATP content using the adenylate kinase equilibrium
constant of 1.05.
Statistical Analysis
All data were tested for normality of distribution and
presented as M ± SE. Time-versus-trial data were assessed
using a two-way ANOVA, and specic differences were
located using the Student-Newman-Keuls post hoc test.
A paired t test was used to compare single-parameter
data between trials. Statistical signicance was accepted
as p < .05.
Results
Trial Conditions
No signicant pretrial differences existed between the
HYD and DEH trials for laboratory temperature (HYD
22.6 ± 0.1 vs. DEH 22.8 ± 0.2 °C, p = .29), relative humid-
ity (32% ± 2.8% vs. 33% ± 2.7%, p = .49), pretrial body
mass (77.1 ± 2.2 vs. 77.5 ± 2.2 kg, p = .10), or hydration
state (USG 1.013 ± 0.003 vs. 1.015 ± 0.003, p = .60).
Body-Mass Loss, Sweat Loss,
and Fluid Intake
Body mass was maintained in the HYD trial by consum-
ing a mean of 2.3 ± 0.2 L of uid over the 120 min of
cycling. In the DEH trial, body mass was signicantly
lower at 40 (0.8 ± 0.1%, p = .04), 80 (1.8 ± 0.2%, p =
.003), and 120 min (2.7 ± 0.2%, p < .001, Table 1). There
was no signicant difference in sweat loss between the
HYD and DEH trials (p = .15). Only 1 subject mictur-
ated after both the HYD trial (350 ml) and the DEH trial
(400 ml).
VO2 and Whole-Body Substrate Use
Subjects started the exercise trial at 63%% ± 0.8%
VO2peak, which then increased to 65% ± 0.7% VO2peak.
There was no signicant difference between trials (Figure
1[a], p = .09). The RER progressively decreased in both
trials over time and was signicantly lower than 20 min at
80, 100, and 120 min in both trials (p < .05), with no sig-
nicant differences between trials (p = .47, Figure 1[b]).
There was no signicant difference in total CHO
(HYD 312 ± 9 vs. DEH 307 ± 10 g, p = .38) or fat oxida-
tion (53 ± 8 vs. 55 ± 17 g, p = .33) between trials. CHO
oxidation signicantly decreased over time from zero to
40 (HYD 116 ± 7 vs. DEH 115 ± 6 g, p = .03), 40 to 80
min (105 ± 5 vs. 107 ± 6 g, p = .05), and 80 to 120 min
(91 ± 4 vs. 85 ± 5 g, p = .03), with no signicant differ-
ences between trials (p = .19). Fat oxidation increased
223
Table 1 Whole-Blood and Plasma Parameters During 120 min of Cycling at ~65% Peak Oxygen Uptake in
the Hydrated (HYD) and Dehydrated (DEH) Trials, M ± SE (N = 9)
Time (min) Trial 0 min 20 min 40 min 60 min 80 min 100 min 120 min
Pvol loss (%) HYD —– 6.0 ± 1.3† 4.9 ± 1.2† 4.5 ± 1.3† 5.1 ± 1.4† 4.9 ± 1.5† 4.1 ± 1.2†
DEH —– 7.1 ± 1.2† 8.3 ± 1.2†* 8.9 ± 1.5†* 8.7 ± 1.5†* 8.5 ± 1.4†* 9.4 ± 1.5†*
Glucose (mM) HYD 4.0 ± 0.2 4.1 ± 0.1 3.9 ± 0.1 3.7 ± 0.1 3.7 ± 0.2 3.7 ± 0.2 3.6 ± 0.2
DEH 4.2 ± 0.2 3.9 ± 0.1 3.9 ± 0.2 3.7 ± 0.2 3.8 ± 0.2 3.8 ± 0.2 3.7 ± 0.3
Lactate (mM) HYD 0.5 ± 0.1 2.0 ± 0.3† 1.5 ± 0.3† 1.4 ± 0.3† 1.4 ± 0.2† 1.2 ± 0.2† 1.5 ± 0.3†
DEH 0.7 ± 0.3 2.6 ± 0.6†* 2.3 ± 0.5†* 1.9 ± 0.3†* 1.9 ± 0.3†* 2.0 ± 0.5†* 2.1 ± 0.5†*
Plasma FFA (mM) HYD 0.15 ± 0.02 —– —– 0.21 ± 0.03† 0.38 ± 0.1† —– 0.89 ± 0.1†
DEH 0.14 ± 0.02 —– —– 0.19 ± 0.04† 0.44 ± 0.1† —– 0.84 ± 0.1†
Plasma EPI (nM) HYD 0.41± 0.03 —– —– 0.93 ± 0.1† 1.13 ± 0.1† —– 1.65 ± 0.2†
DEH 0.41 ± 0.04 —– —– 1.06 ± 0.1† 1.11 ± 0.1† —– 1.66 ± 0.2†
Note. Pvol = plasma volume; FFA = free fatty acids; EPI = epinephrine.
