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1 Department of Human Biology and Nutritional Sciences, University of Guelph, Guelph, Ontario N1G 2W1; 2 Department of Medicine, McMaster University, Hamilton, Ontario, Canada L8N 3Z5; and 3 Division of Clinical Chemistry, Huddinge University Hospital, Huddinge S-141 86, Sweden
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ABSTRACT |
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 TOP
 ABSTRACT
 INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
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This study examined the ability of well-trained
eumenorrheic women to increase muscle glycogen content and endurance
performance in response to a high-carbohydrate diet (HCD; ~78%
carbohydrate) compared with a moderate-carbohydrate diet (MD; ~48%
carbohydrate) when tested during the luteal phase of the menstrual
cycle. Six women cycled to exhaustion at ~80% maximal oxygen uptake
(
O2 max) after each
of the randomly assigned diet and exercise-tapering regimens. A biopsy
was taken from the vastus lateralis before and after exercise in each
trial. Preexercise muscle glycogen content was high after the MD (625.2 ± 50.1 mmol/kg dry muscle) and 13% greater after the HCD (709.0 ± 44.8 mmol/kg dry muscle). Postexercise muscle glycogen was low after
both trials (MD, 91.4 ± 34.5; HCD, 80.3 ± 19.5 mmol/kg dry muscle),
and net glycogen utilization during exercise was greater after the HCD.
The subjects also cycled longer at ~80%
O2 max after the HCD
vs. MD (115:31 ± 10:47 vs. 106:35 ± 8:36 min:s, respectively). In
conclusion, aerobically trained women increased muscle glycogen content
in response to a high-dietary carbohydrate intake during the luteal phase of the menstrual cycle, but the magnitude was smaller than previously observed in men. The increase in muscle glycogen, and possibly liver glycogen, after the HCD was associated with increased cycling performance to volitional exhaustion at ~80%
O2 max.
eumenorrhea; luteal phase; glycogen loading; muscle glycogen supercompensation
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INTRODUCTION |
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TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
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PREVIOUS STUDIES HAVE DEMONSTRATED that a combination of intense aerobic exercise, increased dietary carbohydrate (CHO) intake, and exercise tapering can lead to an increased resting muscle glycogen content or "supercompensation" (3, 7, 8, 13, 21, 34) and ultimately enhance exercise endurance performance (3, 8, 21, 23), as reviewed by Hawley et al. (17). These studies examined primarily male subjects, and the assumption has been that the results also apply to women, although there are few data examining these relationships in aerobically trained women.
Recent, well-controlled, gender-comparison studies have matched male
and female endurance athletes for many of the following parameters:
maximal oxygen uptake
[O2 max;
expressed per kg lean body mass (LBM)]; preexercise dietary
intake; exercise training volume, intensity, and history; and
controlled-for menstrual status (19, 30, 38, 40, 41). Women maintained
lower respiratory exchange ratios (RER), suggesting an increased
reliance on fat oxidation, compared with men during cycling and running
at power outputs between 40 and 75%
O2 max (19, 30, 38, 40,
41). These studies and recent reviews have discussed the potential reasons for the greater reliance on fat oxidation in women (11, 33,
39). In one study, muscle glycogen stores were unchanged in women in
response to a 4-day high-CHO diet (HCD; ~75% CHO) compared with a
moderate-CHO diet (MD; ~57% CHO), whereas the men increased muscle
glycogen content by ~40% after the high-CHO regimen
(40). Furthermore, only men increased cycle time to exhaustion (80-85%
O2 max, after
1 h at 70-75%
O2 max) in response to
the dietary CHO-loading regimen. Therefore, women may not be able to
supercompensate muscle glycogen stores before exercise and increase
exercise performance in response to increases in dietary CHO.
