Carbohydrate
supplementation fails to improve the sprint performance of female cyclists
ANTHONY T. JARVIS, SCOTT D. FELIX, STACY
SIMS, MARGARET T. JONES, MARY ANNE COUGHLIN, and SAMUEL A. HEADLEY
Department of Exercise
& Sports Studies, Springfield College, 263 Alden Street, Springfield,
MA 01109
ANTHONY T. JARVIS, SCOTT
D. FELIX, STACY SIMS, MARGARET T. JONES, MARY ANNE COUGHLIN, and SAMUEL
A. HEADLEY. Carbohydrate supplementation fails
to improve the sprint performance of female cyclists.
JEPonline
Vol
2 No 2 1999. This study was designed to
examine the effect of a 7% carbohydrate electrolyte beverage (CE) on the
sprint performance of trained female cyclists immediately following 50
min of cycle ergometry at 80% VO2
max.
The Wingate Anaerobic Power Test (WAT) was used to measure peak power,
mean power, minimum power, and rate of fatigue. No significant (p > .05)
differences were found between the CE and placebo (PL) treatments for any
WAT indices. Blood glucose was found to be significantly higher (p <
.05) toward the end of the time trial for the CE group than for the PL
group. RPE increased more dramatically from baseline for the PL group than
for the CE group (p < .05). The results suggest that high intensity
exercise performance of female cyclists is not improved with the consumption
of a CE beverage during exercise despite a reduced perception of effort.
Key words:WINGATE,
ANAEROBIC, CARBOHYDRATE-ELECTROLYTE, FEMALES
Introduction
At relatively high workloads, the depletion
of intramuscular glycogen limits prolonged exercise capacity (1).
The elevation of muscle and liver glycogen stores to approximately twice
normal levels has been shown to improve endurance performance (2).
When liver and muscle glycogen levels have been elevated, the ingestion
of carbohydrate does little to improve performance (3).
Also, when liver glycogen levels are low or depleted, as in the case of
an overnight fast (4), performance may be negatively
impacted. The resultant decline in plasma glucose contributes to fatigue
during prolonged exercise by limiting carbohydrate oxidation (5).
In the fasted condition, where glycogen
levels are very low or depleted, the use of an exogenous carbohydrate source
becomes increasingly important. The administration of an exogenous carbohydrate
source allows for the oxidation of carbohydrate from sources other than
muscle glycogen during the later stages of prolonged strenuous exercise
(6).
While most of the available evidence indicates
exogenous carbohydrate ingestion to be beneficial during moderate intensity
aerobic exercise, recent evidence has suggested the use of an exogenous
carbohydrate source to also be of benefit during high intensity exercise
performance (7-10). By mediating favorable alterations
in blood glucose concentration, muscle glycogen depletion during exercise
of shorter (i.e., < 60 min) duration and higher (i.e., > 80% VO2
max)
intensity is reduced. As a result of this glycogen sparing, sprint performance
at the end of an exercise activity is enhanced (10).
Hargreaves et al. (10)
exercised 10 male subjects ( mean VO2 max,
4.43 ± 0.13 L/min) for a total of 4 hr on a cycle ergometer with
intermittent stages of moderate and high intensities. Sprint performance
at the end of each trial was 45% longer when the subjects were supplemented
with carbohydrate.
Below et al. (8) studied
8 endurance-trained (mean VO2 max, 4.44
± 0.08 L/min) males. These subjects cycled for 50 min at 80% VO2
max.
During the trials in which they received a carbohydrate supplement, they
received 79 ± 4 g of a carbohydrate-electrolyte beverage. A cycling
performance test followed in which the subjects were required to complete
predetermined amounts of work in the shortest amount of time possible.
Subjects were asked to perform a cycling task for 10 minutes at a workrate
10% above their individual lactate threshold. Fluid and carbohydrate ingestion
were both found to improve cycling performance, with the effects being
additive. These researchers found carbohydrate ingestion to improve sprint
performance following high intensity exercise.
Ball et al.(7) also examined
the effects of carbohydrate feeding on the cycle ergometer sprint performance
of 8 trained male cyclists. Intensity of exercise prior to measured sprint
performance was also set at 80% VO2 max
for 50 min of cycle ergometry. Sprint performance was improved with the
intermittent ingestion of a carbohydrate supplement providing approximately
53 g CHO/hr.
Most of the previous carbohydrate research
has been focused on male athletes. Since there is little research investigating
the impact of carbohydrate supplementation upon high intensity exercise
performance in females and since our laboratory previously demonstrated
that male cyclists enhance their sprint performance with CHO supplementation
(7), the current study was proposed. It was hypothesized
that the sprint capacity of female cyclists would be enhanced following
exogenous carbohydrate consumption during 50 min of high intensity cycling.