†Signicantly different from 0 min (p < .05). *Signicantly greater than HYD (p < .05).
Figure 1 — (a) Oxygen uptake (VO2) and (b) respiratory-exchange ratio (RER) during 120 min of cycling at ~65% VO2peak
in the hydrated (HYD) and dehydrated (DEH) trials. Data are M ± SE (N = 9). Arrows indicate approximately 1%, 2%, and 3%
body-mass loss.
(a)
(b)
224 Logan-Sprenger et al.
over time from zero to 40 (HYD 12 ± 2 vs. DEH 13 ± 2
g, p = .05), 40 to 80 (18 ± 2 vs. 18 ± 2 g, p = .02), and
80 to 120 min (23 ± 2 vs. 24 ± 3 g, p = .005), with no
signicant trial differences (p = .11).
HR, Tc, and RPE
HR increased signicantly over time in both trials and
was signicantly higher in the DEH than in the HYD trial
from 60 to 120 min of cycling (p = .002, Figure 2[a]).
Tc increased signicantly over time in both trials (p <
.002) and was signicantly higher in the DEH than in the
HYD trial from 75 to 120 min (p = .003, Figure 2[b]).
RPE signicantly increased over time in both trials (p =
.01) and was signicantly higher in the DEH trial from 30
to 120 min (p = .001, Figure 2[c]). The mean RPE over
the entire trial was also signicantly greater in the DEH
than in the HYD trial (14.4 ± 0.6 vs. 12.9 ± 0.3, p = .01).
Blood Measurements
Hemoglobin and hematocrit were signicantly higher
than rest from 20 to 120 min of exercise in both trials. In
the DEH trial, hemoglobin was signicantly greater from
40 to 120 min (p < .001) and hematocrit was signicantly
greater from 60 to 120 min (p = .013). Plasma volume
loss was signicantly greater in the DEH trial from 40 to
120 min of exercise (p < .001, Table 1). Blood glucose
was unaffected by time (p = .07) or trial (p = .41), and
blood lactate was signicantly increased above rest at all
time points in both trials (p < .001) and was greater in
the DEH trial at all exercise time points (p < .001, Table
1). Plasma free fatty acids and epinephrine signicantly
increased from rest in both trials (p < .001), with no
signicant differences between trials (free fatty acids
p = .84, epinephrine p = .44; Table 1).
Muscle Fuels and Metabolites
Skeletal-muscle phosphocreatine content signicantly
decreased in the rst 40 min of exercise (p = .03) and
remained lower than rest at 80 and 120 min of exercise in
both trials, with no signicant differences between trials
(p = .99, Table 2). Skeletal-muscle creatine changes were
reciprocal to the phosphocreatine changes, and muscle
ATP content was unaffected by exercise (p = .21) or
hydration state (p = .10). ADPf and AMPf were signi-
cantly higher than rest at all time points during exercise
in both trials (p = .01), with no differences between trials
(p = .41, Table 2). Muscle lactate content increased with
exercise (p = .02), peaked at 40 min in both trials, and
was signicantly greater in the DEH trial at 40 (p = .02),
80 (p = .01), and 120 min (p = .009) of exercise (Table 2).