The female athletes employed in the above studies were tested during the follicular phase (FP) of the menstrual cycle when circulating reproductive hormones are low (19, 38, 40, 41). Hackney et al. (15) observed 13% higher resting muscle glycogen contents in the luteal phase (LP) compared with the FP of the menstrual cycle when they controlled for diet and exercise in the 36 h before muscle glycogen sampling. In contrast, Nicklas et al. (24) studied moderately trained women and reported no significant difference in muscle glycogen content between phases of the menstrual cycle after a glycogen-depleting exercise bout and 3 days of a controlled diet (~56% CHO) with no exercise. However, a significantly greater amount of muscle glycogen repletion occurred during the 3 days in the LP vs. the FP [379 ± 20 vs. 313 ± 25 mmol/kg dry muscle (dm)]. These studies suggest that glycogen synthesis may be increased during the LP of the menstrual cycle. Animal studies support this suggestion, because elevations in the reproductive hormones, estradiol (E2) and progesterone (P), in the LP increase liver and skeletal muscle glycogen synthesis and the duration of exhaustive exercise in rats (10, 22, 37). Therefore, the testing of women during the LP of the menstrual cycle may demonstrate that female athletes are able to supercompensate muscle glycogen stores and improve endurance performance.
The purpose of this study was to examine the influence of increased
dietary CHO intake, employed during an exercise-tapering program, on
resting muscle glycogen content and performance during cycle exercise
to volitional exhaustion at ~80%
O2 max in aerobically trained women during the LP of the menstrual cycle.
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METHODS |
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TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
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Subjects.
Six well-trained female athletes gave written informed consent and
volunteered to participate in the study after approval by the
University of Guelph Human Ethics Committee. All subjects cycled
regularly because three were triathletes, two were cyclists, and one
was an endurance runner who trained on a cycle twice a week. All
subjects were eumenorrheic with regular cycles of 24-29 days.
Characteristics of the subjects are presented in Table
1.
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Preexperimental protocols.
The subjects performed a progressive
O2 max test on a
cycle ergometer during their initial visit to the laboratory. On a separate day, all subjects performed a practice cycle to voluntary exhaustion at a constant power output of ~80%
O2 max. Exhaustion was defined as the point when the subject could no longer maintain a
cadence 20 rpm below their predetermined cadence (80-100 rpm), despite verbal encouragement. If a subject was unable to cycle for 75 min, a second practice ride to exhaustion was performed at a later date
at a slightly reduced power output.
Diet preparation. The subjects were given detailed instructions to accurately record food intake, and they recorded daily food records for 4-7 days, including 1-2 weekend days. In addition, a list of specific food likes and dislikes were compiled. All food records were analyzed by using the NUTRI-PRO diet-analysis software (West Publishing, ESHA Research, 1991) with manual adjustments when specific foods were not available in the nutrition program. Total daily caloric intake; total grams of CHO, protein, and fat consumed per day; and percent contributions of CHO, protein, and fat to daily total caloric intake were determined. Two 7-day diets, one consisting of MD (~48% total daily energy from CHO) and one of 3 days of MD followed by 4 days of HCD (~78% CHO) were designed and randomly assigned to the subjects.
Diet and exercise tapering regimen.
On the day before day 1 of each of the two diet and
exercise-tapering regimens, all food and drink to be consumed over the next 7 days was purchased and delivered to the subject. The subject followed the designed diet and recorded the actual amount of food eaten. Frequent communication occurred between the subject and researcher during the 7-day regimen to ensure accurate recording of the
diet. After the initial 7-day regimen, the actual dietary consumption
was analyzed, and these results were used to adjust the diet of the
second 7-day regimen. The characteristics of all diets (habitual, MD,
and HCD) are presented in Table 2.
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Experimental protocol. Subjects arrived at the laboratory at the same time of the day for both trials, having eaten the appropriate meal within the previous 2-4 h to simulate precompetition practices. Subjects were weighed on both occasions (Table 1 data represent mean body mass because no difference existed between trials). A catheter with a saline drip was placed in a peripheral arm vein for blood sampling, and a resting sample was taken. One leg was prepared for percutaneous needle biopsy of the vastus lateralis muscle. Two incisions of the skin were made through to the deep fascia under local anesthesia (2% lidocaine without epinephrine) as described by Bergstrom (6). A resting biopsy was taken before exercise and frozen in liquid N2.