METHODS
Subjects
Ten trained eumenorrheic female cyclists
with a VO2
max of at least 40 ml/kg/min
gave their informed consent to participate in this study. All female cyclists
were recruited from the New England and New York areas.
Testing Apparatus
Subject body height (cm) and weight (kg)
were determined using a Detecto scale. Subject body composition was determined
using Lange skinfold calipers. Subject heart rate was measured with a Polar
Vantage XL Heart Rate Monitor (Stamford, CT Model #45900).
A mechanically braked Monark cycle ergometer
(Model #864) was used to perform both the VO2max
test and the cycling protocols. The Monark cycle ergometer was retrofitted
with a Turbo Saddle by Vetta (Vicenza, Italy). Platform pedals were replaced
with strapped toe clips and cycle ergometer handle bars were replaced with
Extreme Handlebars by Scott (Sun Valley, ID). To measure pedal revolutions
an optical sensor was interfaced with an IBM compatible computer using
SportsMedicine IndustriesTM (SMI, St. Cloud, MN) software.
A SensorMedics Energy Expenditure Unit
(2900 System, Yorba Linda, CA) was used to measure expired volumes of oxygen
and carbon dioxide. The plasma concentrations of blood glucose and blood
lactate were measured using a Reflotron (Boehringer Mannheim Corp., Indianapolis,
IN) and YSI Lactate Analyzer (Model 1500-L, Yellow Springs, OH), respectively.
VO2 max
Test
The cycling VO2 max
test was a continuous protocol which consisted of 3-min stages beginning
with a resistance of 1 kilopond (Kp), and increasing by .5 Kp with every
subsequent 3-min stage. Throughout the test subjects pedaled at a cadence
of 90 rev/min. The VO2
max was established
when at least two of the following conditions were met: (a) a plateau or
a decrease in oxygen uptake associated with an increase in workload; (b)
an R value greater than 1.15; (c) a heart rate within 10 beats of the age
predicted maximum (11); or (d) a blood lactate concentration
of at least 8 mmol/L.
Procedures
Prior to testing, all subjects completed
and signed consent and medical history questionnaire forms. Diet
(three day) and training logs were requested from each subject prior to
each experimental trial. The diet logs were analyzed using the Nutritionist
III Version 7.0 (N-squared computing, Salem, OR). This was done in order
to determine the calorie intake and the percentages of carbohydrate, fat,
and protein which were ingested prior to the treatment trials. To ensure
that testing was conducted during the follicular phase of the menstrual
cycle, subjects completed a menstrual history form. Subjects came to the
laboratory to complete their first experimental session during the first
2 days of their menses. Subsequently, the second session was completed
7 days later.
The subjects performed a total of three
tests on a mechanically braked Monark cycle ergometer. The first test was
performed to measure the VO2
max of the
subjects. Subjects fasted for a minimum of 12 hours prior to performing
two separate treatment trials one week apart but administered at the same
time of day. During the treatment trials the subjects were given, in a
counterbalanced and double blind fashion, either a 7% solution of glucose
polymer solution containing maltodextrin (Exceed) or a placebo solution
(PL) containing artificial sweetener and flavoring (Crystal Light). All
subjects consumed the given treatment in a volume that was set at 2 mL/kg
body weight. This equated to the consumption of 440 to 604 mL during exercise.
The CE or PL solutions were given at 10, 20, 30, and 40 min intervals throughout
the 50 min of exercise at 80% VO2 max (based
on workload). All fluids were kept refrigerated until the time of consumption.
Blood glucose and lactate were collected
at baseline, 23 min, and 46 min during exercise via capillary puncture
(12). Both respiratory exchange ratio (RER) (13)
and RPE (14) values were obtained at 2-min intervals
throughout exercise, averaged into 15-min intervals, and subsequently analyzed
at min 15, 30, and 45 of exercise.
Immediately following both 50-min cycle
ergometer sessions at 80% VO2
max, a Wingate
Anaerobic Power Test (WAT) (15) was administered. With
the use of an optical sensor, interfaced with an IBM compatible computer
and software by SportsMedicine IndustriesTM
(SMI,
St. Cloud, MN), flywheel revolutions were counted and recorded by reading
16 evenly spaced reflectors on the flywheel of the Monark cycle ergometer.
As a result, four indices of the WAT were obtained and recorded for analysis:
(a) peak power, highest mechanical power elicited during the test; (b)
mean power, the average power which was sustained throughout the 30-s test;
(c) minimum power, the lowest mechanical power elicited during the test;
and (d) rate of fatigue, the percent difference between the peak power
and minimum power.