Muscle glycogen content was similar in the two trials
before exercise (p = .11) and signicantly lower at 40,
80, and 120 min in both trials compared with rest (p <
.01). Total glycogen use (0–120 min) was signicantly
greater (24%) in the DEH trial (433 ± 44 vs. 349 ± 27
mmol · kg–1 · dm–1, p = .02, Figure 3). However there was
no signicant difference in glycogen use from 0 to 40
(19%, HYD, 209 ± 30 vs. DEH, 249 ± 43 mmol · kg–1 ·
dm–1, p = .13), 40 to 80 (19%, 77 ± 14 vs. 92 ± 19 mmol
· kg–1 · dm–1, p = .47), and from 80 to 120 min (46%, 63
± 12 vs. 92 ± 24 mmol · kg–1 · dm–1, p = .09), although
there were strong trends in each time period.
Discussion
This study investigated the effects of mild progressive
dehydration during exercise at ~65% VO2peak on whole-
body substrate oxidation and skeletal-muscle metabolism,
as well as cardiovascular, thermal, and perceived exer-
tion responses in active, hydrated men. In the control
trial (HYD) of this study, we prevented dehydration by
having subjects drink enough uid to precisely replace
their sweat losses over the 120-min cycling trial. HR
increased from 150 ± 4 beats/min at 15 min to 160 ±
5 and 165 ± 4 beats/min at 60 and 120 min, while Tc
increased from 37.2 ± 0.1 °C at rest to 37.8 ± 0.1 °C at
15 min and reached a plateau of 38.1 ± 0.1 and 38.2 ±
0.1 °C at 60 and 120 min, respectively. In the DEH trial,
the subjects lost approximately 1%, 2%, and 3% body
mass at 40, 80, and 120 min through sweating, adding
to the physiological demands of exercising for 120 min
at ~65% VO2peak. All physiological responses to exercise
were exacerbated in the DEH trial, as HR and Tc were
higher at 60 and 120 min by 6–7 beats/min and by 0.2
°C and 0.5 °C, respectively, in the DEH-versus-HYD
trial. Even in the rst 40 min of exercise in DEH (~1%
body-mass loss), RPE, plasma volume loss, and blood
[La] were all higher and there was a signicantly greater
muscle lactate content and a trend for increased muscle
glycogen use (p = .17). From 40 to 80 and 80 to 120 min,
body-mass loss progressed from 1% to 2% and from 2%
to 3%, and all physiological parameters remained higher
in the DEH trial. The 3% body-mass loss over 120 min of
exercise increased overall muscle glycogen use by 24%
but had no effect on whole-body carbohydrate oxidation
in the DEH trial.
The Effects of Progressive Dehydration
on Muscle Metabolism
Previously, Hargreaves et al. (1996) reported that net
muscle glycogen use was 16% greater over a 120-min
trial at 67% VO2peak when trained men were dehydrated by
2.9% body mass. The RER was signicantly higher in the
uid-restricted trial after 60 and 120 min of exercise, with
the difference between trials being greater in the second
hour of cycling. The current study examined time-course
changes to muscle metabolism at 40, 80, and 120 min of
cycling at ~65% VO2peak with progressive dehydration
to ascertain how much dehydration was necessary to see
a shift in substrate oxidation and muscle glycogen use.
There were no differences in whole-body substrate oxida-
tion when subjects were dehydrated by 1%, 2%, or 3%
body mass. In addition, there were trends for accelerated
muscle glycogen use in each 40-min exercise segment,
resulting in a signicant 24% increase over the entire
225
Figure 2 — (a) Heart rate during 120 min of cycling at ~65% peak oxygen uptake (VO2peak) in the hydrated (HYD) and dehydrated
(DEH) trials. Heart rate was signicantly greater than 15 min at all time points in both trials (p < .05). bpm = beats per minute.
(b) Core temperature during 120 min of cycling at ~65% VO2peak in the HYD and DEH trials. Core temperature was signicantly
greater than 15 min for all time points in both trials (p < .05). (c) Rating of perceived exertion (RPE) during 120 min of cycling
at ~65% VO2peak in the HYD and DEH trials. RPE was signicantly greater than 15 min from 45–120 min in both trials (p < .05).