Subjects cycled at ~80%O2 max to voluntary
exhaustion determined by the inability of the subject to maintain a
cadence 20 rpm below the predetermined cadence, despite verbal
encouragement by a researcher blinded to the preexercise dietary
regimen. The saline drip was maintained during exercise, and water was
given ad libitum. Total volume intake (saline and water) was constant between trials. Expired air was sampled for the determination of oxygen
uptake (O2), carbon dioxide
output (CO2), and RER at 5, 15, and every 15 min throughout exercise, and at exhaustion. Blood
samples were taken every 20 min during exercise and at exhaustion. At
the point of fatigue, a second biopsy was immediately taken and frozen
in liquid N2.
Analyses. Expired gas samples were analyzed on-line with a calibrated metabolic measurement cart (model 2900Z, Sensor Medics) .
Blood samples were fluorometrically assayed for whole blood glucose, lactate, and glycerol as described by Bergmeyer (5). A Wako NEFA C kit (Wako Chemicals, Dallas, TX) was used to measure free fatty acids (FFA). Plasma P, E2, and insulin concentrations were determined by using radioimmunoassay (125I-labeled hormone) Coat-A-Count kits (Diagnostic Products, Los Angeles, CA). Muscle biopsies were stored in liquid N2, freeze-dried, and powdered, and visible blood and connective tissue were removed. Samples were stored in desiccant at
80°C until analyzed. Three aliquots of muscle (~3 mg) were incubated with 0.1 M NaOH for 10 min
at 80°C to destroy background hexosemonophosphates and glucose and
were neutralized with 0.1 M HCl and 0.2 M citric acid. Glycogen was enzymatically converted to glucose with amyloglucosidase and assayed spectrophotometrically (model DU-70, Beckman Instruments, Mississauga, ON) as described by Harris et al. (16).
Calculations.
Total CHO and fat utilization during the initial 75 min of all trials
(and entire trial) were calculated by using the measured O2 and
CO2 data, by using the
following equations (29)
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O2 max in
well-trained men (32).
Statistical analysis. Pulmonary and blood measurements were analyzed by using a two-way analysis of variance with repeated measures. When a significant F ratio for time or interaction of time and diet was found, a Newman-Keuls post hoc test was applied to locate differences between specific means. Pre- and postexercise muscle measurements, glycogen-utilization data, exercise performance, and all exhaustion pulmonary and blood (because samples were not available from all subjects, and exhaustion times were different between trials) data were analyzed by using a one-tailed paired t-test. Statistical significance was accepted at P < 0.05. Data are expressed as means ± SE.
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RESULTS |
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TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
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Performance variables.
The women cycled longer at ~80-82%
O2 max after the HCD
(115:31 ± 10:47, min:s) compared with the MD (106:35 ± 8:36 min:s). There were no significant differences in
O2 or heart rate throughout the exercise between trials (Table 3).
However, RER was significantly higher during the initial 75 min of the
HCD vs. MD trial (significant main effect for trial).
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Muscle glycogen measurements.
Preexercise glycogen content increased in five of the six athletes
after the HCD and tapering regimen, resulting in a significant increase
of 84 mmol glucosyl units/kg dm (13%) over the MD regimen (Table
4, Fig. 2).
Postexercise muscle glycogen contents were below 100 mmol/kg dm in
both trials and were not significantly different (Fig. 2). Net glycogen
utilization during the HCD trial was also significantly greater than
that during the MD trial.
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Blood measurements.
Plasma insulin concentrations were constant during the initial 60 min
of exercise and decreased at exhaustion in both conditions (Fig.
3). Whole blood glucose increased during
exercise and returned to rest levels at exhaustion in both conditions
(Fig. 4). There were no differences between
dietary conditions in insulin or glucose. Whole blood lactate was
significantly elevated in response to exercise and was significantly
higher during the HCD compared with the MD trial (Fig.