Statistical Analysis
A series of dependent samples t-tests
were used to analyze the dietary intakes 3 days prior to each of the two
experimental testing sessions involving either the CE or an aspartame flavored
water placebo. A series of 2 X 3 (treatment X time) repeated measures
ANOVAs were used to analyze blood glucose, blood lactate, RER, and RPE
obtained for the CE and PL treatment conditions. A dependent groups t-test
was performed to analyze the Wingate Anaerobic Power Test indices of peak
power, mean power, minimum power, and the rate of fatigue for the CE and
PL treatment conditions. A Fisher’s LSD was run as a post hoc multiple
comparison method to determine which mean values differed for variables
that had a significant treatment x time interaction. The 0.05 alpha
level was used for all statistical comparisons.
RESULTS
All data are presented as means ±
SD. Descriptive data for subjects are presented in Table 1. The subjects
of the study were moderately endurance trained with above average VO2
max
and below average body fat values. The results of the paired samples
t-tests yielded no significant (p > .05) differences between the two treatment
conditions for total calories; grams of carbohydrate, fats, and proteins;
and percentage of calories from carbohydrates, fats, and proteins.
Table 1. Descriptive
statistics of female cyclists (N = 10)
Variable
Mean ± SD
Minimum
Maximum
Age (yr)
30.40 ± 7.90
20.00
42.00
Height (cm)
168.15 ± 4.29
160.02
175.26
Weight (kg)
63.38 ± 7.28
55.00
75.50
Body Fat (%)
17.42 ± 2.69
14.00
22.90
VO2 max (ml/kg/min)
47.13 ± 3.75
40.34
52.89
Blood Glucose
A significant interaction for treatment
condition by time was found for blood glucose levels [F = 5.93, Table F
(2, 16) = 3.63, p = .012]. No significant (p > .05) difference was found
between the carbohydrate- electrolyte beverage (CE) and the placebo (PL)
at baseline and min 23. However, mean blood glucose was found to be significantly
(p < .05) higher for CE than PL at min 46 of the time trial (see Figure
1).
Figure 1. Blood glucose
levels (mmol/L) across time
for the carbohydrate-electrolyte
and placebo treatment
conditions. *
= significant difference between PL and
CE (p < 0.05).
Blood Lactate
There was neither a significant treatment
by time interaction nor a treatment effect (CE vs PL) for blood lactate;
therefore, data were collapsed across groups. There was a significant time
effect. Lactate levels were not different between min 23 and min 46, but
levels were elevated above baseline at minutes 23 and 46 (see Figure 2).
Figure 2. Plasma lactate
(mmol/L) over time collapsed
baseline (p < 0.05).
RER
There was neither a treatment by time
interaction nor a treatment effect on RER. A significant (p < .05) time
effect was found in mean RER across the three averaged 15 min intervals.
Significant (p < .05) differences were found between all comparisons.
Minutes 30 and 45 were both found to be significantly (p < .05) higher
than min 15. Min 45 was also found to be significantly (p < .05)
higher than min 30 (see Table 2).
Table 2. RER collapsed
across
treatments and RPE data
during CE and PL trials
Variable
15 Min
30 Min
45 Min
RER
.95 ± .06*
.96 ± .07
.99 ± .07**
RPE (PL)
3.9 ± 1.1§
5.1 ± 1.5
5.5 ± 1.9
(CE)
4.2 ± 1.4
4.7 ± 1.6
5.1 ± 2.1
* Min 15 less than minutes
30 or 45 (p < .05)
** Min 45 higher than min
30 (p < .05)
§ Min 15 less
than minutes 30 or 45 (p < 0.05)
RPE
A significant treatment by time interaction
was found for RPE [F = 6.07, Table F (2, 18) = 3.55, p = .010]. Post
hoc analyses indicated that there was no significant difference in
RPE values across time for the CE treatment condition [F = 2.62, Table
F (2, 18) = 3.55, p = .10]. However, for the PL treatment condition, significant
(p < .05) differences were found for RPE scores across the time intervals
[F = 8.73, Table F (2, 18) = 3.55, p = .00]. No significant (p > .05) difference
was found in mean RPE between min 30 and min 45. However, both minutes
30 and 45 were found to be significantly (p < .05) higher than min 15
for the PL group (see Table 2).
Sprint Performance
Four indices of performance were obtained
from the WAT and evaluated. No significant difference was found between
the CE and PL treatment conditions. Data are presented in Table 3.
Table 3. Data
from the Wingate test
Variable
Pl
CE
Peak power (watts)
491.5 ± 101.9
508.2 ± 99.4
Mean power (watts)
425.7 ± 72.7
431.7 ± 63.3
Minimum power(watts)
356.6 ± 47.5
361.9 ± 36.0
Rate of fatigue (%)
26.0 ± 9.2
27.2 ± 10.7
DISCUSSION
Following a 12 hour fast, liver glycogen
is dramatically reduced (4). Therefore, there would be
a tendency for plasma glucose levels to be reduced if an exogenous source
of glucose is not ingested during prolonged exercise. In this study,
as expected, blood glucose levels were higher in the CE trial than in the
PL trial. This did not result in higher WAT scores, although it might have
affected the perceived effort (i.e., RPE) of the subjects. In addition,
fatigue rate was not significantly different between the two trials.