Data are M ± SE (N = 9). *Signicantly higher than HYD trial (p < .05). Arrows indicate approximately 1%, 2%, and 3% body-
mass loss.
(a)
(b)
(c)
226 Logan-Sprenger et al.
DEH trial compared with HYD. An interesting nding
in this study was the greater total muscle glycogen use
in the DEH trial with no difference in whole-body carbo-
hydrate oxidation. This suggests that the extra glycogen
metabolized to pyruvate was not oxidized but converted
to lactate. This speculation is supported by the augmented
blood and muscle lactate contents throughout the DEH
trial. There are at least three potential mechanisms that
may account for the increased muscle glycogenolysis
and glycogen phosphorylase (PHOS) activity in the DEH
trial without also affecting the activity of pyruvate dehy-
drogenase. They include an increased sympathoadrenal
response leading to elevated circulating epinephrine and
activation of PHOS, a decreased energy status in the
cell manifested by elevated ATP:ADP ratio and AMPf
levels (as they act as allosteric activators of PHOS), and
increased muscle temperature (Tm; Febbraio, 2000).
Hargreaves et al. (1996) reported no difference in
plasma epinephrine at 60 or 120 min of cycling with
2.9% body-mass loss at 120 min, and in the current study
we also found no signicant difference in epinephrine
response with uid restriction up to 3% body-mass loss
in men. Estimates of free ADP and AMP in the DEH
and HYD trials of the current study did not reveal any
apparent differences, downplaying the rst suggestion
that these allosteric regulators can explain the increased
PHOS activity and glycogenolysis in the DEH trial.
Second, studies examining the importance of local
hyperthermia to muscle glycogenolysis increased the Tm
of one leg during exercise and reported increased muscle
glycogenolysis and [La] in the hot leg only, independent
of changes to Tc or circulating epinephrine. This demon-
strated that increasing local Tm increased glycogenolysis
at a given Tc, when both legs were exposed to the same
Table 2 Skeletal-Muscle Fuel and Metabolite Contents (mmol · kg–1 · dm–1) During 120 min of Cycling at
~65% Peak Oxygen Uptake in the Hydrated (HYD) and Dehydrated (DEH) Trials, M ± SE (N = 9)
HYD DEH
0 min 40 min 80 min 120 min 0 min 40 min 80 min 120 min
PCr 80.3 ± 4.9 63.8 ± 4.7† 59.4 ± 4.6† 58.0 ± 4.7† 84.6 ± 3.3 57.1 ± 4.3† 58.7 ± 5.0† 61.2 ± 6.2†
Creatine 74.7 ± 3.2 90.8 ± 7.0† 95.2 ± 6.7† 93.0 ± 5.3† 70.5 ± 2.8 98.1 ± 7.1† 96.8 ± 6.2† 94.1 ± 6.2†
ATP 25.6 ± 1.2 26.4 ± 1.1 25.2 ± 0.9 24.8 ± 0.8 26.9 ± 0.8 26.5 ± 0.6 26.1 ± 0.9 26.1 ± 0.8
ADPf 144 ± 19 246 ± 48† 231 ± 24† 205 ± 31† 128 ± 10 238 ± 32† 247 ± 52† 182 ± 28†
AMPf 1.0 ± 0.3 2.8 ± 0.9† 2.0 ± 0.4† 2.9 ± 0.9† 0.6 ± 0.3 1.8 ± 0.5† 2.9 ± 1.1† 1.5 ± 0.4†
Lactate 1.8 ± 0.4 9.3 ± 2.6† 8.1 ± 1.7† 5.2 ± 1.5† 2.1 ± 0.5 13.1± 3.3*† 12.4± 2.2*† 9.9± 2.4*†
Glycogen 522 ± 58 313 ± 63† 228 ± 54† 165 ± 43† 572 ± 61 323 ± 67† 240 ± 53† 147 ± 41†
Note. PCr = phosphocreatine; ATP = adenosine triphosphate; ADPf = free adenosine diphosphate; AMPf = free adenosine monophosphate.