5). Plasma FFA remained at low resting concentrations in both trials throughout exercise (Table
5). Whole blood glycerol concentrations
increased significantly above rest in both trials but were unaffected
by diet (Table 5).
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Reproductive hormones.
Resting plasma P and E2 were high on days 1 and
7 of the diet and tapering regimen in both trials and
indicative of the LP of the menstrual cycle (Table
6). During exercise on day 7,
plasma E2 and P were generally unchanged over time,
although plasma E2 increased above rest at 40 and 60 min in
HCD (Table 6).
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Estimated CHO and fat oxidation. The subjects oxidized significantly more CHO during the initial 75 min of the HCD (2.79 ± 0.22 g/min) compared with the MD trial (2.35 ± 0.19 g/min). The total estimated CHO oxidation in the initial 75 min of cycling was 209 ± 16 g in HCD and 176 ± 14 g in MD. Fat oxidation was significantly less during the HCD (0.25 ± 0.05 g/min) compared with the MD trial (0.42 ± 0.04 g/min), resulting in estimated total fat utilization of 19 ± 4 g in HCD and 31 ± 3 g in MD during the first 75 min of exercise.
Total CHO oxidation during the entire trial was also significantly greater after the HCD (317 ± 28 g) compared with the MD (252 ± 33 g). Total fat oxidation was significantly lower during the HCD (34 ± 8 g) compared with the MD trial (49 ± 7 g). CHO accounted for 81% of the oxidized substrate in the HCD trial and 70% during the MD trial.| |
DISCUSSION |
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TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
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A 7-day diet and exercise-tapering regimen, containing 48% of the
total caloric intake as CHO, resulted in high skeletal muscle glycogen
contents in well-trained eumenorrheic women during the LP of the
menstrual cycle. Glycogen content increased by 13% (84 mmol/kg dm)
when the diet was altered to contain 78% CHO in the final 3-4
days of the regimen. The increase in muscle glycogen, and possibly
liver glycogen, was associated with increased CHO oxidation and cycle
time to volitional exhaustion at ~80%
O2 max. Therefore, the
women were able to supercompensate glycogen but not to the same
magnitude as generally reported in men.
Dietary CHO and resting muscle glycogen content. The present findings of a small but significant increase in glycogen content with an increased dietary intake of CHO differ from an earlier study reporting an inability of trained women to supercompensate glycogen (40). There are several potential explanations to account for the differing effectiveness of the HCD regimen in the two studies. The most obvious is the testing of subjects in the LP of the menstrual cycle in the present study, as opposed to the FP in the previous study (41). Others include the higher muscle glycogen values obtained after the MD in the present study and the higher amounts of CHO consumed during the HCD in the present study.
During the LP, the reproductive hormones E2 and P are elevated, in contrast to during the FP when the hormones are suppressed. Rodent data suggest that the reproductive hormones may augment glycogen synthesis and resting muscle glycogen contents (10, 22, 37). In humans, only two studies have measured resting glycogen concentration in both menstrual phases in the same individuals, and neither study employed well-trained endurance athletes. Hackney (15) measured a higher resting muscle glycogen content in the LP compared with the FP (439 ± 21 vs. 390 ± 15 mmol/kg dm) while controlling for diet and exercise in the 36 h before muscle sampling. Nicklas et al. (24) measured muscle glycogen content after a bout of depleting exercise and again after 3 days of rest and a controlled diet (~56% CHO) in both menstrual phases in moderately trained women (O2 max 44.9 ml · kg
1 · min1).
Glycogen repletion was significantly greater during the 3 day in the LP
vs. the FP (379 ± 20 vs. 313 ± 25 mmol/kg dm), although the
resulting muscle glycogen was not statistically higher (LP, 446 ± 24 vs. FP, 400 ± 25 mmol/kg dm). Therefore, whereas the LP may be
associated with marginally higher rates of glycogen synthesis and
higher glycogen contents, no study has examined the ability to
supercompensate muscle glycogen in both phases of the menstrual cycle
in the same women.