RPE is a variable that gives an indication
of the perceived difficulty of the work performed by the subjects (14).
The significant treatment by time interaction indicated that RPE increased
more during the PL trial than the CE trial. Therefore, the subjects did
not feel as if they were working as hard during the later stages of the
50 min ride as a result of the periodic consumption of the CE drink during
exercise. Consumption of a carbohydrate beverage appears to decrease the
perception of fatigue.
During the 50 min ride, there was an increase
in RER. This would suggest that over time, there was a relative increase
in the rate of carbon dioxide production compared to oxygen consumed. Carbon
dioxide is produced as a byproduct of respiration, with more resulting
from carbohydrate than fat catabolism, or as a result of the buffering
of free protons by bicarbonate. From the results of this study, it
is very difficult to determine which of these is responsible for the observed
rise in RER.
Gender differences may account for the
discrepancy between the results of the current investigation, which used
female cyclists, and previous studies of a similar nature, which used male
cyclists (7,8). In the current study
as well as those of Ball et al.(7) and Below et al.(8),
subjects intermittently ingested a carbohydrate electrolyte beverage and
performed 50 min of cycle ergometry at 80% VO2 max
followed by a high intensity “sprinting” test. Unlike the current study,
carbohydrate ingestion enhanced sprint performance in the aforementioned
cyclists (7,8).
It is also important to note that in this
study, as was the case in the Ball et al.(7) study, subjects
were given the same relative dose (i.e., 2 ml/kg) of the identical carbohydrate
electrolyte beverage. For the males, this was equivalent to the consumption
of carbohydrate at the rate of approximately 53 g/hr compared to 36 g/hr
in the females. Therefore it is possible that the female subjects may have
received the carbohydrate supplement at a rate that was below the minimum
threshold needed to have an ergogenic effect (16).
Tarnopolsky et al. (17,18)
have shown that trained females oxidize lipids at a greater rate during
submaximal exercise than equally trained males. This same group of researchers
have also shown that trained female endurance athletes show a blunted increase
in intramuscular glycogen and virtually no increase in performance following
a period of carbohydrate loading when compared to males (17).
These findings are consistent with the observations of the current study.
The ovarian hormones (estrogen and progesterone)
are thought to have significant effects upon substrate utilization during
exercise (19). Estrogen has been shown to enhance lipid
oxidation which has a carbohydrate sparing effect (20).
The levels of these steroid hormones vary in a relatively predictable manner
during a 28-34 day cycle. Therefore, substrate use is influenced by the
phase of the menstrual cycle in which exercise is performed (21).
In the present study, all subjects were tested during the follicular phase
of their menstrual cycles at a time when estrogen levels are thought to
be low and the confounding effects of progesterone are minimal (21,22).
During the luteal phase of the menstrual cycle, both estrogen and progesterone
levels are high, and glycogen sparing is enhanced (23).
However, even during the follicular phase (when estrogen is generally lower),
females still derive a greater proportion of energy from fat and utilize
less carbohydrate than males (18). This greater reliance
upon fat may be directly related to estrogen's effect upon fat metabolism
or indirectly via its action upon human growth hormone (22).
The use of an oral contraceptive (OC) by
a female athlete can have an effect on energy substrate usage during exercise.
OCs have been shown to influence growth hormone, and plasma glucose levels
(24). These observed responses suggest that OCs have
a carbohydrate sparing effect (24). Since five
of the ten subjects in the present study were using some form of an OC
this may have had an effect on substrate metabolism and the WAT scores.
It is also possible that the inclusion
of more subjects would result in a different outcome. A power analysis
indicated that it would have taken approximately 15 (power = 70%) to 23(power
= 90%) female subjects to detect a treatment difference in this study.
In contrast, in males, treatment effects have been detected with the use
of as few as 8 subjects (7)
In conclusion, the feeding of a 7% CE solution
(in the same relative dosage as in males) does not significantly improve
short term (i.e., < 60 min) high intensity (i.e., > 80% VO2
max)
exercise performance of trained female subjects. This may be caused by
gender differences in substrate utilization during exercise. A higher relative
dose of carbohydrate may be required to produce an ergogenic effect during
high intensity exercise in females.
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Corresponding Author:
Samuel
A. Headley, Allied Health Sciences Center, Springfield
College, 263 Alden Street, Springfield, MA 01109, Tel # 413-748-3340
Copyright
©1999
American
Society of Exercise Physiologists
All Rights Reserved