†Signicantly different from 0 min (p < .05). *Signicantly greater than HYD (p < .05).
Figure 3 — Muscle glycogen use during 120 min of cycling at ~65% peak oxygen uptake in the hydrated (HYD) and dehydrated
(DEH) trials. Data are M ± SE (N = 9). *Signicantly greater than HYD (p < .05).
Effects of Dehydration on Muscle Metabolism 227
epinephrine (Febbraio, Carey, Snow, Stathis, & Harg-
reaves, 1996; Starkie, Hargreaves, Lambert, Proietto,
& Febbraio, 1999). In a review article, Febbraio (2000)
stated that increases in Tc of >0.5 °C increased intra-
muscular CHO utilization consistently during moderate-
intensity exercise in the heat. In the current study, Tc was
0.3–0.5 °C higher in the nal 30–45 min of exercise in the
DEH trial (~20 °C). While Tm was not measured in this
study, the work of Hargreaves et al. (1996) and Febbraio
(2000) predicts that Tm would have been higher during
exercise in the DEH trial of the current study (Starkie et
al., 1999). Similar results without Tm measures have been
reported in a DEH versus HYD trial when exercising in
the heat (35 °C), as Gonzalez-Alonso, Calbet, and Nielsen
(1999) had male subjects cycle until volitional exhaustion
(135 ± 4 min) while progressively dehydrating to ~3.9%
body-mass loss. They reported a 45% increase in muscle
glycogen use across the contracting leg compared with
the euhydrated trial.
Therefore, the last mechanism proposed—an
increased Tm and the Q10 effect—appears to be the
most plausible explanation for the increased muscle
glycogenolysis reported during progressive dehydration
in men in the current study. It is currently unknown why
dehydration preferentially increases CHO metabolism
and not fat metabolism, but it may be related to the abil-
ity to quickly mobilize muscle CHO versus the relatively
slower mobilization of fat fuels not coming from intra-
muscular triglycerides.
The current data also suggest that activity of PHOS
may be more sensitive to increased Tm (increased pyru-
vate production) as compared with the activity of pyruvate
dehydrogenase, resulting in no increase in pyruvate oxi-
dation and more lactate formation. As this was the case,
the increased glycogen use with dehydration appeared
to be wasted as the excess pyruvate produced was con-
verted to lactate and not oxidized. Others have reported
that dehydration in the heat increased muscle glycogen
use (45% greater), muscle lactate accumulation, and net
lactate release across the contracting leg compared with
the euhydrated trial (Gonzalez-Alonso et al., 1999), but
there was also an increase in CHO oxidation, unlike the
current study. At the present time, there does not appear to
be an explanation for this nding except that the subjects
in Hargreaves et al.’s (1996) and Gonzalez-Alonso et al.’s
(1999) studies were trained cyclists, and the subjects in
the current study were only recreationally trained.
Effects of Dehydration on Cardiovascular
and Thermal Responses
It is well established that uid ingestion attenuates the
increases in HR and Tc and the decreases in stroke
volume and cardiac output that occur during prolonged
exercise without uid ingestion (Armstrong et al., 1997;
Cheuvront, Keneck, Montain, & Sawka, 2010; Feb-
braio et al., 1996; Hamilton, Gonzalez-Alonso, Montain,
& Coyle,1991; Morimoto, 1990; Nadel et al., 1980;
Sawka et al., 1985). An early study demonstrated that
when heat-acclimatized male subjects were dehydrated
to 3%, 5%, and 7% body-mass loss by an exercise-heat
regimen and then walked in a hot environment (49 °C)
at a low intensity for 140 min, HR and Tc increased
linearly with the severity of dehydration (Sawka et al.,
1985). In a similar way, our results demonstrated that as
dehydration increased from zero to 1%, 1% to 2%, and
2% to 3% body mass during exercise in the DEH trial,
HR and Tc became progressively higher than the eleva-
tions in the HYD trial.