The subjects in the present study were well trained and included
cycling in their normal training programs. After the MD, they achieved
resting vastus lateralis muscle glycogen contents that were ~50%
higher than those reported by Tarnopolsky et al. (40). Although the
training status and
O2 max of the two
groups of subjects were similar, it may be that including cycling as a
regular component of the training programs in the present study contributed to the difference. Cycling places localized stress on the
vastus lateralis muscle and may have augmented the ability to increase
glycogen during the HCD and tapering regimen. In support of this,
Greiwe et al. (14) reported muscle glycogen contents of ~800 mmol/kg
dm in a group of four women and two men who trained for 10 wk on a
cycle ergometer.
Other explanations relate to the larger difference in the percent CHO
intake between the MD and HCD in the present study or, more
importantly, the attainment of higher absolute and relative amounts of
ingested CHO in the HCD. In the present study, the percent CHO intake
was 48 and 78% in the MD and HCD, respectively, as opposed to 57 and
75% in the previous study (40). The absolute and relative CHO
consumption in the HCD was 464 g and 10.1 g/kg LBM in the present study
and was 370 g and 7.9 g/kg LBM in a previous study, respectively (40).
Earlier studies with men reported that a CHO intake >500 g/day and
8.5 g/kg LBM was consumed to increase glycogen by 300 to >400 mmol/kg
dm, compared with MDs (7, 13, 20, 21, 34). In the present study, the
women approached this absolute level and achieved this
relative level of CHO intake, possibly accounting for the
supercompensation that did occur.
However, as discussed by Tarnopolsky et al. (40), it may be difficult
for women whose habitual caloric intakes are <2,400 kcal/day (12, 28,
40) to achieve higher CHO-intake values. This is lower than male
athletes who consume >3,000 and often >5,000 kcal/day during CHO
loading and can easily consume >500 g CHO/day during a 70-75%
CHO diet. To test whether higher dietary CHO intake would increase
muscle glycogen supercompensation, female athletes would need to
increase their caloric intake to ~2,700 kcal/day for 3-4 days,
with ~78% of the energy derived from CHO.
Dietary CHO, resting muscle glycogen content, and endurance
performance.
The increase in muscle glycogen in the HCD and tapering regimen was
associated with a prolonged cycle time to exhaustion at ~80%
O2 max,
during the LP in the female athletes. The performance increase (8%)
was smaller than reported in some studies with men (20%) but was
consistent with the general pattern of performance increases in
CHO-loading studies where exercise lasts >90 min and muscle glycogen
concentration is below ~110 mmol/kg dm at exhaustion (see Ref. 17 for review).
O2 max after
three dietary conditions (72, 54, and 13% of the total caloric intake
as CHO). They reported improved cycle performance after the HCD and MD
compared with the low-CHO diet (113 ± 28, 98 ± 13, and 60 ± 12 min, respectively). The cycle times were longer on the HCD
because five of seven subjects increased their performance after the
HCD compared with the MD, but the increase was not statistically
significant. However, muscle glycogen was not measured and menstrual
status was not controlled for in this study (27).
It is unlikely that the increased cycle performance in the present
study after the HCD regimen was solely dependent on an increased muscle
glycogen store, because the HCD may have increased liver glycogen
stores. Both liver glycogen and glucose production by the liver are
very responsive to dietary CHO intake in men. A
moderate-CHO diet (~40% CHO) resulted in a threefold greater hepatic
glucose release than after a CHO-poor diet (25), and substantial
increases in liver glycogen content were observed after a high-CHO diet
compared with starvation or a CHO-poor diet (26). Although the
influence of the HCD vs. MD regimens on liver glycogen in women is
unknown, the HCD would be expected to increase resting liver glycogen
content. This may have resulted in a higher rate of glucose release
from the liver in the latter stages of exercise (when muscle glycogen
supply is low) and contributed to the longer cycle time in the HCD
performance ride.