Hypovolemia and the displacement of blood to the
skin for evaporative cooling make it difcult to maintain
central venous pressure during exercise when uid is
restricted (Sawka, Montain, & Latzka, 2001). Central
venous pressure is regulated by the continuous adjustment
of blood volume to the changing size of the vascular bed
to maintain cardiac output, and heat stress or exercise-
induced dehydration provides a threat to this control
as inadequate uid intake during periods of sweat loss
reduces plasma volume (Morimoto, 1990). In light of the
signicantly greater loss in plasma volume found in the
DEH versus the HYD trial after ~40 min of cycling, a
reduction in central venous pressure and stroke volume
may account for the signicantly elevated HR to maintain
cardiac output when stroke volume was compromised. An
accompanying baroreex that would decrease cutaneous
blood ow leading to heat storage may account for the
augmented Tc found in the DEH trial. In support of this,
Nadel et al. (1980) reported that diuretic-induced dehy-
dration of 2.7% body mass led to restrictions in core-to-
skin heat transfer, which forced esophageal temperature
to nearly 39 °C during 30 min of cycling at 55% VO2peak
in the heat. Further support was provided by Montain and
Coyle (1992a), who investigated whether uid ingestion
attenuated the hyperthermia and cardiovascular drift that
occurred during exercise dehydration due to increases in
blood volume. Subjects exercised at ~65% VO2peak for
2 hr in three conditions: no uid replacement, infusion
with a blood-volume expander, or given uid to replace
~80% of sweat loss. They reported that uid replace-
ment and the blood-volume expander maintained blood
volume compared with the no-uid trial, but only uid
replacement resulted in lower Tc. The authors argued that
the decreased hyperthermia during exercise in the uid-
replacement trial was due to the measured increase in skin
blood ow. In the current study, subjects had higher Tc
values in the last 45 min of exercise in the DEH (38.7 °C)
versus HYD (38.1 °C) trials, while the sweat rates were
the same, suggesting that the lack of heat transfer to the
periphery accounted for the elevated Tc in the DEH trial
with as little as ~1–2% body-mass loss.
Effects of Dehydration on RPE
In this study, RPE became signicantly higher in the
DEH trial after only 30 min of cycling when subjects had
lost <1% body mass. Similar results have been reported
in other studies investigating the effects of progressive
dehydration on RPE (Ishijima et al., 2009; McGregor,
228 Logan-Sprenger et al.
Nicholas, Lakomy, & Williams, 1999). It is speculated
that hypovolumia associated with exercise dehydration
leading to a reduction in brain blood ow may exacerbate
the sense of effort associated with exercising without
uid leading to greater perceived exertion (Maughan,
Shirreffs, & Watson, 2007). More simply, it may be that
the temperature and cardiovascular centers that sense
elevations in Tc, HR, and reduced plasma volume feed
back to the brain and increase the RPE during exercise at
the same relative intensity in a mildly dehydrated state.
Shirreffs, Merson, Fraser, and Archer (2004) reported
that as subjects became progressively more dehydrated to
2.7% body-mass loss they reported feelings of headache,
reductions in their ability to concentrate, and reduced
alertness, which may all contribute to an elevated RPE
during exercise. Finally, it is not possible to blind sub-
jects, so the mere knowledge that they are not drinking
during the exercise trial may affect their perceived-
exertion measures.
Summary and Conclusions
This study investigated the time course of changes in
whole-body substrate oxidation and skeletal-muscle
metabolism, as well as cardiovascular, thermal, and
perceived-exertion responses in recreationally active,
hydrated men with progressive mild dehydration during
exercise at ~65% VO2peak. All changes in physiological
parameters accompanying exercise in a hydrated state
were exacerbated with mild dehydration of ~1–2%
body-mass loss. Muscle glycogenolysis was signicantly
increased in the DEH versus HYD condition over the
entire trial (0–120 min) with no difference in whole-body
CHO oxidation between trials. We speculate that the
increased glycogenolysis was due to increases in Tm
and the Q10 effect, as there appear to be no differences
in plasma epinephrine or the energy status of the cell
(free ADP or AMP) between the HYD and DEH trials.
There does not appear to be an obvious explanation for
the lack of increased whole-body CHO oxidation in
the face of the dehydration-induced increase in muscle
glycogenolysis.
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