Whole blood venous glucose concentrations were not different between
the MD and HCD trials during cycling and were not at hypoglycemic
levels at exhaustion in either trial. However, venous concentrations do
not reflect the rate of appearance and disappearance of glucose during
exercise. Furthermore, CHO oxidation was higher during cycling after
the HCD regimen, implying that liver glucose provision may have
contributed to the increased CHO oxidation throughout the trial.
Additional studies examining glucose rates of appearance and
disappearance during exercise after HCD and MD regimens are required to
clarify this issue.
Potential mechanisms for reduced muscle glycogen synthesis in women. The present study and an earlier study (40) both suggest that well-trained women supercompensate glycogen to a smaller extent than do men. An explanation would likely involve gender differences related to the uptake and storage of glucose. These may include differences in the uptake of glucose during the basal state via GLUT-1; glucose uptake after exercise via the exercise and insulin-sensitive GLUT-4; phosphorylation of glucose by the enzyme hexokinase (HK); and factors associated with the synthesis of glycogen, including the activity of glycogen synthase (GS), the availability of glycogenin, and the formation of pro- and macroglycogen.
Although still controversial, there is strong evidence that the rate-limiting step for glucose uptake and glycogen synthesis in muscle is glucose transport (as reviewed in Refs. 4, 18). However, we are unaware of any gender comparisons of GLUT-1 and -4 contents in human skeletal muscle, although no difference was reported in the rate of muscle glycogen synthesis in the 4-h period after 90 min of cycling in well-trained men and women (41). This argues that glucose uptake from the basal GLUT-1 and GLUT-4 remaining in the plasma membrane and T tubules was not limiting glucose uptake in the women during this 4-h postexercise period. There have been reports of higher maximal HK activities in untrained men vs. women, but no differences were reported between active men and women (as reviewed in Refs. 33, 39). However, we were unable to find comparisons of maximal HK and GS activities in well-trained men and women, although the presence of testosterone has been shown to increase GS activity and glycogen synthesis in rat skeletal muscle (2). Lastly, the importance of glycogenin availability and the formation of pro- and macroglycogen for maximal glycogen storage have recently been investigated in human skeletal muscle (1), but gender comparisons do not exist.Summary.
We observed that well-trained, eumenorrheic female endurance athletes
who follow a 7-day diet and exercise-tapering regimen containing 48%
of the total caloric intake as CHO achieved high muscle glycogen
contents (625 mmol/kg dm) during the LP of the menstrual cycle. When
the dietary CHO content was increased to 78% in the final 3-4
days of the diet and tapering regimen, muscle glycogen content
increased a further 13%. The increase in muscle glycogen, and possibly
liver glycogen, after the high-CHO regimen was associated with
increased CHO oxidation and cycling time to exhaustion at ~80%
O2 max. Therefore,
female athletes were able to supercompensate glycogen but to a smaller
extent than generally reported in male athletes undergoing the same
dietary and exercise-tapering regimen. Little information presently
exists to explain this gender difference in the ability to store muscle glycogen.
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ACKNOWLEDGEMENTS |
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We thank Dr. D. Friars for assistance with the muscle biopsy sampling.
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FOOTNOTES |
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This work was supported by research grants from Sport Canada and the Gatorade Sport Science Institute. J. L. Walker was supported by a Postgraduate Scholarship from the Natural Sciences and Engineering Research Council of Canada. G. J. F. Heigenhauser is a Career Investigator of the Heart and Stroke Foundation of Ontario (I-2576).
The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. §1734 solely to indicate this fact.
Address for reprint requests and other correspondence: L L. Spriet, Dept. of Human Biology and Nutritional Sciences, Univ. of Guelph, Guelph, Ontario, Canada N1G 2W1 (E-mail: lspriet.ns{at}aps.uoguelph.ca).
Received 18 October 1999; accepted in final form 31 January 2000.
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REFERENCES |
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TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
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